Minerals – Coconino Lapidary Club

Pyrite

Mineral Man Mel — Sun, 13 Oct 2024 00:12:47 +0000

This is a brief introduction to the mineral Pyrite, or when we were young, known as Fools Gold. I’m also going to tell you about one mineral species which forms kidney stones in people.

Pyrite is an iron sulfide with two atoms of sulfur for every iron atom, giving it the formula FeS_2.[Ref 1] It is found in varied geological environments, such as in the hydrothermal veins of metal mines, [Ref 1] or in black shales, having as its sources the iron and sulfur from former sea life e.g. [Ref 2]

Because of its brilliant golden color, and often pristine crystal forms, it is a favorite among collectors. Specimens of pyrite from metal mines are often quite spectacular, as seen in the photos below, of Pyrite accompanying the zinc ore mineral Sphalerite and quartz, and alone in crystal clusters as found in metal mines.

Pyrite and Sphalerite on Quartz, Alimon Mine, Peru

Pyrite, Huanzala mine, Peru

Pyrite crystallizes in the cubic crystallographic system. (Ref 1) It exhibits a number of forms, as seen in Figures 1, 2, 3 & 4, either singularly such as the cube, or in combinations of these forms. Scrolling through the gallery of images available when Googling “Pyrite” will show you specimens exhibiting such forms from many worldwide locations.

Pyrite octohedra, Huanzala Mine, Peru

Pyrite pyritohedrons, Huanzala Mine, Peru

Pyrite Crystal forms

Pyrite specimens found in black shales rich in organic-materials, occur in a number of interesting forms, found when replacing an animal and forming a fossil or in sun shapes formed of tiny radiating crystals, and as aggregates of small crystals as seen in the photos below.

Fossil Ammonites in Black Shale, Bavaria, Germany

Pyrite sun, Sparta, Ohio

Aggregate of Cubic Pyrite Crystals, Pilbara, Western Australia

Above, I’ve emphasized pyrite accompanying other ore minerals. However, pyrite itself used to be an important ore for the production of sulfuric acid and sulfur. (Ref 3) The first step in these recoveries was roasting the ground pyrite in the presence of air. (Ref 4) The past and present most important use of pyrite is as an ore of gold. (Ref 3) In these ores, gold occurs as an impurity. (Ref 5)

Ending on a different note, one type of kidney stone is chemically a calcium phosphate which has the same composition as the mineral Apatite, (Ref 6), an example of one of the minerals present in life forms.

Ref 1 https://www.mindat.org/min-3314.html

Ref 2 http://www.indiana.edu/~sepm04/PDF/JS-J28-pyrite_balls.pdf

Ref 3 http://geology.com/minerals/pyrite.shtml

Ref 4 http://www.saimm.co.za/Conferences/Sulphur2009/101-110_Runkel.pdf

Ref 5 Abstract in: http://www.otago.ac.nz/geology/research/gold/geology-and-gold/gold-and-arsenic.html

Ref 6 https://www.mindat.org/min-29229.html

Opals – As Gemstones & it’s Biological Role in Grasses

Mineral Man Mel — Sun, 13 Oct 2024 00:12:47 +0000

Today I want to talk about opal as a gemstone and it’s, not expected, biological role in grasses and algae.

Opal as a Gemstone

Gem-quality opals are characterized as Precious Opal, also called Fire Opals and Common Opals. Precious Opal as a gemstone is prized by collectors and can display all the colors of the rainbow, (Figures 1-4), or present only one (Figures 5 & 6). The differences in the presentations of colors lies in their structural and chemical differences.

Figure 1. Black Opal, Lightning Ridge, Coober Pedy, South Australia

Figure 2. Opal in Matrix, Queretaro, Mexico

Figure 3. Black opal (Fossilized tree limb), Virgin Valley, Nevada

Figure 4. Black Opal, Constellation Mine, Spencer, Idaho

Figure 5. Faceted Opal, Mexico

Figure 6. Opal, Queretaro, Mexico

As seen in [Ref 1], chemically, opal is a hydrated silicon dioxide with the formula SiO₂-nH₂O [Ref 2] in which “n” denotes a variable water content. In Fire Opal, (Opal AG), the weight percentage of water as estimated from Figure 3 in [Ref 2], is about 5%. Opal is not structured like Quartz, (SiO₂), in which the silicon and two oxygen atoms are arranged in a fixed array throughout the crystal. In opal the silicon dioxide is amorphous with no crystalline order and forms spheres in the 10s-100s nanometers size-range.

In Fire Opal the spheres are typically arranged within a layer in a very nearly ordered fashion, either in a square or hexagonal array, as shown in Figure 7, in which both are in different regions within the layer. These layers, when stacked, present an orderly arrangement as shown in Figure 8.

Figure 7. Regions of both square and hexagonal packing within a layer of Fire Opal. The regions may display different colors or “fire” (See later in Blog).

Figure 8. Stacked layers of silica spheres in “fire” Opal.

In Common Opals the spheres are not arranged in an orderly fashion, but are in a disorderly array as seen in Figure 9. And, in some instances, spheres with large differences in their diameters and with an orderly arrangement as seen in Figure 10. These features prevent light diffraction.

Figure 9. Disorderly array of spheres in common Opal.

Figure 10. Common Opal with large disparity in sphere diameter.

Source of Color in Preciouse Opal

In Precious Opal the uniform diameters of the silica spheres result in a uniform thickness of the layers as seen in Figure 8. This structural feature of precious opal allows it to diffract light [Ref 6]. As in diagram below, the wavelength of the light given by the symbol λ which is diffracted depends directly on the distance the light travels between each layer as given in the diffraction equation: λ =2dsin(α) where d is the diameter of the spheres and α is both the angle that is incident and diffracted away [Ref 6]. The total distance traveled by both incident and diffracted light depends upon the angle α at which the total incident and diffracted light is travels and equals 2d sin(α). When the distances traveled by both their incident and diffracted light waves are equal they reinforce each other. This effect gives the “fire” in the color as no other color is selected. When the light strikes perpendicular to the layers λ = 2d. When the light strikes at any other angle α, the distance traveled is lessened by the factor sin(α) and the corresponding wavelength which is diffracted is larger.

In Precious Opals the uniformity of the diameter of the spheres and in various regions of the gem the spacing of the spheres is not necessarily uniform over the entire volume of the opal. These factors will result in the diffraction of different wavelengths over the surface of the gem as evident in Figures 1-4.

Schematic of light diffraction in precious Opal

Sources of color in Fire Opal

In Fire Opal, a variety of Common Opal, the diameters of the spheres are in the range o5 to 25 nanometers. Spheres this size being only about one-tenth that of visible light do not diffract and do not present the play of colors as does Precious Opal; they possess the disordered structural features of Common Opal as shown in Figures 9 & 10. Instead the color is uniform over the volume of the opal, and is imparted by inclusion of the iron compound Fe₂O₃ as a pigment [Ref 7]. As examples: yellow, orange, red, & brown Common Opals of Mexico have nanometer-sized iron inclusions. Similarly the blue-colored opals from Peru do not diffract and are colored by a copper silicate mineral [Ref 7].

Opals in Biology

Having made a transition from the material sciences to biophysics along my career path, I became intrigued with the interrelationships between the worlds of the inorganic and the organic. Among these interrelationships are the roles of minerals in biological organisms. In addition to the vital roles of the elements such as iron, iodine, calcium, potassium, sodium, phosphorous, in the biology of the cell and in organism, minerals also play important roles as skeletal structural elements across the animal and plant kingdoms. The roles of these structural elements are to support and/or protect the soft tissues and organs of the organism.

How Opal Protects the Cell Walls of Grasses and Diatoms

As an example of opal in biology I will describe the mechanisms whereby opal serves to protect grasses and the soft cellular components of diatoms, a major group of microalgae.

In grasses

Grasses, by virtue of their environments, are subject to attack and consumption by both insects and grazing animals. As a protective mechanism, leaf, stem, and roots of many grasses have adapted by hardening their outermost layer of tissue, the epidermis [Ref 8 & 9]. Hardening of the tissue is achieved through deposition of hydrated silica (SiO₂-nH₂O) in the walls of the epidermal cells [Ref 8]. The scanning electron micrograph shown in Figure 11, [Ref 10] show the deposition of silica in the walls of grass cells and forming rows of stony phytoliths [Ref 10]. The phytoliths, arranged in rows, provide hardened regions along the length of leaf, stem and root. The phytoliths deter attack by herbivores by reducing the digestibility of the grass tissue, and in insects, they exact wear on their mandibles. The jagged shapes of phytoliths in grasses, such as those in sorghum, [Ref 10], also suggest that the grasses eaten by grazing animals may be an irritant to the soft tissues of the mouth.

Figure 11. Rows of phytoliths in a sorghum leaf.

In diatoms

Diatoms are single-cell microalgae and are among the most common of plant plankton, distributed ocean-wide and in fresh waters. Their species exhibit a myriad of shapes, some quite beautiful. (Figure 12) [Ref 11]

Figure 12. Diatoms in myriad shapes. The colors arise from diffraction by their lacey shells with small pores.

Their cell walls, of hydrated silica, both protect the soft contents of the cell and allow the in-flux of nutrients. They also provide a route for the out-flux of gases and waste products [Ref 11]. The cell wall comprises two cap-like halves known as frustures as shown in the examples in Figure 13. The outer appearance of the cell wall is one of a ribbed structure with numerous pores distributed between the ribs.

Figure 13 Diatoms showing the two frustures decorated with pores enclosing the cell.

The pores overlie a chamber, as seen in Figure 13-b. The innermost surface of the cell wall lies against the outer membrane of the cell where they are taken inside the cell via the membrane of the cell. The small pores allow entrapment of nutrients within the chamber and act as valves to protect against loss of nutrients on the occasion of lessened concentrations of nutrients outside the cell wall. [Ref 13]. This provides the ability to compete with nearby feeding diatoms.

Figure 13. Stricture of a diatom cell wall with the pore structures referred to as as valves. The pores in the outer surface allow influx of nutriments from the surrounding water into the chamber enclosed by the silica wall. The small pores impede reverse flow of the nutrients from the chamber.

Ref 1. https://www.gia.edu/opal-description

Ref 2. https://www.mindat.org/min-3004.html

Ref 3. https://www.tandfonline.com/doi/abs/10.1080/00167617108728743

Ref 4. https://www.blackstaropal.com/pages/opalessence-about-opal-the-queen-of-gems

Ref 5. http://nhminsci.blogspot.com/2012/07/loving-ethiopian-opals.html

Ref 6. http://www.uniqueopals.ch/opal-play-of-colour.htm

Ref 7. http://www.ingentaconnect.com/contentone/asp/jctn/2016/00000013/00000003/art00080

Ref 8. https://www.ncbi.nlm.nih.gov/pubmed/16638012

Ref 9. http://wfu.me/andersonlab/data/role-of-silica-in-serengeti-grasses-2/

Ref 10. http://rivkaelbaum.wixsite.com/rivka-elbaum/silica

Ref 11. https://en.wikipedia.org/wiki/Diatom

Ref 12. http://science.jrank.org/pages/2051/Diatoms.html

Ref 13. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0059548

Glorious Garnets

Mineral Man Mel — Sun, 13 Oct 2024 00:12:47 +0000

In this blog, the subject is the Garnet Family and their six members, (See Figure 1), which are gemstones and mineral specimens highly prized by collectors, as well as by those that wear them, because of their colors. Their colors, as seen in Figure 1, span the rainbow, and some unexpectedly undergo a change in color, depending on the wavelength of the light, (daylight or incandescent light), that illuminates them, as seen in Figure 2.

I will discuss the relationships between the colors of the garnets and their chemical compositions and how their colors are perceived, due to absorption of light by specific colors and by included chemical species. Such as a tomato absorbing mostly green light and reflecting red light. As an example of another optical effect, (and other gemstones can exhibit this effect also), I’ll discuss asterism, the cause of the formation of a star-shaped figure, seen on the surface of some Star Almandine garnets.

Figure 1. Garnets and their color [Ref 2].

Figure 2. Color change in a garnet under two different light sources [Ref 3].

GARNETS AS GEMSTONES

Since antiquity garnets have been admired and worn as gemstones in jewelry such as: the 7^th century Anglo Saxon sword pommel found as part of the Staffordshire Hoard in July 2009, [Fig 3]; And in Figure 4, the 1^st -2^nd century Roman necklace; and in Figure 5 the Roman ear rings; and in Figure 6, the 30 – 323 BCE Egyptian ring; the Byzantine pendant shown in Figure 7; and the Victorian necklace shown in Figure 8. References for each piece of jewelry accompany its picture.

Figure 3. Garnet & gold sword pommel [Ref 4] [Ref 5].

Figure 4. Third to Fifth Century garnet and gold with glass and chalcedony pendant [Ref 6].

Figure 5. Second Century Greco-Roman carved garnet and gold earrings [Ref 7].

Figure 6. 30-323 BCE Egyptian garnet & gold ring [Ref 8].

Figure 7. Byzantine 4th century garnet & gold pendant [Ref 9].

Figure 8. Victorian Age garnet and gold necklace [Ref 10].

CHEMISTRY OF THE GARNET FAMILY

The chemical relationships between the six, end members, of the garnet family are summarized in the two, phase diagrams, shown below in Figure 9, [Ref 11]. The shaded regions of each diagram show compositions where the metal ions, calcium (Ca), magnesium (Mg), and iron (Fe), can substitute for each other. For example, increasing substitution of Mg²⁺ for Fe²⁺ in Almandine leads to a garnet increasingly approaching, and ultimately equal to, Pyrope in composition. Appreciation of the shaded regions of mixed composition is important in relating colors of the gemstone to the relevant metals presented in its ionic form, as shown in Tables 1 and 2.

The formulas of the end members of each of the families are also shown in Table 1. The formula cited for the end member Uvarovite, in Figure 9, is erroneous in this old version, (The only one I could dig up), of its compositional group. Only the metals calcium (Ca) and chromium (Cr) are present in this garnet.

Figure 9. Compositional arrangements within the garnet family.

The three garnets of first group, the Pyralspites, contain the metals magnesium (Mg), Manganese (Mn), and iron (Fe), and aluminum (Al). Each garnet of the other group, the Ugrandites, contains calcium. The other metals are aluminum (Al), iron (Fe), and chromium (Cr). In common, all six members of the garnet family contain the element silicon (Si), bound with Oxygen (O), and are known as silicate minerals. These phase diagrams are from an older unreferenced source with an erroneous formula for Uvarovite garnet – Its correct formula can be seen in Table 1.

The formulas for each of the members of the garnet family are listed in Table 1. In the formulas, the charge of the ion of the metal is shown for each metal. For example, the divalent iron ion is written as Fe²⁺ and the trivalent chromium ion as Cr³⁺.

Table 1. Garnet family members and formulas [Ref 1].

SOURCES OF COLORS IN GARNETS

Of the garnet family, Almandine and Andradite owe their colors to the iron present in their formulas, Spessartine to manganese, Uvarovite to chromium, all in ionic form. Pyrope and Grossular, in pure form, are colorless. Various combinations of iron, manganese, titanium, and vanadium ions are the causal agents of color in both Pyrope and Grossular. The formulas for each of the members of the garnet family are summarized in Table 1. The colors; the responsible metal; and its ion forms, which are present in garnets, are given in Table 2.

HOW GEMSTONE COLORS ARE PERCEIVED

The colors of the light we see, either transmitted or reflected by a gemstone, stem from that part of the spectrum of the incident light which is not absorbed within the gemstone. The light, incident on the gemstone, is ambient light. Depending on the source of the incident light, its intensity over the blue to red range of the spectrum can be weighted more in the blue than in its red regions. As examples, light from a halogen lamp, or a white light diode have a spectrum richer in the blue region than an incandescent lamp which has a spectrum richer in the red region.

The effects of the spectral content of the incident light and its absorption at some wavelengths and not at others on perceived color can be demonstrated in a study done on color change in a Pyrope garnet [Ref 13]. Changes in the colors of a gemstone with illumination lend drama to the gemstone. A study was undertaken of a type of Pyrope color-change garnet from Tanzania so that the thickness of a cut gem was optimized in a way that the color change, with a change in illumination, could be maximized.

Figure 10. Calculated absorption spectra for three pyrope color change garnet samples of on millimeter thicknesses [Ref 13].

Figure 11. Three samples of Pyrope color change garnets under bluer and redder light [Ref 13].

STAR GARNETS: ASTERISM IN GARNETS A BEAUTIFUL EFFECT

Some gemstones, such as the Almandine garnet, Moonstone, Spinel, Rose Quartz, Citrine, Diopside, Emerald, Sapphire and Ruby may exhibit asterism in displaying a rayed star, best viewed when centered on the dome of a cabochon-cut gem, as in Figure 12. The star effect is caused by the scattering of light from nano-sized crystals of the mineral rutile, oriented in parallel fashion to each other [Ref 14]. In Almandine garnet the star may be either 4 or 6-rayed according to which directions in the crystallographic lattice the rutile crystals are located [Ref 15]. Star garnets are typically a purple shade, as seen in Figures 12 & 13. Idaho and India are the major, if not the only suppliers, of starred garnets [Ref 16].

Figure 12. Six-rayed Almandine Star garnet.

Figure 13. Four-rayed Alamandine Star garnet.

A Feast for Your Eyes – A Gallery of Garnet Family Specimens

Mineral Man Mel — Sun, 13 Oct 2024 00:12:47 +0000

In the presentation of photos of specimens of various garnets the source of the photo is referenced. For more or supporting images, Google the name of the mineral and the word image to see a gallery of additional images.

Almandine Garnets

Specimens from the classic localities of Wrangall Island, Alaska and Tyrol, Austria as well as those of very large size from the Salida Mine, Salida, Chaffee County, Colorado have long attracted collectors.

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Pyrope Garnets

Rhodolite garnet, the raspberry red to purplish red variety of Pyrope, as shown in figure 20, was first discovered and described from Cowee Valley, Macon County, North Carolina. Now considered a classic locality. Bohemian garnets from the Czech Republic, with their glossy red color, as shown in Figure 21, were first marketed in the 17^th century. Newer finds in Madagascar, Brazil, and Arizona are current sources of Pyrope garnets.

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Spessartine Garnets

Orange Spessartine Garnets associated with Smokey quartz from China are greatly desired by collectors. The vividly orange-colored Spessartine, single crystal \pessartine garnets, many displaying perfect forms, from both Tanzania and Nigeria are also prized. Crystals of red Spessartine Garnets, from Brazil, with their complex multi-faced faces present unique crystal forms.

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Andradite Garnets

The gem varieties of Andradite garnets, the green demantoid, the honey-colored topazolite, and black melanite, respectively from Madagascar, Canada, and California are not only prized as gemstones, but also by collectors. Also coveted are the beautiful green Andradite specimens from Stanley Butte in Arizona as well as the brown andradite garnets from Mali and Greece.

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Grossular Garnets

The classic Grossular garnet specimens, from the Jeffery Mine in Quebec, with their pristine crystals and colors, which range from colorless through pink, honey-brown, and green, are highly regarded by specimen and gemstone collectors. The pink ,manganese-rich, Grossular garnets and their iron-rich companions from Coahuila, Mexico, are also prized by collectors. The startling green Tsavorite gem variety of Grossular garnets from Tanzania also appeal to both specimen and gemstone collectors.

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Uvarovite Garnet

The two hallmark localities for Uvarovite specimens are located in Russia and Finland, and are associated with chromite, the oxide mineral of chromium. Specimens from Russia typically present dense fields of small perfect crystals on a chromite matrix. Some specimens also contain a drusy (dense array of small crystals) made up of the lavender mineral Amesite. Specimens with larger crystals occur in Finland.

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Who Is That Masked Mineral Man?

Mineral Man Mel — Sun, 13 Oct 2024 00:12:46 +0000

I’ve been an avid collector of mineral specimens from around the world since my experience, at the age of 13, of finding a beautiful black tourmaline crystal while on a Boy Scout hiking adventure. We were in the upper limits of the gem mining Pala District in San Diego County, California. I’m now 84 and still greatly enjoying my collection and sharing it with friends. In my blog posts, I want to share with you my joy in collecting these beautiful works of the Earth and hope to interest you in collecting them as well. My great pleasure has evolved from their aesthetics – enjoying the beautiful color and crystal forms of minerals, to learning about their geological histories — where and how they formed, their chemistries and crystal forms in relationship to minerals of similar composition, their mining history, and their frequent influence in geopolitics.

In sharing ideas about these subjects I will, because of space limitations, provide short but meaty encapsulations. I will draw abundantly from resources on the web. To complement my input, I will usually provide links to the subject for your further exploration. In a lighter vein, I plan to frequently include the rich lore of mining and of mining men, of prospectors, and of Lost Gold and Silver Mines and of the historic mines, particularly in the Southwest and Mexico.

To begin, what is a mineral? Drawing from the site, Webmineral, I find a number of definitions cited from scientific literature. To synthesize: “a mineral is a naturally occurring homogeneous solid with regularly ordered crystalline structure and a definite chemical composition. They can be distinguished from one another because of these definite characteristics”. Knowledge of these ideas are powerful tools in identifying a mineral specimen. The mineral’s chemical composition leads directly to its color, internal atomic arrangement, and crystal form. For example, the beautiful Rhocochrosite crystal from the Sweet Home Mine in Colorado, shown above, is manganese carbonate, having the chemical formula MnCaCO₃. Its deep red color is due to its manganese content and its rhombohedral form comes from the internal arrangement of atoms.

Because of the importance of chemical and crystallographic relationships in defining a mineral, I’m providing a link to an introductory course to minerology and crystallography offered by the Open University, a long known and excellent United Kingdom source of quality courses offered, at no cost, to world-wide users. I encourage you to open the link and scan the topics offered, as well as the internal links to tools for accessing a comprehensive body of reference material.

I hope you will share your questions and comments with me, submitting them to our “Ask An Expert” feature.

In my next post, I’ll share with you ideas offered by the most senior of collectors on how to build your own collection. Those ideas will include: collecting one mineral species; collecting many; collecting from one locality; collecting worldwide; where to find bargains and much more.

Until then, have fun learning about minerals and collecting.

What is the Difference Between a Rock and a Mineral?

Mineral Man Mel — Sun, 13 Oct 2024 00:12:46 +0000

According to the USGS website, “A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties”. “A rock is an aggregate of one or more mineral, or a body of undifferentiated mineral matter”. For more information about rocks and minerals, be sure. . . Read More

On Wulfenite

Mineral Man Mel — Sun, 13 Oct 2024 00:12:46 +0000

Figure 1 Wulfenite, Rowley mine, Maricopa County, AZ

Today my main topic is the mineral Wulfenite which has a strong association with Arizona. Lead mines, in which Wulfenite has been found, are numerous, with some of them offering such beautiful examples of the mineral that they have become classic localities. Among these are the Rowley Mine and the Glove Mine. More photos of Wulfenite

Wulfenite, having lead in its composition, is found primarily in lead mines and is widely distributed among Arizona mines as shown on the map of occurences: Map

There are 137 of these mines, with distribution from North to South and East to West. An article by the former Curator of the Arizona Mining and Mineral Museum, Jan C. Rasmussen, identifies eight mines noted for the aesthetics of the Wulfenite specimens found in them, and describes the physical and historical geology of the region, as well as the geochemistry describing the mineral deposition. The author also includes photos of specimens from each mine in this downloadable pdf document. Arizona Wulfenite by Jan C. Rasmussen

Figure 2: Wulfenite, Glove Mine, Santa Cruz County, AZ

The wide range of yellow to red colors is notable among Arizona specimens. However, for completeness sake, Wulfenite specimens come not only in shades of red, orange, and yellow – as this one in figure #2, from the Glove Mine – but they can also be made black by manganese inclusions, such as the one in figure #3. Other grey to black inclusions of such ore minerals as metal sulfides would have a similar effect.

Figure 3: Wulfenite with Manganese oxide inclusions, Glove Mine, Santa Cruz County, AZ

The beautiful yellow to red colors of Wulfenite deserve mention of their origin. I will paraphrase the explanation in this post by Fred Haynes

Since lead molybdate is colorless or white: the color must arise from another metal with the same valence as lead. Trace amounts of vanadium, manganese, chromium, and titanium are the source the colors. These elements do this by absorbing the violet, blue, and green parts of the visible spectrum. Some of the crystal forms evidenced by Wulfenite range between tabular, through blocky, to highly elongated as shown by examples in the Atlas of Crystallographic Forms of Wulfenite and the vast gallery of photographs at its Mindat site, which I linked to at the beginning of this blog. Just click on the icons in the atlas to view these forms. Some of the basic forms evidenced are modified by beveled edges and corners which add to their interest.

On Building a Collection: In my first blog I stated that I would provide some insights on how to build a collection that would maximize your enjoyment. Rather than paraphrase their content I’ve provided links, below, to the websites which offer sound advice on subjects ranging from the aesthetics of a specimen through pragmatic How To’s, to how to build a collection on a budget. I hope these ideas serve you well in the process of building your collection

Desirable to mandatory specimen attributes: http://farlang.com/how-to-build-a-mineral-collection

Pragmatic advice: http://www.johnbetts-fineminerals.com/jhbnyc/articles/advice.htm

Pragmatic advice including building upon locality, one species, etc:

http://www.mcdougallminerals.com/blog/seven-keys-to-building-a-great-mineral-collection/

AND http://www.minerals.net/resource/Organizing_Mineral_Collection.aspx

Building a collection on a budget: http://www.treasuremountainmining.com/index.php?route=pavblog/blog&id=133

A short personal note: Shortly after I began collecting, I learned of a mineral dealer named Jack Filer who, with son Russell, dealt in mineral specimens and were located close to my home. Frequently, on a Saturday, with or without money in my pocket, I would go over to visit Jack. Even without purchasing anything, I was welcome to visit, ask questions, eagerly sop up information and hear their stories of mining and collecting, and so wonderfully allowed to hold and pore over specimens, as well as help Jack in curating his collection. All of this was an incredible privilege. I include these remarks not only to reminisce, but to also point out the importance and joys of having a mentor who knows a lot about minerals, communicates well, particularly in question-answer form, and who really cares about you and your education about minerals. Jack and Russell started me out in becoming the collector I remain today. I will always be grateful for them.

USE OF COLOR IN MINERAL IDENTIFICATION

Mineral Man Mel — Thu, 03 Jun 2021 18:48:41 +0000

In some minerals color is directly related to a metallic element, is characteristic, and can be useful in identification. As examples, azurite as shown in Figure 1A, is always blue due to the presence of copper, and rhodochrosite, shown in Figure 1B, is always pink to red due to the presence of manganese,. However minerals such as fluorite, colorless in it self, can be yellow, blue, purple, or green due to low concentrations of metal impurities.

Accordingly color alone, generally is insufficient for identification, but when linked with knowledge of other properties of a mineral can be useful in its identification.

Understanding of the roles of metals as sources of colors in a mineral, particularly when augmented with knowledge of its location where found, can be useful in identification. In this Blog I’ll describe how the presence of a transition metal element, present intrinsically or as an impurity, and certain structural features within the mineral are sources of colors or when present, can modify the appearance of the mineral.

Figure 1A. Azurite, Czar Mine, Copper Queen Mine, Queen Hill, Bisbee. Cochise County, Arizona[Ref1]

Figure 1B. Rhodochrosite, N’Chwaning Mines, Kuruman. Kalahari Manganese
Field, Northern Cape, South Africa[Ref2]

CAUSES 0F COLORS AND COLOR EFFECTS IN MINERALS

The color of a mineral that we see arises from transmission of light over the visible wavelengths of light within the visible light spectrum which are framed by one or more regions of wavelengths that are absorbed as shown in Figure 1C.

Figure 1C. With absorption of specific wavelength{s) of white light spanning wavelengths of red to purple results in transmission of light over a wavelength range imparting color[Ref3]

Extensive studies have shown that light absorption by a number of processes are among the causes of the colors in minerals as summarized in TABLE I[Ref4].

Absorption of light by electrons in transition metals present intrinsically or as impurities are involved in light absorption processes and the excitation of electrons within their intrinsic energy distribution within conductors and semiconductors upon light absorption also imparts colors in other minerals which are conductors and semiconductors. Excitation of an electrons or a hole (hole = absence of a negatively charged ion or electron) associated with color centers which reside in defects within the crystal lattice imparts colors in some minerals

Structural features in minerals with sizes of the order of the wavelengths cause physical optical effects: light scattering, interference, and diffraction of light. These can either introduces colors or add optical features to the appearance of the mineral.

TABLE I. CAUSES OF COLORS AND COLOR EFFECTS IN MINERALS[Ref4].

Light Absorption by Transition Metals[Ref4,5,6]

Among the causes of color in minerals listed in TABLE I[Ref4] are the ions of transition metals that contribute to the color of many minerals through selective light absorption over the visible light spectrum. They can be present as a major constituent such as manganese (Mn) in rhodochrosite(MnCO₃) giving it its pink-red color, present as a low concentration impurity in a mineral such as chromium (Cr)substituting for aluminum (Al) in Ruby giving its red color and the formula {Al,Cr_minor)₂O₃, and as ions of iron, chromium , manganese, titanium(Ti) participating in intervalence charge (electron) transfer between two of their ions and with the oxygen ion[Ref5].

Transition metals are present as a formulaic constituent and some as an impurity are shown along with their associated colors in TABLES II and III.

In a transition metal ion with a partially filled d-shell the electrons in the outer region of the d-shell are unpaired. The surrounding oxygen ions of the crystal lattice exert forces on the outer d-shell electrons dermining their occupied and empty energy levels[Ref5]. The forces and energy levels depend on the strength and nature of the bonding as well as the valence of the transition metal ion. These energy levels are quantized so that a absorption of a specific amount energy is required to to increase an electrons energy to another level. Absorption of light of that energy at the corresponding wavelength provides the energy required for excitation of the unpaired electron to a higher level.

As an example consider absorption and transmission of light in the ruby. The sumation of surrounding forces on the chromium ion Cr³⁺ result in two energy levels, C and D, avialable for an excited electron as shown in Figure 2[Ref3]. Symmetry conditions deny occupancy of level B by an excited electron [Ref].

As shown in Figure 3 light of 414 and 561 nm wavelengths is absorbed . With abosrbtion of these wavelengths light in the regions of 480 nm and greater than 620 nm are transmitted imparting the red color of the ruby.

Figure 2. Excitation of d-elecrons in Cr³⁺ in a ruby crystal upon absorption of 414 nm (2.99 eV) ultraviolight light and of 561 nm (2.21 eV) yellow- green light[Ref]. The peak abosorption and transmission of light within the ruby at these wavelengths are shown in the two spectra shown in Figure [Ref3]

Figure 3. Transmittance spectrum (Right) of a 1-cm thick ruby crysta[Ref4]. Absorptioon of light is greatest at 414 nm and 561 nm and transmittance in the blue at 480 nm and in the red at wavelengths greater than 620 nm in the red.

TABLE II. TRANSITION METALS PRESENT AS AN INTRINSIC CONSTITIUENT IN A MINERAL AND ASSOCIATED COLORS[Ref5,6]

TABLE III. TRANSITION METALS PRESENT AS AN IMPURITY IN A MINERAL AND ASSOCIATED COLORS[Ref5,6].

Light Absorption By Charge Transfer Between Transition Metal Ions and Between oxygen ions and Transition Metal Ions[Ref7,8].

Transition metals can exist in different valence states. These ions can form covalent bonds with oxygen in which electrons in outer shells can travel between the ions upon being supplied sufficient energy. This can result in charge transfer in the form of an electron upon absorption of light with wavelengths in the Ultraviolet through Visible into Near Infrared light ranges. Charge transfer from an adjacent oxygen to a transition metal ion and charge transfer between neighboring transition metal ions connected by an oxygen atom can occur. The direction of transfer charge is from the ion of least positive valence number to the neighbor of higher valence. For example the deep violet-blue color of cordierite var. Iolite as shown in Figure 4 results from the neighboring absorption peak centered at 600 nm due to charge transfer from a ferrous ion Fe²⁺ to a neighboring ferric ion Fe³⁺, and extending through the blue range of the spectrum as shown in Figure 5[Ref10]. Other examples of the processes of charge transfer in minerals with their associated colors are given in TABLE IV.

Figure 4. Cordierite var. Iolite, Mogok, Pyin-Oo-Lwin District, Mandalay Region, Myanmar[Ref9].

Figure 5. Absorption spectrum of cordierite var. Iolite from Tamil Nadu, India[Ref10].
Note that the wide absorption peak in the 400 to 600 nm range results in a transmission window in the violet.

TABLE IV. CHARGE TRANSFER PROCESSES IN SOME MINERALS WITH THEIR COLORS

Charge Transfer	Color and Mineral*	Reference
Oxygen to Metal Transfer
O^2-  Fe³⁺	Yellow to Brown: beryl/heliodor, quartz/citrine	11
O^2–  Fe⁴⁺	Purple: quartz/amethyst	11
O^2-  Cr⁶⁺	Yellow to red: Crocoite	11
O^2- U⁶⁺	Yellow: uranophane	11
Intervalence Charge Transfer
Fe²⁺–O—Fe³⁺	Violet: jadeite Blue: lazulite, vivianite,	11,12
Fe²⁺–O—Ti⁴⁺	Blue: berlaquamarine, corundum/sapphire, cordierite, amphibole,	11
Mn²⁺–O—Ti⁴⁺	Greenish-yellow: tourmaline	11
Others
Fe²⁺Ti⁴⁺ and both Fe³⁺ and Cr³⁺	Orange, sapphire	13

*For information on any mineral Google “Mindat.org + Mineral name”

Light Absorption By Color Centers[Ref4]

Unpaired electrons can be located on a non-transition metal ion or on a defect in the crystal lattice form color centers as shown in the Fluorite structure and quartz structure shown in Figures 5 and -6. An electron present at a lattice vacancy forms an electron color center and absence of an electron from where an electron pair is normally present forms a hole (lack of a negatively charged ion) center.

In fluorite the absence of a fluorine ion F^-1 from its normal position forms an F-center occupied by an electron as shown in Figure 6. Absorption at longer wavelengths by the F-center is responsible for a purple color in fluorite[Ref5].

In smoky quartz the presence of an aluminum ion Al³⁺as a low level impurity substituting for a silicon ion Si⁴⁺is required[Ref5]. Removal of an electron from an oxygen ion O^2- adjacent to aluminum ions by radiation results in the formation of a singly negative oxygen forming an electron-hole. To preserve charge neutrality a positive hydrogen ion H+ is present. Aluminum not substituting for silicon does not form smoky quartz. The same mechanism accounts for the yellow color of citrine[Ref11].

Figure 6. Schematic of normal fluorite structure (A) and fluorite with an
F-center occupied with an electron (B)[Ref5].

Figure 7. Schematic of the normal quartz structure (A) and containing impurity aluminum ions Al³⁺ replacing silica ions Si⁴⁺in the lattice (B)[Ref5]. Radiation has removed an electron from oxygen leaving a hole center of smoky quartz. To preserve charge neutrality a positive hydrogen ion H+ is present. Aluminum not substituting for silicon does not form smoky Quartz.

Light Absorption in Metals and Semiconductors: Band Theory[Ref5]

The band theory treats electrons as belonging to the crystal as a whole and capable of occupying bands or ranges of energies as shown for metals and semiconductors in Figures 8 and 9.[Ref5]. Colors of representative minerals described by band theory are shown in TABLE IV.

TABLE IV. COLORS OF METALS AND SEMICONDUCTORS DESCRIBED BY BAND THEORY[Ref5].

The Band Theory of Metals[Ref5]

In a metal such as copper or iron each metal atom contributes its outer electrons to a common pool in which they are free to move freely through the crystal accounting for the large electrical conductivity of metals as well as the metallic luster and specular reflection of metals. In a typical metal which contains 10²³ electrons per cm^-3 all essentially equivalent to each other, such that quantum mechanically

the energy levels are broadened into bands as shown in Figure 8A. The density of electrons each energy level is limited and states are filled to the maximum Fermi surface (level). Upon absorption of light electrons are excited to empty bands of higher energy with density of states varying with as shown in Figure 8B.

If the efficiency of absorption decreases with increasing energy in the blue-green spectrum as in gold and copper the spectrum features the yellow and reddish colors of gold and copper upon reemission of the electrons as shown by the reflectance spectra shown in Figure 9[Ref18]. The spectra of silver and aluminum in the figure show that light across the visible spectrum is absorbed and re-emitted leading to their white color.

Figure 8. Energy band of a typical metal (A) and showing energy transitions upon light absorption (B)[Ref5]. The Fermi Surface is the maximum energy of an electron in the valence band.

Figure 9. Reflectance (or fraction of light reflected) spectra of aluminum (Al), Silver (Ag), gold(Au), and copper(Cu)[Ref14] as also shown in the specimens in Figures 10-12.

Figure 10. Native silver on native arsenic, Pohla-Tellerhauser mine, Pohla, Saxony, Germany[Ref15].

Figure 11. Group of twinned native gold crystals, Round Mountain Mine, Round Mountain, Round Mountain Mining District, Toquima Range, Nye County, Nevada[Ref16].

Figure 12. Twinned crystals of native copper, Chino Mine, Santa Rita, Santa Rita mining district, Grant County, New Mexico[Ref17].

Figure 13. Energy bands in a typical semiconductor (A), energy transitions upon light absorption in (B), the dependence of color on energy(C), and the color stemming from a band gap of that energy (D)[Ref5].

The Band Theory of Semiconductors[Ref5]

Many sulfide minerals are semiconductors that exhibit substantial electrical conductivity, high refractive indices, and often a metallic luster[Ref4]. In minerals where bonding is predominantly covalent or ionic the sulfur electrons occupy a valence band as shown in Figures 13A and 13B. A band comprised of empty metal electron orbitals exists at higher energies separated from the valence band by an energy gap as shown in Figure 13. Absorption of light by electrons of energy greater that the gap energy excites them into the conduction band as shown in the Figures 13C and 13-D. If the band gap is smaller that the energy of the visible light range all of the light is absorbed and the mineral behaves like a metal and appears black or grey as shown in Figures 13–C and13 -D and by tetrahedrite with a band gap of 0.000 eV[Ref21] and galena with a band gap of 0.993 eV[Ref 22] and shown in Figures 14 and 15. As the band gap energy increases the wavelengths excited in the spectrum become shorter and colors shift through red to yellow. The band gaps of those semiconductors such as diamond and sphalerite with band gaps of about 5.5 and 3.5 eV are greater than available energy at the lowest wavelength (highest energy) of the visible spectrum and the light is absorbed. With an intermediate band gaps of 1.99 eV and 2.197eV, respectively cinnabar appears red and orpiment yellow-orange as shown in Figures 16 and 17[Ref23].

Figure 14. Hopper crystal of tetrahedrite, Romania[Ref26]. The crystal is black because with a band gap of 0.000 eV tetrahedrite absorbs all visible wavelengths[Ref18,19].

Figure 15. Galena, Sweetwater mine, Ellington, Reynolds County, Missouri[Ref20].

Figure 16. Cinnabar, Almade’n mine, Almade’n, Almade’n Mining District, Ciudad Real, Castle-La Manch, spin[Ref21].

Figure 17. Orpiment, Twin Creeks Mine, Potosi Mining District, Osgood Mountains,
Humboldt County, Nevada[Ref22].

Light Absorption and Colors in Impurity and Defect Semiconductors[Ref5]

Certain impurities can cause light absorption in large band gap minerals. Absorption of light nitrogen and boron impurity centers as shown in Figure A and –B results, respectively in yellow and blue colored diamonds.

Yellow Diamonds

Figure 18. Light absorption in the blue-violet spectral range by a
nitrogen impurity results in the yellow color of a diamond[Ref23].

Figure 19. Yellow diamond of 32.77 carats[Ref27].

Blue Diamond

Figure 20. The boron acceptor atom accepts electrons with energies corresponding to the green to red region of the visible spectrum resulting in transmission of the blue color[Ref23].

Figure 21. The blue Hope diamond[Ref28].

Green Diamond

A green color in a diamond arises from absorption of light in the yellow to red wavelength region by electrons associated with defects in the periodicity of the diamond crystal lattice as shown in Figure [Ref24].

Figure 22. The defects in the diamond lattice that are responsible for a green color in a diamond: In Figure 2.A, absorption by unbonded electrons surrounding a lattice vacancy (missing carbon atom); in 2.B light absorption by unbound electrons surrounding a lattice vacancty associated by a pair of neighboring nitrogen atom impurities; in 2.C light absorption by electrons associated with a hydrogen atom with a neighboring nitrogen atom impurity; in 2.D absorption of light by a defect comprised of two neighboring carbon lattice vacancies with an associated nickel atom[Ref24].

Pink Diamond

Color defect centers developed along deformation planes in the diamond crystal lattice diamond are responsible for the coloration in pink diamonds as shown in Figure23[Ref25,26].

Figure 23. Pink Diamonds, Argyle Mine, East Kimberly region, Australia[Ref39].

Colors and Features Caused by Physical Optics

Play of color due to structural characteristics of the gemstone can be useful in mineral identification. Iridescence, opalescence and labradorescence, chatoyancy, and asterism are characteristics of a limited number of minerals.

Opalescence

Opalescence is an opal-like play of light producing flashes of different colors as in an opal as shown in Figure 24[Ref40]. The various colors are produced by diffraction from regions with layers of different thickness of formed by silica spheres of different uniform diameters[Ref41]

Figure 24. Structure of precious opal showing uniform layers of silica spheres[Ref30].

Figure 25. The “Virgin Rainbow” opal, Coober Pedy, Australia[Ref31]

Labradorescence

Labradorite is a feldspar mineral comprised having striations of exsolved lamellae of nanometer of albite in the 10s to 100sof nanometers size range in anorthite with both having different calcium and sodium concentrations. Accordingly their refractive indices are unequal[Ref39,40] and the striations diffract light giving the colors according to the thicknesses of the lamellae giving the labradorescence as shown in Figure 25[Ref44].

Figure 26. Labradorite cabochon showing striations with alternating lamellae of albite and anorthite of different thicknesses, hence different colors[Ref32].

Iridescence

Iridescence is the play of colors produced in a thin film of different refractive index than in the surroundings and varying thickness such as the wall of a soap bubble or layer of oil on water[Ref45]. Like diffraction the colors depend on thickness and angle at which viewed. The iridescence of some specimens of the copper mineral chalcopyrite as shown in Figure is a classic example of iridescence on a mineral surface where the colors range from blue to red with increasing thicknesses of the film.

Figure 27. Iridescent chalcopyrite, Greystone Quarry, Lezant, Cornwall,
England[Ref34].

Chatoyancy

Chatoyancy is an effect of preferential light scattering by parallel aligned rod like mineral inclusions like the fibers of crocidolite (asbestos) cemented in quartz of the gemstone tigers eye as shown in Figure 28[Ref35,36]. Aligned needle-like microcrystals crystals of rutile are responsible for the chatoyancy of chrysoberyl as shown in Figure .[Ref37]

Figure 28. Chatoyant tigers eye showing aligned structure of crocidolite[Re38].

REFERENCES

Ref 1. https://www.mindat.org/photo-257370.html

Ref 2. https://www.mindat.org/photo-83179.html

Ref 3. http://gemologyproject.com/wiki/index.php?title=Causes_of_color

Ref 4. . https://en.wikipedia.org/wiki/File:Ruby_transmittance.svg

Ref 5. http://www.minsocam.org/msa/collectors_corner/arc/color.htm

Ref 6 https://en.wikipedia.org/wiki/Transition_metal

Ref 7. https://www.higp.hawaii.edu/~gillis/GG671b/Week02/Readings/Deep%20Reading/Burns_CrystalFieldTheory_Ch2and3.pdf

Ref 8. https://www.gia.edu/doc/SP88A1.pdf

Ref 9. https://www.mindat.org/photo-650677.html

Ref 10. http://minerals.gps.caltech.edu/files/Visible/cordierite/Cordierite2016_India.gif

Ref 11. https://www.gia.edu/doc/SP88A1.pdf

Ref 12. http://minerals.gps.caltech.edu/color_causes/ivct/index.htm

Ref 13. file:///Users/millardjudy/Downloads/[20834799%20-%20Advances%20in%20Materials%20Science]%20Heat%20treatment%20enhancement%20of%20natural%20orange-red%20sapphires.pdf

Ref 14. https://eng.libretexts.org/Bookshelves/Materials_Science/Supplemental_Modules_(Materials_Science)/Optical_Properties/Metallic_Reflection

Ref 15. https://www.mindat.org/photo-109627.html

Ref 16. https://www.mindat.org/photo-112379.html

Ref 17. https://www.mindat.org/photo-78563.html

Ref 19. https://materialsproject.org/materials/mp-647164/

Ref 20. https://materialsproject.org/materials/mp-21276/

Ref 21. https://i.pinimg.com/originals/72/26/67/7226674458e6dd34ba15b72f4034130f.jpg

Ref 22. https://en.wikipedia.org/wiki/Hopper_crystal

Ref 23. https://materialsproject.org/materials/mp-641/

Ref 24. https://www.mindat.org/photo-85601.html

Ref 25. https://www.mindat.org/photo-83818.html

Ref 26 https://www.mindat.org/photo-56538.html

Ref 27. http://www.webexhibits.org/causesofcolor/11A0.html

Ref 28. https://www.gia.edu/gems-gemology/spring-2018-natural-color-green-diamonds-beautiful-conundrum

Ref 29. https://www.gia.edu/doc/Characterization-and-Grading-of-Natural-Color-Pink-Diamonds.pdf

Ref 30. https://br.pinterest.com/pin/27092035231593578/

Ref 31. https://www.smithsonianmag.com/travel/the-hope-diamond-102556385/

Ref 32. https://www.pinterest.com/pin/AaGzMFSFFrs9vFLxcvJNl4g6FGLK1W2_i5FGc6i82BuZSmK7fjG7pJ0

Ref 33. http://www.uniqueopals.ch/opal-play-of-colour.htm

Ref 34. http://www.uniqueopals.ch/opal-play-of-colour.htm

Ref 35. https://www.geologyin.com/2016/07/worlds-most-expensive-opal-literally.html

Ref 36. https://www.mindat.org/min-2308.html

Ref 37. https://www.britannica.com/science/exsolution

Ref 38. https://www.google.com/search?client=firefox-b-1-d&q=refractive+index+of+albite

Ref 39. https://www.google.com/search?client=firefox-b-1-d&q=refractive+index+of+anorthite

Ref 40. https://www.sciencedirect.com/topics/medicine-and-dentistry/diffraction-grating

Ref 41. https://austinbeadgallery.com/what-to-look-for-in-labradorite-cabochons/

Ref 42.

https://www.asu.edu/courses/phs208/patternsbb/PiN/rdg/interfere/interfere.shtml

Ref 43. https://www.crystalclassics.co.uk/product/chalcopyrite-(irridescent)/

Ref 44. https://en.wikipedia.org/wiki/Chatoyancy

Ref 45. https://www.asbestos.com/blog/2020/04/20/asbestos-jewelry-mesothelioma/

Ref 46. https://blogs.scientificamerican.com/rosetta-stones/tiger-s-eye-a-deceptive-delight/

Ref 47. https://en.wikipedia.org/wiki/Chrysoberyl#Cymophane

USE OF CRYSTAL FORMS AND HABITS IN MINERAL IDENTIFICATION

Mineral Man Mel — Mon, 01 Feb 2021 17:32:13 +0000

The atoms within the crystal of a mineral are arranged in a regular fashion to form a lattice, and the crystal exhibits a shape with surface regularity which reflects its internal symmetry[Ref1]. The shape of a crystal is often typical of a mineral. and often typical the location where found; thus, crystal shape comprised of crystallographic forms modulated by crystal habit can be a useful tool in mineral identification.

The crystals of all minerals fall into seven families defined by their required symmetries as given in the table in Figure 1[Ref2]. The hexagonal family comprises two crystal systems as seen in Figure 1. Planes and shapes which enclose space as shown in Reference 3 are the crystallographic forms which comprise the shapes of crystals exhibited by minerals. The basic forms exhibited by the seven crystal systems are shown in Figure 2. Environmental conditions during deposition can influence the both the forms present on the crystal and the habit of a crystal in influencing its shape [Ref5].

Figure 1. The crystal families, systems, and their required symmetries[Ref2].

Figure 2. Forms of the basic prisms exhibited by the six crystal systems[Ref4].

Gallery of Crystal Systems, Forms, and Habits

In order to introduce some of the forms and habits of crystals I’ll use examples of minerals we often have enjoyed seeing in the literature and at lapidary and mineral shows as shown in Figures 3-9.

CUBIC CRYSTAL SYSTEM

Figure 3. Skeletal or hoppered crystals of galena on sphalerite, Madan Ore Field, Rhodope Mountains, Bulgaria [Ref5,8].

TETRAGONAL CRYSTAL SYSTEM

Figure4. Tabular crystal of wulfenite, Los Lamentos Mountains, Chihuahua, Mexico[Ref5,9].

ORTHORHOMBIC CRYSTAL SYSTEM

Figure 5. Acicular crystals of mesolite on green hydroxyapophylite, Pashan quarries,
Pashan Pune District, Maharashtra State, India[Ref5,10].

HEXAGONAL CRYSTAL SYSTEM

Figure 6. Bipyramidal crystals of quartz paramorph after hexagonal beta quatz, with hematite crystals, Florence Mine, Egremont, Cumbria, England, UK[Ref3,11].

Figure 7. Crystal of Beryl var. emerald displaying faces of the hexagonal and dihexagonal prisms, of the hexagonal pyramid, and of the basal pinacoid, Muzo Mine, Muso Municipality, Boyaca’ Department. Colombia[Ref3,12].

TRIGONAL CRYSTAL SYSTEM

Figure 8. Phantomed schalenohedral crystals of calcite, Mariposa Mine, Terlingua District, Brewster County, Texas[Ref 6,13]

Figure 7. Rhombohedral crystals of calcite, Gonsen Mine, St. Gallen, Switzerland[Ref3,14].

MONOCLINIC CRYSTAL SYSTEM

Figure 8. Crystal of gypsum (selenite), with faces comprising two domes and six pinacoids, Gilbralter Mine, Naica, Chihuahua, Mexico[Ref3,5,15].

TRICLINIC CRYSTAL SYSTEM

Figure 9. Crystal of Axinite-(Fe) with 7 pinacoidal faces, Pulva Mount. Tyumenskaya, Urals Region, Russia, Asia[Ref5.16].

REFERENCES

Ref 1. https://www3.nd.edu/~amoukasi/CBE30361/Lecture__crystallography_A.pdf

Ref 2. https://en.wikipedia.org/wiki/Crystal_system

Ref 3. https://www.tulane.edu/~sanelson/eens211/forms_zones_habit.htm

Ref 4. http://www.geologyin.com/2014/11/crystal-structure-and-crystal-system.html

Ref 5. https://en.wikipedia.org/wiki/Crystal_habit

Ref 6. http://www.galleries.com/minerals/property/habits.htm

Ref 7. http://www.minsocam.org/msa/collectors_corner/id/mineral_id_keyi8.htm

Ref 8. https://www.youtube.com/watch?v=f_g3r79mG9s

Ref 9. https://www.pinterest.com/pin/369506344411688204/

Ref 10. https://www.mindat.org/photo-303305.html

Ref 11. https://www.irocks.com/minerals/specimen/42517

Ref 12. https://www.rockngem.com/uncommon-emerald-exhibit-opening-sept-26/

Ref 13. https://www.spiritrockshop.com/Calcite_Mariposa.html

Ref 14. https://www.fabreminerals.com/LargePhoto.php?FILE=Calcite-SH47AB1f.jpg&LANG=EN

Ref 15. https://www.irocks.com/minerals/specimen/38370

Ref 16. https://www.crystalclassics.co.uk/product/cc19390/

USES OF CLEAVAGE, PARTING, AND FRACTURE IN MINERAL IDENTIFICATION

Mineral Man Mel — Mon, 25 Jan 2021 16:52:30 +0000

CLEAVAGE IN A MINERAL CRYSTAL[Ref1]

Cleavage in a mineral is the tendency for the crystal to split along definite crystallographic planes as exemplified by the rhombohedron cleaved from a calcite crystal shown in Figure 1[Ref1]. These planes of weakness are present within a regular repeating array of atoms and ions within the crystal and are always parallel to a potential face of the crystal. The weakness arises from the chemical bonds between cleavage planes being fewer in number or weaker than those between ions and atoms within the cleavage planes. Accordingly the crystal will tend to split between the cleavage planes and along a plane of relative weakness. When present, it is the uniqueness of the mode of cleavage in a mineral that makes it a tool in its identification.

Mineral crystals exhibit modes of cleavage along six families of planes as shown in Figure 1. Identification of the mode of cleavage and descriptions of its appearance and ease of cleaving can be used as steps in the use of Mineral Identification Keys and data compendiums as listed in References 2-7

The excellent YouTube video of Reference 2 shows how to experimentally develop and evaluate modes of cleavage, parting, and fracture in crystals in their identification. Note that use of eye protective wear is a must while performing these experiments.

Figure 1. Cleavage rhombohedron from a calcite crystal from Naica, Chihuahua, Mexico[Ref9].

Figure 1. Six families of cleavage planes exhibited by minerals[Ref10]

As demonstrations of the effects on cleavage of the density and strength of chemical bonds crossing the cleavage plane compared to the cleavage plane itself, consider its behavior in the mica family of minerals, in calcite, and in the diamond.

Cleavage in Muscovite Mica

Muscovite of the mica family, as the other members, only cleaves along planes parallel to the basal plane[Ref11] of a crystal with resulting thin sheets as shown in Figure 2. The structural relationship of the cleavage plane to the relative strengths of chemical bonds between the atoms in the lattice of the muscovite crystal is shown in Figure 3.

Figure 2. Sheets of muscovite mica parallel to the basal crystal plane[Ref12].

Figure 3. The basal plane of the muscovite crystal is parallel to the layers of potassium ions (blue) and to the intervening layers of silicon and aluminum atoms covalently bonded to oxygen atoms to form layers[Ref13]. The strengths of these covalent bonds are greatly stronger than the electrostatic attraction between the positive charges of potassium ions and the negative charges of the hydroxyl ions OH^2—on the surfaces of the layers; thus, cleavage occurs between the covalently bonded sheets, not through them.

Cleavage in Calcite

Calcite crystallizes in the trigonal crystal system[Ref15] and cleaves along six planes into rhombohedrons as shown in Figure 4.

Figure 4. Rhombohedrons with shiny flat cleaved surfaces as cleaved from a calcite crystal[Ref15].

Figure 5. Calcite cleavage rhombohedrons and lattice structure of calcite showing the relative arrangement of the calcium ions Ca²⁺ and carbonate ions CO₃^2_[Ref9] .

The angles between the legs of the rhombohedral faces are 78.5° and 101.5°.[Ref16] Note that the six cleavage planes lie between extended chain-like arrays of calcium ions and of carbonate ions, and minimal density of bonds crossing the cleavage planes.

Cleavage in Diamond

Cleavage in a diamond crystal can proceed any on of eight octahedral planes which parallel the octahedral faces of a diamond crystal as shown in Figures 6-9. The video shown in Reference 17 demonstrates cleaving of a diamond crystal and the relationship the cleavage plane to the density of bonds crossing the plane.

Figure 6. The basic cubic lattice structure of the diamond crystal with
the diagonal octahedral plane is shown in blue[Ref18].

Figure 7. View of the diamond crystal lattice showing planes with fewer
bonds crossing it lying between two parallel octahedral planes, each
with larger densities of atoms and bonds than the plane between them[Ref19].

Figure 8. View of the diamond crystal showing that cleavage occurs between the octahedral planes[Ref19].

The parallelism of both a cleavage plane and octahedral faces of a diamond crystal is shown in Figure 9. A representative perfect cleaved surface of a diamond crystal is shown in Figure 10. The white linear features are presne on the outer surface of the cleaved crystal.

Figure 9. Octahedral diamond crystal with cleavage along a plane parallel to two of
Its octahedral faces[Ref20].

Figure 10. Smooth and shiny cleavage surface of
Diamond[Ref20].

Descriptors of Cleavage for Use With Mineral Identification

In Mineral Identification Flow Charts [Ref3] and Mineral Data Tables[Ref4,5,6] not only the mode of cleavage, but descriptions of the quality and ease of cleavage for each mineral also are used as part of the body of information used in identification.

Descriptors of the quality of cleavage are summarized in the table in Figure 11. In addition the difficulty in achieving is described as easy, hard, and difficult to produce[Ref3].

Figure 11. Descriptions of cleavage used in mineral identification[Ref].

REFERENCES

Ref 1. https://en.wikipedia.org/wiki/Cleavage_(crystal)

Ref 2. .http://www.minsocam.org/msa/collectors_corner/id/mineral_id_keyi6.htm

Ref 3. http://www.minsocam.org/msa/collectors_corner/Id/mineral_id_keyi1.htm#TOC

Ref 4. www.mindat.org

Ref 5. http://webmineral.com/

Ref 6. https://www.minerals.net/MineralMain.aspx

Ref 7. Mineralogical Society of America – Mineral-Related Links

Ref 8. https://www.jstor.org/stable/30063973?seq=15#metadata_info_tab_contents

Ref 9. https://www.treasuremountainmining.com/4.3%22-Gemmy-Double-Refracting-PINK-ICELAND-SPAR-Calcite-Crystal-Mexico-for-sale

Ref 10. https://images.slideplayer.com/20/5954014/slides/slide_21.jpg