Diamonds – Coconino Lapidary Club

USE OF COLOR IN MINERAL IDENTIFICATION

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

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Diamonds IV

Tue, 28 May 2019 18:25:44 +0000

Lattice Defects, Impurities and Color

As stated in references 17, 18 of my DIAMONDS II blog, impurity atoms and lattice vacancy defects are responsible for the coloration of diamonds [Ref 1]. A lattice vacancy (V) without a carbon atom in the diamond lattice which partners with one to four neighboring nitrogen atoms (N), in colored diamonds, as well as the space taken by an adjacent pair of them partnering with a nickel atom (Ni), are present in colored diamonds. These atom-vacancy structures are shown in Figures 1 to 6 of this Blog. Boron atoms (B), substituting for carbon atoms in the crystal lattice, give rise to the blue color. Hydrogen atoms, possibly associated with lattice vacancies, and nitrogen atoms may also be responsible for imparting color [Ref 2].

Figure 1. The N3 -nitrogen Center [Ref 1]. The vacancy is the space at the tips of four unfilled carbon bonds.

Figure 2. H3-nitrogen Center [Ref 2]. The vacancy is at the tips of the four unfilled nitrogen bonds.

Figure 3. The NV^O-nitrogen Center [Ref 3].

Figure 4. The NV^-1-nitrogen Center. Only the carbon bond of one carbon atom is unfilled [Ref 4].

Figure 5. Boron substituting for a carbon atom in diamond. [Ref 5].

Figure 6. Nickel atom occupying the space of a di-vacancy
[Ref 6].

References for the N3-nitrogen center, the H3-nitrogen center, the NV^O-nitrogen center, the NV^-1-nitrogen center, the boron atom substation for a carbon atom, and the Nickel Di-vacancy center are described respectively in Refs 3, 4.

The colors of diamonds associated with these lattice defects and impurities are referred to in my DIAMONDS II Blog and shown below in Table I.

TABLE I

DEFECT/IMPURITY	DIAMOND COLOR
N3-nitrogen center	Yellow
H3-nitrogen center	Green
NV^O-nitrogen center	Pink
NV^-1-nitrogen center	Pink
Boron atom substation for carbon	Blue
Nickel Di-vacancy	Green
Uncertain defect due to Hydrogen	Grey-brown, Yellow, Pink

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Diamonds III

Thu, 09 May 2019 22:13:42 +0000

A Gallery of Natural Diamond Crystals

The gallery includes clear crystals and those exhibiting a range of colors (Figures 2-13), a range of shapes (Figures 2-15), examples of inclusions found in Diamonds (Figures 1-3), examples of shaping and etching by resorption (Figures 3-15), and examples of twinned diamonds (Figures-14,15). Well-developed crystal forms, typical of diamonds, are shown in Figure 1. References to the impurity and structural defects as causes of differently colored diamonds, to twinning of diamonds, and their deposition including resorption can be found in DIAMONDS II.

Figure 1. Crystal forms of native diamonds and the tetrahedral arrangement of carbon atoms in the crystal lattice [Ref 1].

Figure 2. Pink cubic diamond crystals, Argyle mine, East Kimberley region, Western Australia [Ref 2]. Slight rounding and etching due to resorption is present [Ref 3].

Figure 3. Colorless rounded dodecahedral diamond crystal, location not given [Ref 4]. Some rounding due to resorption is present [Ref 3].

Figure 4. Rounded yellow octahedral diamond crystal (The Oppenheimer Diamond), Kimberley region, Republic of South Africa [Ref 5]. Rounding due to resorption is present [Ref 3].

Figure 5. Octahedral diamond crystal with included garnet crystal [Ref 6]. Slight rounding of the octahedron’s corners is present due to resorption [Ref 3].

Figure 6. Green clinopyroxene crystals in an irregular brown diamond, unstated location [Ref 7]. A large degree of rounding due to resorption is present [Ref 3].

Figure 7. Sulfide mineral inclusions in octahedral diamonds and in a twinned crystal, Diavik mine, Northwest Territories, Canada [Ref 8]. A slight degree of rounding by resorption is present [Ref 3].

Figure 8. Hydrogen inclusions appearing as a brown cloud in a diamond formed in liquid metal in the deep mantle of the earth [Ref 9].

Figure 9. Diamond octahedrons shaped and etched by resorption [Ref 3], [Ref 10].

Figure 10. Rounded brown diamond octahedron [Ref 3][Ref 11]

Figure 11. Rounded green diamond octahedron [Ref 3],
[Ref 12].

Figure 12. Rounded grey diamond octahedron [Ref 3] [Ref 13]

Figure 13. Rounded blue diamond rough [Ref 3]
[Ref 14] with cut gem.

Figure 14. Rounded twinned crystal with some rounding. [Ref 3]
[Ref 15]

Figure 15. Sixling diamond crystal formed by two interpenetrating diamond twins which are mutually twinned on octahedral planes; some rounding by resorption is present. [Ref 3] [Ref 16]

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Diamonds II

Mon, 22 Apr 2019 20:19:32 +0000

In this blog, my second on diamonds, I’ll discuss the physical and chemical properties of a diamond, then its optical properties in relation to its crystal structure and chemistry, and then their formation and associated geology.

CRYSTAL STRUCTURE AND PROPERTIES OF DIAMOND

To understand the physical properties of the diamond it’s important to understand its crystal structure, which underlies its strength, and its other mechanical properties. To understand the sources of the various colors observed in diamonds, it’s necessary to relate them to localized defects in the periodic array of carbon atoms within the crystal lattice, as well as to the impurity atoms in the lattice.

Physical Properties of Diamonds

In this section those physical properties affecting diamond as a gemstone are described. Diamond, unique among minerals, is comprised of only one element, carbon. In the crystal lattice of diamond, the carbon atoms can be pictured as being located at each of the four corners of a tetrahedron, embedded in a cube, as seen in Slide 7 of [Ref 1] and shown in Figure 1.

Figure 1. Tetrahedral arrangement of carbon atoms
in a Diamond.

Extension of the arrangement of the four carbon atoms in the tetrahedron to the lattice, results in the arrangement of carbon atoms in the cubic unit cell of the crystal lattice, as also shown in Figure 1. Bonds between the carbon atoms extend throughout the diamond crystal, as shown in Figure 3. Their strength and arrangement underlie the physical properties of the diamond.

Strength of a Diamond

With a hardness of 10 on the Mohs Hardness Scale [Ref 2], and a relative hardness scale of minerals in the range of 1-10, diamond is ranked as the hardest of minerals. The scale is approximately logarithmic. As a quantitative example, the tensile strength, or resistance to pulling, by natural diamonds has been measured within the range 2.8 – 2.93 GPa (452,776-473797 psi) [Ref 3]. In parallel, these large values of tensile strength are exceeded by values of the compressive strength of diamond, in the range 10-20 GPa (1,617,057-3,234,114 psi) [Ref 3]. This large compressive strength underlies the uses of diamond windows in the diamond anvil cell, as shown in Figure 2. Used for exerting very large pressures on a material for study of its properties [Ref 4]. Recently the formation of metallic hydrogen, by breakdown of molecular hydrogen, (under familiar conditions, a gas), at a pressure of 495 GPa (71.700,000 psi) was achieved using anvils of a specially treated synthetic diamond [Ref 5]. Interestingly, this result supports the hypothesis that the core of the planet Jupiter may be comprised of liquid metallic hydrogen [Ref 6].

The large strength values derive, in part, from the strength of the carbon-carbon bond in the diamond, having the energy of 346 (kilojoules)/(mole of carbon), [Ref 7], and from the pattern of orientation of carbon-carbon bonds within the lattice, with the exception of the pattern of those bonds crossing the octahedral planes of a crystal, along which the crystal can be cleaved. As seen below.

Figure 2. Diamond anvil cell for studies of the properties at ultrahigh pressures. Both anvils and the gasket for confining the specimen are shown. Application of pressure along the axis of the cell is transmitted to the specimen [Ref 4].

Cleavage in a Diamond

Despite the strength of the carbon bonds in the diamond, its lattice does possess a specific set of planes of weakness along which the severing of the bonds requires the least energy. In all other directions, with the application of shear force, the diamond lattice will resist compression or tension. Diamonds exhibit a perfect and easy cleavage or separation along planes parallel to any one of the eight faces of an octahedron, as can be seen in Figure 3 [Ref 8]. In any other direction the number of bonds necessary to be broken for cleavage are larger than along this octahedral plane. This feature underlies the use of intentional cleavage of a rough diamond in removing material prior to further steps in preparing a gem [Ref 9].

Figure 3. Octahedral cleavage planes in diamond.

Visualizing cleavage in diamond

In Figure 4, a partial cleavage is seen to be exactly oriented, parallel to the opposing octahedral face [Ref 10]. Additionally, the view of the diamond lattice parallel to an octahedral plane and the slightly tilted view in Figure 3, shows that the minimum possible number of carbon-carbon bonds per unit area cross this plane, thus, requiring the least possible energy for cleavage.

Figure 4. Octahedral cleavage in diamond.

As a caveat, because of its easy and perfect cleavage, and despite its hardness, toughness or tenacity, the ability to resist fracture is only fair to good, because its easy cleavage renders it susceptible to breaking.

Twinning in Diamonds

Twinned crystals of diamond present a unique form, quite unlike that of a single crystal. Conceptually, twinning in diamonds occurs between the octahedral faces of two crystals, as shown diagrammatically in Figure 5 [Ref 11]. This twin form is also known as a macle. The reentrant angle at the base of the triangular faces of the macle is shown on the twin in Figure 6. Two macles can pair as shown in Figure 7, to form a Star-of-David twin [Ref 12].

Figure 5. Octahedral twinning (Spinel Law twinning) in diamonds [Ref 11]

Figure-6. Twinned diamond crystals (a macle). In this twin two octahedral faces face each other [Ref 11].

Figure 12. Star of David Twin, Northern Cape Province, RSA [Ref 12]. The two macles are twinned on mutual octahedral planes.

Optical Properties of Diamond

The appreciation of diamonds as gemstones, lies in their sparkling brilliance, and in their colors, which span the rainbow. Their brilliance is due to their large refractive index and dispersion of colors. Their colors are due to the presence of impurities and defects in their crystal lattice.

Refractive index and dispersion

The refractive index of a gemstone is a measure of the magnitude of the angle at which light is bent, (refracted), as it enters from the outside [Ref 13]. If the refractive index of the material at which the light enters exceeds that at the exterior, the light will bend more towards the perpendicular (Figure 1 in Ref 13).

The brilliant appearance of a diamond, compared to other gemstones, stems from its larger refractive index and degree of dependence on the wavelength of the incident light. The brilliance, or dance of colors seen on a cut diamond stems from its large dispersion, which is a measure of splitting light into its constituent colors after entering the gemstone [Ref 14]. Such dispersion can be demonstrated by the rainbow of colors which exit from a glass prism, as shown in Figure 13. As a uniform measure for all gems, the dispersion is calculated as the difference in refractive index values measured with light at 430.8 in the violet and nm ad 686.7 nm in the red region [Ref 14]. The difference of the two values gives the dispersion of the diamond d = 2.451-2.407 = 0.044 [Ref 14]. This value is very large compared to other gemstones. For example, this value far exceeds the value of 0.014 for difference in the refractive index of light determined at two wavelengths, 430.8 nm in aquamarine (and also for emerald, heliodor, and morganite), the value of 0.018 for corundum (and also for ruby and sapphire) [Ref 15].

Figure 13. Dispersion of white light into colors.

The angle at which visible light is bent as it enters the diamond depends directly on the wavelength-dependent refractive index. The cut of the gem, or the overall placement of the facets on the rear, sides and face, is designed to optimize the play of colors developed within the gem by dispersion for the viewer. The facets must be placed at angles calculated to optimally reflect the light back into the gemstone and position it to escape from the face of the gemstone. The American Standard Brilliant Cut [Ref 16], among others, maximizes the total amount of light and the play of its colors, leaving the gem as shown in Figures 14 and 15.

Figure14. The angles of the facets of the gem comprising the bottom of the gem are determined to maximize the amount of light re-emerging from the stone [Ref 17].

Figure 15. “Fire in colors by dispersion”.

Impurities, Crystal Lattice Defects, and Color of Diamonds

Many of the structural impurities, which are sources of colors in diamonds, are described in the Shigley Chart [Ref 18]. In the list of color sources, nitrogen atoms are labeled N, as impurities substituting for carbon atoms in the diamond lattice, alone or associated with an adjacent lattice vacancy labeled V, and are seen to cause many of the colors which range over the green to red portion of the visible spectrum. Additionally, boron atoms, labeled B, as impurities substituting for carbon atoms, impart a blue color, while a single lattice vacancy labeled V, imparts a green color. Groups of vacancies can impart a yellowish-brown color. Also, interstitial hydrogen atoms lying in spaces between carbon atoms, impart a yellowish-brown color. In another study, an interstitial nickel atom as an impurity, (with an associated large-sized split-vacancy Ni-2V), was found to impart a green color [Ref 19].

The Formation of Diamonds [Ref 20]

Diamonds are formed within the earth’s mantle at remarkable depths of 80 kilometers and greater, and at notable pressures of 2.6 Gigapascals or 377098 psi and greater, as shown in Figure 22. They are violently projected to the surface by magma rising at velocities of 200 meters per second, which fractures the overhead rock and forms a crater at the surface containing a plug of the host rocks of diamonds, as shown in Figure 23. The major host rocks are kimberlite [Ref 21] or lamproite [Ref 22] as shown in Figures 23 and 24. Both rocks are breccias formed of cemented fragments, which resulted from the rock fracturing. Figure 27, taken of the Diavik mine in Canada, shows the circular contours of two kimberlite ore bodies on the surface [Ref 20].

Figure 22. Pressure and depth relationships for the formation of diamonds [Ref 20].

Figure 23. Formation of a kimberlite or lamproite ore body [Ref 20].

Figure 23. Kimberlite comprised of cemented rock fragments
[Ref 21].

Figure 24. Lamproite comprised of cemented rock fragments
[Ref 22].

Figure 27. Workings of the Diavik Diamond Mine [Ref 20].

Resorption with etching and rounding of the surfaces of diamonds can occur due chemical attack by the acidic environment, due to the presence water (H₂O), and oxidation of the diamonds carbon to form carbon dioxide (CO₂) within the melt of the magma [Ref 23]. Examples of the rounding of the shapes of octahedral and cubic crystals with increasing resorption are shown in Figure 28.

Figure 28. Shape changes of diamonds indicating rounding of their shapes due to resorption increasing with pressure at depth and temperature [Ref 23].

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Diamonds I

Tue, 26 Mar 2019 22:04:28 +0000

Having greatly enjoyed writing my last blogs on garnets as gemstones and mineral specimens, topics different from my long-time interests in the ore-forming minerals, I am starting a series of blogs in the same vein on the precious gems: diamonds, emeralds, rubies, and sapphires.

In the first three blogs the topic is diamonds. In this first blog, I will describe some famous diamonds and their histories, and follow up with a gallery of diamond jewelry and diamond-decorated art objects from over the years. In the second blog I will briefly discuss the crystal structure of diamonds, which underlies their physical and optical properties, including their brilliance and colors, and how diamonds are formed deep within the earth’s mantle and then ejected to the surface. In the third blog I’ll end the series on diamonds with a photo gallery of natural uncut diamond specimens, emphasizing varieties of crystal forms and colors.

THE BEAUTY AND ALLURE OF DIAMONDS

Not only do diamond gemstones display a remarkable sparkling brilliance, their colored varieties, perhaps unexpectedly, span the spectrum of the rainbow (Figure 1). These properties underlie their being esteemed over the years by collectors and wearers of these gems. Diamond’s beauty and early rarity due to limited sources, along with their aura of great wealth and power, underlay their being worn as crown jewels, being set as the gemstones of royals and glitterati, and being set in jewelry, which are also art objects. Some famous gemstones and other art objects are described below.

Figure 1. Colors of diamonds.

The Koh-i-noor Diamond [Ref 1]

The rough stone was mined in the alluvial sands in the Golconda, the ancient diamond-mining region of India [Ref 2]. Its name is Persian for “Mountain of light”, attesting to its brilliance. Prior to entering the written record, it was shown in 1628 as being set at the top of the gem-laden throne of the Mughal ruler Shah Jahan (Figure 1) [Ref 3].

Figure 2. Peacock Throne of the Mughal ruler Shah Jahan [Ref 3].

Following its possession by Mughals, it was taken from India in 1739 by the Persian ruler Nadar Shah among the spoils of war. The gem remained in what is now Afghanistan until, after decades of fighting, it was returned to India by the Sikh ruler Ranjit Singh in 1813. After Singh’s death in 1839 the gem passed from ruler to ruler before being possessed by a boy, Duleep Singh and his mother, Rani Jindin. Following her imprisonment by the British, who, as the British East India Company, expanded into central India, ten-year old Duleep was forced to give away the Koh-i-noor and his claim to sovereignty. From there the diamond came into the possession of Queen Victoria after it was recut to enhance its brilliance (Figure 3) [Ref 4]. It was set in a brooch, (Figure 4) [Ref 5], and became part of the British Crown Jewels [Ref 6]. After resetting, the diamond now resides at the front of the crown of the Queen Mother, mother of Queen Elizabeth II (Figure 5).

Figure 3. The Koh-i-noor diamond before and after re-cutting [Ref 4].

Figure 4. Queen Victoria wearing the brooch with the recut Koh-i-noor diamond [Ref 5].

Figure 5. Queen mother’s crown with the round brilliant recut Koh-i-noor Diamond [Ref 6].

The Cullinan Diamond [Ref 7]

More recently found than the Koh-i-noor, the Cullinan diamond (Figure 6) was found in 1905 at the, (then called), Premier Mine in Johannesburg, South Africa. At 3106 carets (1.371 lbs.) it was the largest rough diamond ever found, until the discovery of a 1109 caret diamond in Botswana. Cutting it provided seven gemstones (Figure 7) including the Cullinan I and Cullinan II diamonds. These were set respectively in the Sovereign’s Scepter (Figure 8) and in the Imperial State Crown (Figure 9), both among the British Crown Jewels.

Figure 6. The rough Cullinan Diamond as discovered [Ref 7].

Figure 7. The major gems from the Cullinan diamond [Ref 8].

Figure 8. The Sovereign’s Scepter of England with the Cullinan I Diamond [Ref 9].

Figure 9. The Imperial State Crown of England with the Cullinan II diamond, the St. Edward’s Sapphire and the Black Prince’s “Ruby” (actually a red spinel) are also set on the crown [Ref 10].

The Hope Diamond [Ref 11]

Reportedly cursed, after being stolen from the eye of a holy Hindu statue, the original 115.16 ct stone was likely purchased by Jean-Baptiste Tavener, a diamond merchant, from the Kollur Mine in Golcanda, India and sold in 1668 to King Louis XIV of France, and became known as the “French Blue”. It was recut to a 67 ct gem by the court jeweler [Ref 11]. During the French Revolution the French Blue was stolen. Later the stone changed hands and went to Henry Phillip Hope. It is now famously known as the Hope Diamond. After a series of purchases, it was donated to the Smithsonian Institution in 1958 where it resides today. After its acquisition by the museum the weight of the stone was determined to be 45.52 carats. Its intense blue color is seen in Figure 10.

Figure 10. The Hope Diamond [Ref 11].

The Orlov Diamond [Ref 12].

The rough diamond ancestor of the Orlov Diamond was mined in the Kolliur Mine in the Golcanda region of India [Ref 12] and weighed 787 carats. Following it’s residence as an eye of a deity, in a temple on the island of Srirangam, in South India, it made its way to Amsterdam, and around 1768 was sold to Count Grigoryevich Orlov, a Russian nobleman and officer who helped Catherine the Great overthrow Peter III and gain the throne of Russia. Catherine had the diamond mounted on the Imperial Scepter (Figure 11). The diamond has a non-traditional shape, resembling half of a bird’s egg. It is a white diamond [Ref 13] with a bluish-green tint and weighs 189 cts.

Figure 11. The Olov Diamond atop the Russian Imperial Scepter [Ref 13].

The Tiffany Yellow Diamond [Ref 14]

This rough diamond, weighing 287 cts, is the largest yellow diamond ever mined. Purchased by Charles Lewis Tiffany, founder of Tiffany & Co., in 1877. It was fashioned into a cushion-cut brilliant, weighing 128 cts. The gemstone was fashioned into a diamond necklace (Figure 12) and worn by Audrey Hepburn in promotional photos for the film Breakfast at Tiffanys. Later the gemstone was mounted to create the whimsical “Bird on a Rock” brooch (Figure 13). The stone was reset into a diamond and platinum necklace in 2012 to commemorate the 175^thanniversary of Tiffany & Co.

Figure 12. Necklace with the Tiffany Yellow Diamond as worn by Audrey Hepburn [Ref 16].

Figure 13. Bird on a Rock brooch with Tiffany Yellow Diamond [Ref 15]

Gallery of Diamond Jewelry as Art Objects

Figure 14. Mukut (Crown) head ornament of Lord Vishnu with diamonds and other precious stones (1300-1500) [Ref 17].

Figure 15. Mughal turban ornament with diamonds, rubies, emeralds [Ref 18].

Figure 16. (Inscribed Shah diamond) [Ref 19]

Figure 17. Indian Prince Diamond necklace [Ref 20]. The triangular gems are worked twinned crystals (Macles). See gallery of specimens in the Diamonds III blog.

Figure 18. Nephrite jade box with diamonds, rubies, and emeralds [Ref 21].

Figure 19. Art Deco diamond, emerald, and platinum bracelet, ca. 1915 [Ref 22].

Figure 20. Art Deco Diamond, Coral, Onyx and Platinum Ring [Ref 23].

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