Ask An Expert – Coconino Lapidary Club

Need Help Identifying These Specimens

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

What Are You Curious About?

I am from Wisconsin, staying in Winslow with girlfriend who is a nurse.. I just became interested in rocks and minerals and need help identifying some specimens…. thank you

Your specimens are three forms of quartz, in which the crystals are of microscopic size so you can’t see their shape.

This lightest colored specimen, is called chalcedony (http://www.quartzpage.de/chalcedony.html).

This pinkish-beige specimen is also chalcedony, with its pink-beige color coming from the presence of iron oxide.

This specimen, with the areas of darkly colored orange and red, is jasper, a variety of chalcedony (http://www.quartzpage.de/jasper.html).

Each of your specimens is typical of its type and could give you a nice beginning in collecting varieties of quartz.

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

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

Ref 11. https://www.mindat.org/min-2815.html

Ref 12. https://www.pitt.edu/~cejones/GeoImages/1Minerals/1IgneousMineralz/Micas.html

Ref 13. https://www.semanticscholar.org/paper/How-flat-is-an-air-cleaved-mica-surface-Ostendorf-Schmitz/9138459a4a23daf157b9cfd22e08eee2aaf32087/figure/0

Ref 14. https://www.mindat.org/min-859.html

Ref 15. https://www.witchcraftsartisanalchemy.com/viking-sunstone-natural-rhombohedral-iceland-spar-optical-calcite-crystal/

Ref 16. https://chemdemos.uoregon.edu/demos/Properties-of-An-Ionic-Salt-0

Ref 17. https://www.britannica.com/science/calcite

Ref 18. https://jgs.lyellcollection.org/content/176/2/337

Ref 19 . https://vimeo.com/20281170

Ref 20. https://www.google.com/search?q=image+of+a+diamond+clevage+surface&client=firefox-b-1-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjS3pLQ_drtAhWSLH0KHemBBy4Q_AUoAXoECBEQAw&biw=1382&bih=911#imgrc=jAnPo_wrqLUWNM

FRACTURE IN MINERAL IDENTIFICATION

Mineral Man Mel — Fri, 22 Jan 2021 18:54:24 +0000

Fracture in mineralogy is the texture and shape of the surface formed when the mineral is fractured. Fracture differs from cleavage and parting, which involve clean splitting along a plane surface, as it produces rough irregular surfaces [Ref1]. The appearance of fracture surfaces among minerals is highly varied and is a useful tool in identification. In this part of my Blog I’ll describe the fracture surfaces broadly seen in minerals.

Conchoidal fracture is characterized by smoothly curving nested arcs as those on a seashell[Ref2].

Figure 1. Conchoidal fracture in rose quartz[Ref3,4].

Earthy fracture results in dull, clay-like surfaces without crystalline appearance[Ref2].

Figure 2. Earthy fracture in massive limonite[Ref5,6].

Fibrous fracture is typified by elongated crystal forms[Ref2].

Figure 3. Fibrous fracture in chrysotile (asbestos) [Re7,8].

Granular fracture is produced in aggregates of crystals[Ref2].

Figure 4. Granular fracture in an aggregate of arsenopyrite crystals[Ref9,10]

Hackly fracture produces torn edges and surfaces[Ref2].

Figure 5. Hackly fracture in copper producing torn edges[Ref11,12].

Irregular fracture presents an irregular fracture pattern[Ref2]

Figure 6. Plagioclase feldspar showing an irregular fracture surface[Ref13,14].

Splintery fracture produces thin long cleavages or partings[Ref 2].

Figure 7. Splintery fracture in actinolite [Ref15,16].

Uneven fracture features flat surfaces in a random pattern[Ref2].

Figure 8. Flat surfaces unevenly arrayed on blue sodalite[Ref17,18].

REFERENCES

Ref1 . https://en.wikipedia.org/wiki/Fracture_(mineralogy)

Ref 2. http://csmgeo.csm.jmu.edu/geollab/kearns/Minerals/Fracture.html

Ref 3. https://www.pinterest.com/pin/109564203407418361/

Ref 4. https://www.mindat.org/min-3337.html

Ref 5. https://en.wikipedia.org/wiki/Fracture_(mineralogy)#/media/File:Limonite_bog_iron_cm02.jpg

Ref 6. https://www.mindat.org/min-2402.html

Ref 7. https://en.wikipedia.org/wiki/Fracture_(mineralogy)

Ref 8. https://www.mindat.org/min-975.html

Ref .9 https://sanuja.com/blog/ore-minerals-and-rocks

Ref 10. https://www.mindat.org/min-305.html

Ref 11. https://geology-fundamentals.fandom.com/wiki/4241978/hackly-fracture

Ref 12. https://www.mindat.org/min-1209.html

Ref 13. http://www.pitt.edu/~cejones/GeoImages/1Minerals/1IgneousMineralz/Feldspars.html

Ref 14. http://webmin.mindat.org/data/Plagioclase.shtml#.YADaz-BlCV4

Ref 15. https://www.minerals.net/Image/2/9/Actinolite.aspx

Ref 16. https://www.mindat.org/min-18.html

Ref 17. https://www.minerals.net/mineral_glossary/uneven_fracture.aspx

Ref 18. https://www.mindat.org/min-3701.html

Meteorites

Mineral Man Mel — Sat, 27 Jun 2020 18:18:06 +0000

Meteorites are stuff from outer space; each is a solid piece of debris which formed from dust within the protoplanetary disc or from object such as an asteroid, planetesimal, or planet, and which travels through space and falls through the atmosphere to the surface of earth. As alien objects which fall dramatically they have evoked interest and wonder among many people. I remember vividly at the age of ten watching with my dad the meteorites fall during a Perseid Meteor Shower such as shown in time lapse photography in Figure 1. We both were greatly excited by the sights of the darting meteor trails and the awe felt by both of us felt was almost palpable. This fond memory has prompted me to write about meteorites in hopes of further acquainting my readers with them.

I will write three blogs about meteorites; in the first I’ll consider both iron and stony-iron meteorites, and the following blogs will be about chondrites and achondrites, both classes of stony meteorites, and lastly about chondrules, which are constituents of chondrites and are sub-millimeter to millimeter-sized spherules containing silicates and another minerals and are often quite beautiful.

The three major classifications of meteorites based on their mineral compositions are: stony meteorites which are comprised of silicate minerals, iron meteorites which primarily are comprised of an alloy of iron and nickel, and stony-iron meteorites which are comprised of a mixture of iron-nickel alloy and silicate minerals. Iron-nickel meteorites can also contain sulfide, carbide, phosphide minerals of iron, nickel, cobalt, and chromium, as well as native carbon. The class of stony-iron meteorites are comprised of two subclasses, pallasites and mesosiderites[. The distribution of classes of meteorites found world-wide are shown in TABLE 1 and examples of them are Figures 2-5.

Figure 1. Time-lapse photograph showing meteors falling during a Perseid Meteor Shower[

TABLE 1. DISTRIBUTION OF CLASSES OF METEORITES BY FALLS FOUND THROUGHOUT THE WORLD

METEORITE CLASSIFICATION	PERCENTAGE
Stony	97.4
Iron-Nickel	2.0
Pallasite	0.2
Mesosiderite	0.4

IRON AND STONY-IRON METEORITES

Meteorites containing an iron-nickel alloy are classified as the iron meteorites, and the stony iron meteorites, the latter including the pallasites and the mesosiderites.

In the iron meteorites the iron-nickel alloy is present as two minerals: kamacite which contains up to 7.5% dissolved nickel in solution with iron and taenite with more than 25% nickel in solution. Other minerals which contain iron, nickel, cobalt, phosphorous, oxygen, sulfur, and carbon in lesser abundance may be present in iron meteorites. These minerals with their compositions are summarized in Table 2. Each minda.org reference contains representative photographs of the mineral.

TABLE 2. MAJOR MINERALS FOUND IN IRON AND STONY IRON METEORITES

MINERAL	CHEMICAL FORMULA	MINDAT.ORG REFERENCE
Kamacite	(Fe,Ni)	8
Taenite	(Fe,Ni)	9
Troilite	FeS	10
Daubre’elite	Fe²⁺Cr³⁺₂S₄	11
Cohenite	(Fe,Ni,Co)₂₃C₆	12
Schreibersite	(Fe,Ni)₃P	13
Chromite	Fe²⁺Cr³⁺O₄	14
Magnetite	Fe₃O₄	15
Corundum	Al₂O₃	16
Graphite	C	17
Diamond	C	18
Lonsdaleite	C	19

Figure 2. Canyon Diablo meteorite, Barringer Meteor Crater region, Coconino County, Arizona. Note the sculpting of its form by ablation and loss of the molten surface layer of iron alloy formed by frictional forces during its fall through the air.

Figure 3. Etched slice of a Canyon Diablo meteorite, Barringer Meteor Crater Region, Coconino county, Arizona[Ref28]. Note both the brown crystal of troilite with a grey rim of cohenite and the The angular Widmenstatten are comprised of angular interleaving bands of the iron-nickel alloys kamacite and taenite referenced in TABLE 1.

The Structure and Metallurgy of Iron-Nickel Meteorites

Iron-nickel meteorites are thought to be remnants of the cores of planetesimals following collision between them . With slow cooling during solidificaiton of the core the liquid alloy mixture of iron and nickel results in a mixture of the two minerals kamacite and taenite. With sufficiently slow cooling to 912 deg C and below discrete visible crystals of each mineral form separately as shown in the iron-nickel phase diagram. The interleafed arrangement of the crystals results in the arrangement called Widmanstatten Figures as shown in Figure 3 which become visible upon etching with an acid. The different grey-tones of the crystals develop from different etching susceptibilities which arise from the different atomic arrangements within face-centered cubic kamacite and body-centered cubic taenite.

Structure, Mineral Composition, and Formation of Stony-iron Meteorites

Stony-iron meteorites consist of approximately equal parts of iron-nickel alloy and silicate minerals as opposed to stony meteorites which primarily are made up of silicate minerals. Stony-iron meteorites are divided into two groups, the pallasites and the mesosiderites Pallasites have silicate minerals, mostly olivine embedded in the meteoric iron. Mesosiderites are breccias comprised of fragments of meteoric iron interspersed with fragments of silicate minerals.

Pallasites

Results of a recent study suggest that pallasite meteorites may have formed in a glancing impact between a body with a largely solidified core surrounded by a molten layer and a larger body. It is assumed that loss of the solid core and formation a body comprised of aggregates of fragments of olivine surrounded by molten iron-nickel alloy ensued. Solidification of the molten alloy results in cementation of the olivine fragments which results in the structure shown in Figure 4

Figure 4. The olivine usually is present in a 2/1 volume ratio with the iron-nickel matrix. The boundary regions between an olivine crystal and metal may contain thin layers of a mineral such as troilite, schreibersite, or chromite.

Mesosiderites

Mesosiderites are considered to have formed from fragments of iron-nickel alloy and silicate minerals which resulted from collision of metal-rich and silica-rich asteroids. The fragments may be of the igneous rocks, basalts, gabbros, and pyroxenites of igneous rocks as well as fragments of the minerals orthopyroxene, olivine, and plagioclase. The Iron-nickel metal is mostly in the form of millimenter or sub-millimeter grains admixed with grains of silicate minerals in the same size range surrounding the larger fragments of rocks and minerals. These structural features are shown in Figure 5.

Figure 5 . Mesosiderite, found at Gillio, Libya in 1985. Metal nodules, pyroxene, on Ca-pyroxene fragments, plagioclase, pyrrhotite, chromite, and both kamacite and taenite are present.

HARDNESS: A MECHANICAL PROPERTY OF USE IN IDENTIFYING MINERALS

Mineral Man Mel — Mon, 16 Mar 2020 16:29:18 +0000

Along with the colors and crystal forms of minerals, their mechanical properties such as the hardness. tenacity, and habits of structural failure can be very useful tools in identifying an unknown mineral. In this first Blog on the use of mechanical properties in identifying minerals I’ll focus on hardness and describe the Moh’s Hardness Scale, the standard reference scale for minerals, and briefly describe how to use it. I’ll also list those often available objects which can be usedinstead of a mineral member of the scale as hardness references for comparison with the mineral.

The Mohs Hardness Test and Scale were invented in 1812 by the German mineralogist Fredrich Moh[Ref1,2] as a means to rank minerals according to their relative hardness in resisting scratching. Its ease of use has resulted in its wide application by mineral collectors and lapidarists. The ten-point scale begins with the softest mineral talc assigned a hardness of 1 and ends with the hardest mineral diamond assigned a hardness of 10. Minerals with intervening hardness values between 2 and 9 are: gypsum (selenite) at a hardness of 2, calcite at 3, fluorite at 4, apatite at 5, orthoclase (feldspar) at 6, quartz at 7, topaz at 8, and corundum (ruby, sapphire) at 9. In Figure 1 shows the hardness scale, and as well, includes everyday objects of known hardness which can also be used in hardness testing[Ref2]. The relative hardness values are obtained by determining which mineral scratches one of lesser hardness and is scratched by a mineral or an object of greater hardness. Testing is done by placing a sharp point of one mineral or another testing agent against the surface of another and attempting to scratch it. One of the following results may be observed:

If mineral A scratches mineral B then A is harder than B.

If A doesn’t scratch B then B is harder than A.

If A and B are relatively ineffective in scratching each other they are of equal hardness.

If A can be scratched by B but not by C the hardness o A is between the hardness of B and C.

The Mohs Hardness Test is performed using the steps and tips as described in

Figures 2 and as shown live in the YouTube video accompanying this brief presentation. An example of testing using a knife and a penny are described in Figure3.

Figure 1. Mohs Hardness Scale collection and mineral identification[Ref3]. The copper penny must be dated no later than 1982; pennies issued subsequently were minted using a softer bronze alloy[Ref4].

Figure 2. Steps in performing Mohs Hardness Test[Ref5].

Figure3. An example of a Mohs Hardness test using a knife and a penny[Ref5].

REFERENCES

Ref 1. https://en.wikipedia.org/wiki/Friedrich_Mohs

Ref2. https://geology.com/minerals/mohs-hardness-scale.shtml

Ref 3. https://www.lindahall.org/friedrich-mohs/

Ref 4. https://www.nbc12.com/story/17003531/jens-coin-story/

Ref 5. http://www.rocksandminerals.com/hardness/mohs.htm