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 AustraliaFigure 2. Opal in Matrix, Queretaro, MexicoFigure 3. Black opal (Fossilized tree limb), Virgin Valley, NevadaFigure 4. Black Opal, Constellation Mine, Spencer, IdahoFigure 5. Faceted Opal, MexicoFigure 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.
We had thirteen club members drive out to Dobell Ranch to collect petrified wood. Everyone was in high spirits, and the weather was cooperative, providing us with a gorgeous day. We had so much fun both Linda & I forgot to take photos. Here’s a link to the Dobell Ranch website’s photo gallery: https://azpetrifiedwood.com/gallery
Noah Dobell was incredibly helpful and generously donated a bucket of petrified wood and a bunch of dinosaur bones for us to use in our Kids Zone at our show in June.
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 FeS2. [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.
Thanksgiving Day 2017, my son Greg and I traveled to the old copper mining town of Bisbee for a two day exploration of the Bisbee Mining and Historical Museum, observing the, now non-operational, Lavender Open Pit Copper Mine, and soaking up the ambiance of this charming town perched on the low hills of the Mule Mountains. [Ref 1]
Mines
We found that mining activity in Bisbee began with the staking of mining claims in 1877. It evolved from underground mining at the Copper Queen Mine to operation of the Lavender Pit and its cessation of operation.
The Copper Queen Mine – The Copper Queen was mined over an approximately 100 year period. Beginning with the staking of its claim in 1877 and ending in 1974. [Ref 2] During its operation, the Copper Queen Mine produced over eight billion tons of copper, gold production of almost three million ounces and over seven and a half million ounces of silver. [Ref 3] It has also yielded spectacular specimens of copper minerals.
Azurite crystals from the Copper Queen MineMalachite replacing Azurite from the Copper Queen MineEntry to the Copper Queen MineEarly ore transport in the Copper Queen MineSupport timbering in the Copper Queen Mine
The Lavender Pit – The Lavender Pit was named in honor of Harrison M. Lavender, Vice President and General Manager of the Phelps Dodge Corporation. [Ref 5] He conceived and carried out the open pit plan for continuing the mining activity at the site of the, former high-grade, Sacramento Hill Mine. The open pit mine opened in 1950 and continued until 1974. During this period the mine yielded about 600,000 tons of copper with ancillary production of gold and silver from ore averaging 0.7% copper. During operation about 250 million tons of waste were striped. Mining advanced by dynamiting 50-foot high ledges. Each blast resulted in the removal of about 75,000 tons of rock. Use of the1.2 tons of blasting material was strongly leveraged.
The Lavender Pit from the beginning to the endBisbee Blue turquoise with veins
The gem mineral turquoise formed as a secondary mineral through the chemical reaction from the primary copper sulfide and oxide ores. It occurred as stringers up to a few inches wide and small nuggets, were dispersed randomly throughout the ore body, and was recovered as a product of the mining activity by company personnel. [Ref 6] The typically deep blue in color, with red-brown veins or a chocolate brown matrix, is called Bisbee Blue in the trade. This is in recognition of its often deep blue color. [Ref 6]
Bisbee Blue turquoise with matrix
The Bisbee Historical and Mining Museum – The Museum offers exhibits that trace the initial settlement of Bisbee upon the finding of copper and staking of mining claims in 1988, through the closing of mining activity in 1970. [Ref 7] The Museum also houses a world-class collection of copper minerals gathered early in the mining history of Bisbee and features, what must be called awesome, specimens. [Ref 8]
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
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.
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 to check out our Blog posts by Mineral Man Mel.
December 20, 2017 – We had seven club members head out to Gray Mountain to collect Petrified Wood and Jasper. The morning started out a bit cooler than most of us found comfortable, but by the time we got to our second stop the wind had died down and the temperature was as nearly perfect as I’ve ever felt it out there.
We followed several dirt tracks just to see where they went and found plenty of the rocks we were looking for – including a few specimens of Petrified Wood sporting delightful druzy crystals. The Jasper was plentiful in a couple of spots just off the main road into the BLM area we were collecting in.
As several of us hadn’t followed the main road all the way out to the Little Colorado river before, we took that drive, which is well worth it if you enjoy amazing sandstone sculptures.
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.
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,Crminor)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
*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 O2- 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.
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].
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].
Summit Fire District Station 31 6425 N Cosnino Rd, Flagstaff, AZ corner of Townsend-Winona & Cosnino roads
Agenda:
Along with the usual club business, the meetings consist of discussions of upcoming field trips, local club shows, ‘show and tell ‘of members’ recent finds and expositions. As always the club library is available for browsing and snacks for munching.
Fieldtrips are generally scheduled on the Saturday after the monthly meeting. The plans for the fieldtrip are discussed at that meeting. Details include time and location for caravaning to the search site, types and examples of specimens we’ll be looking for, location map, and any personal items needed for the exposition.
Anyone who plans on participating on the club sponsored trip needs to be a member.
January Fieldtrip
The club did not plan a trip for January. However, they agreed that the Quartzsite Show would be the destination for those who could make it. The show runs through January and February. Information for the show may be found if you Google “Quartzsite Gem and Mineral Showcase”.
