Among the native elements, Gold [Ref 1] because of its beautiful golden color, its rarity, and its aura of wealth and power, is a favorite among collectors and museum-goers [Ref 2] [Ref 3] [Ref 4]. Specimens of electrum, the lighter colored of alloy of gold, containing silver, [Ref 2] are also favorites. Specimens of both native gold and electrum from around the world occur in a variety of aesthetic and interesting forms ranging from single crystals and their groupings (Figs 5-9), twinned crystals (Figures 10-14), intricate dendrites, which are fern-like single crystals (Figures 14-15), and in spectacular sheet forms (Figure 16).

Some Properties of Gold [Ref 1] 

With a Mohs hardness in the range 2.5-3 gold is malleable which makes it easy to work into decorative forms by a goldsmiths. It also doesn’t oxidize, which facilitates melting it for casting, and soldering. In thick form, gold exhibits a rich yellow color due to its high reflectance of light in the yellow-red spectral range. In sufficiently thin form, as gold leaf, it transmits blue and green light. Its high specific gravity measures in the range 15-19.3 grams/(cubic centimeter), which allows efficient recovery of gold in placer deposits such as gold panning.

Basic Gold Crystal Forms [Ref 1]

Gold, (and Electrum), crystallize in the isometric crystal system in it’s typical forms, shown in Figures 1-2. Gold forms twinned crystals about an octahedral plane as shown inFigures 3-5 [Ref 5 ]. Dendritic crystals result from repeated Spinel-twinning in a branched structure with branches at 60 degrees relative to each other. A native gold specimen referred to as wire gold, is not a wire in the sense of the native silver wire, (See the Native Silver Blog), but is an extended single group of multiple Spinel-Twinned crystals as shown in the specimens of Figures 12-13. Gold deposited in fractures with the host mineral, when exposed, possesses leaf-like forms as in Figure 15.

Gallery of Native Gold Specimens

In many gold specimens the crystals do not display the perfection of form typical of some other minerals such as pyrite, but are skeletal with depressions, or [Ref 6], are also referred to as being hoppered [Ref 7]. Gold crystals can also exhibit interesting, complex twinned and dendritic forms. These departures from ideal forms stem from conditions of rapid deposition in absence of thermodynamic equilibrium [Ref 8]. Gold specimens featuring octahedral, cubic, wire, dendritic. and leaf forms, the latter, which in many instances is formed in interstices in fractured quartz [Ref 9].

[metaslider id=1914]

Deposition of Gold By Bacteria

In searching the web, I found recent studies which, surprisingly to me, demonstrated the deposition of particulate gold by the action of specialized bacteria in alluvial deposits [Ref 10] [Ref 11]. In the studies, the presence of toxic gold complexes was shown to initiate the formation of a population of bacteria which excretes enzymes that catalyze the formation of nanoparticles of gold, [Ref 12]. Aggregation of nanoparticles of silver and gold have been shown to participate in crystallization of these metals, which allow growth of gold on a nugget [Ref 13]. Bacteria resident in a biofilm [Ref 12], on the surface of a gold nugget could function as a source of gold which forms a gold coat on the crystal with sustained activity of the bacteria. A scanning electron microscope image of the bacteria in the biofilm on a gold nugget is shown in Figure16. The deposition process takes place for years to decades in order to accrue on a gold grain, suggesting that if the process can be speeded up, bacterial deposition could improve ore-processing [Ref 14].

References

Ref 1. https://www.mindat.org/min-1720.html

Ref 2. https://www.amnh.org/exhibitions/gold/

Ref 3. https://en.wikipedia.org/wiki/Gold_Museum,_Bogota

Ref 4. https://adrianhepworth.photoshelter.com/image/I0000f0UBlFQgu3I

Ref 5. https://www.mineral-forum.com/message-board/viewtopic.php?t=3044

Ref 6. https://www.mindat.org/glossary/skeletal_crystal

Ref 7.https://en.wikipedia.org/wiki/Hopper_crystal

Ref 8. https://en.wikipedia.org/wiki/Crystal_growth

Ref 9. https://pubs.geoscienceworld.org/gsa/geology/article-abstract/16/6/551/190624

Ref 10. https://phys.org/news/2009-10-bacterium-formation-gold.html

Ref 11. http://www.mdpi.com/2075-163X/3/4/367/htm

Ref 12. https://en.wikipedia.org/wiki/Cupriavidus_metallidurans

Ref 13. http://iopscience.iop.org/article/10.1088/0957-4484/17/23/021/meta

Ref 14. https://phys.org/news/2017-04-role-microorganisms-industrial-gold.html

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

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

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

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

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

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

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

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

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

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

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

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

[metaslider id=1788]

References

Ref 1. https://en.wikipedia.org/wiki/Twenty-sixth_Dynasty_of_Egypt

Ref 2. https://artdeco.org/what-is-art-deco/early-20th-century-timeline

Ref 3. https://en.wikipedia.org/wiki/Silversmith

Ref 4.https://en.wikipedia.org/wiki/Inlay

Ref 5. http://www.busaccagallery.com/catalog.php?catid=141&itemid=5619

Ref 6. http://www.ancientresource.com/lots/ancient_jewelry/diannesommelet/diannesommelet2.html

Ref 7. https://www.secretenergy.com/illustrations/cultures/mesopotamia/

Ref 8. http://www.getty.edu/art/exhibitions/ancient_luxury/

Ref 9. http://www.historyandcivilization.com/Picture-Gallery—Early-Mesopotamia-from-Sumer-to-Assyria—Artifacts–Objects—Sculpture.html

Ref 10. https://www.pinterest.com/pin/354799276868988412/?lp=true

Ref 11. http://www.antiques.com/classified/1112715/Antique-Huge-Roman-Silver-Ring-w–Portrait#

Ref 12. https://www.ngccoin.com/news/article/6078/ancient-coins/

Ref 13. http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?assetId=973486001&objectId=154939&partId=1

Ref 14. https://www.archaeology.org/issues/149-1409/artifact/2388-denmark-viking-figurine

Ref 15. https://art.thewalters.org/detail/40080/signet-ring-2/

Ref 16. http://www.getty.edu/art/exhibitions/ancient_luxury/

Ref 17. https://boylerpf.com/products/antique-victorian-silver-italian-coral-bracelet

Ref 18.https://boylerpf.com/products/vintage-italian-silver-carnelian-art-deco-bracelet

Among the native elements, silver, [Ref 1], is a favorite among mineral collectors, as it is for me. Silver offers variations in color from metallic silver-white to the optical interference colors of a thin coating film, (such as on copper sulfide minerals – see my earlier Copper Blog), to the sooty black of a thick coating. From locations around the world it occurs in a number of aesthetic and geometrically interesting forms, ranging from groupings of single crystals (Figures 1-2), spectacular fern-like dendritic arrays of crystals (Figures 3 & 4), and striking wire and sheet forms (Figure 5 & 6). The relative arrangement of crystals in, and the shape of, the dendrites has been found to depend on the conditions of the surrounding silver-bearing solution during deposition of the silver, [Ref 2]. Specimens of wires attached to the silver sulfide acanthite, (Ag₂S), grow on oxidation of the sulfide mineral, which liberates the silver, as described below, both by roasting, [Ref 3], or by chemical reactions in solutions within both the oxidized and replacement zones of an ore body, (Slide 9 of [Ref 4]). The wires grow at the interface between the acanthite and silver by continuing the face centered cubic lattice shared by both the acanthite and the silver, [Ref 1]. The sulfur of the acanthite occupies the interstices between the silver atoms. X-Ray diffraction and microscopy have demonstrated the crystallinity of a native silver wire.

In order to share with you these beautiful and intriguing forms of native silver, I’ve included a comprehensive gallery of these forms from around the world, (Figures 3-17). I’ve also taken the liberty of including a favorite specimen from my silver collection in the gallery, (Figure 10).

Because the lore of lost precious metal mines, particularly those in Arizona, New Mexico, and Nevada fascinate many of us; I’ll begin referencing descriptions and histories of these mines and provide brief excerpts from the references. In this blog, the emphasis will be on lost silver mines and in future blogs on silver minerals. Future blogs on Gold and Gold minerals will also include lore & history of lost Gold mines.

Silver Crystal Forms

Silver belongs to the isometric crystal system, [Ref 1], and crystallizes in cubic and octahedral forms as shown in Figures 1 and 2. The forms reflect the symmetry of the isometric crystal system. Silver crystals form twins on the octahedral surfaces of two crystals resulting in a Spinel-Twin, [Ref 6], with the remainders of each of the octahedrons visible, as seen in Figure 3.

GALLERY OF NATIVE SILVER SPECIMENS

LOST SILVER MINES

The location of the Lost Duppa Silver Mine in Arizona, [Ref 8], lies in the numerous mines and ore deposits of the heavily mineralized Bradshaw Mountains, (Figures 11-16). When discovered, the deposit was a ledge of silver-bearing quartz located in one of the many steep canyons located on the east side, of the northern Bradshaw Mountains. The ore was native silver. After his initial find, Duppa failed to relocate his original path to the deposit and never found it again.

The Lost Silver Lode of Carbonate Creek, New Mexico, [Ref 11]

The discovery of lode was in the Kingston Mining District, located in the southern region of the Black Range in Southwestern New Mexico. Located within the range are the Chloride, Kingston, and Hermosa Silver Distracts which have been rich producers of the metal. The lost lode lies in the Kingston Silver Mining District shown in [Ref 12], which eventually produced silver amounting to over Six Million in USD. The lost lode was discovered along Carbonate Creek near the town of Kingston as surface float of acanthite (silver sulfide). The weights of pieces of the float ranged up to 250 pounds. Ultimately float yielding over 80,000 ounces was found, but the source of the float was never discovered.

Porphyry Copper Deposits

Porphyry Copper deposits are the world’s largest source of copper [Ref 1] and are distributed globally (Figure 1).

World wide, production by the ten largest producers amounted to 15.4 million tons in 1917 [Ref 2]; of these, the United States ranked fourth in production at 1.27 million tons, of which 68% was produced by mines in Arizona [Ref 3].

Numerous deposits are located in the geological Basin and Range Province of the Southwest, as shown in Figure 2. Among the currently active mines in Arizona are those operated by Freeport-McMoRan Inc, the Morenci, Bagdad, Safford, Sierrita and Miami mines [Ref 4] and those operated by ASARCO LLC, which are the Silver Bell, Mission complex mines, and the Ray complex mines [Ref 5].

Formation of Porphyry Copper Deposits and Ore Minerals

The ore bodies of Porphyry Copper deposits are formed by the intrusion of hydrothermal fluids emanating from a magma chamber several kilometers below the earth’s surface and the deposition of ore minerals as veins in pressure-induced fractures within a granitic porphyry (See figures and text in Ref 6). Chalcopyrite is the major copper mineral deposited [Ref 7, Page 4]. This initial mineralization results in grades of 0.3 to 0.9% copper and almost always less than 1% [Ref 6]. It is by Supergene Enrichment (Slides 6, 7, 8 in Ref 8], a secondary enrichment process, that the deposition and the accumulation of copper ore minerals above and below the water table increases the ore grade. As shown in Slide 8 of [Ref 9], oxidizing conditions in the ore body above the water table result in deposition of copper minerals such as azurite, malachite, and chrysocolla, and the sulfide minerals, chalcocite and bornite, form under a lesser concentration of oxygen below the water table.

The Ore Minerals

Peacock Ore

Many young mineral collectors, drawn by the brilliant spectrum of colors on their surfaces, have collected specimens of either oxidized Chalcopyrite or Bornite (Figures 10 and 12); these specimens typically are labeled as “Peacock Ore” or ‘Peacock Copper”. The color stems from a thin film formed by the oxidation of the mineral surface. The colors are caused by an optical effect due to light waves reflected by both the underlying surface of the mineral and the surface of the film, which reinforce each other. Reinforcement of the waves depends on the thickness of the film and the wavelength of the light [Ref 8, Equation 4(41)]. In thinner film the reflected light tends to the blue end of the spectrum and in thicker film, to the red end of the visible light spectrum [Ref 8, Equation 4(41)]. The copper sulfides Chalcocite and Covellite can also demonstrate blue to red reflections as seen in Figures 5 and 6.

Reference 1.  https://www.geologyforinvestors.com/porphyry-largest-source-copper/

Reference 2. https://investingnews.com/daily/resource-investing/base-metals-investing/copper-investing/copper-production-country/

Reference 3. http://azgs.arizona.edu/minerals/king-copper

Reference 4. https://www.fcx.com/operations/north-america

Reference 5. http://www.asarco.com/about-us/

Reference 6. https://www.911metallurgist.com/blog/geology-of-porphyry-copper-deposits

Reference 7.  https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0ahUKEwiGrK2ouK7aAhVK7IMKHc9lDTsQFghAMAY&url=https%3A%2F%2Fofmpub.epa.gov%2Feims%2Feimscomm.getfile%3Fp_download_id%3D517008&usg=AOvVaw2hyme1RJiqvtz5uQ0dreVM

Reference 8. https://www.slideshare.net/hzharraz/topic-9-supergene-enrichment

Reference 9. http://pages.physics.cornell.edu/p510/w/images/p510/1/14/Ss14_heavens.pdf

Generally slab saws are heavy, so find a covered area indoors to place it, (or outside if you have to). You probably won’t be moving it unless yours has wheels. Because your saw uses oil as a coolant/lubricant take that into consideration when deciding where to place it. The area around the saw will be messy no matter what you do. A nearby source of electricity, good overhead lighting and a workbench next to the saw are essential for obtaining the best results.

Safety is number one when working with any type of saw. Your electric cord should be a grounded three prong plug, no bare wires showing.  Depending on the size of the blade your saw uses, 12”, 14”, 16”, 18” on up to 36”, your motor size will vary accordingly. From a ½ hp to 1 ¾ hp electric motor, so if you need to use a drop cord, the bare minimum to use is a SJTW 16awg cord, the older saw motors, unless they were upgraded, will use a lot of amps just turning them on. Sometimes a saw-cut can take an hour or better to complete so using a cord that is too small for the amperage will overheat the motor and wear it out prematurely. Never leave a running saw unattended, even if it has an automatic shut-off, the moment you walk away bad things tend to happen, maybe you didn’t secure that large rock in the saw vice well enough and it moves, binding that $400.00 blade in a running motor, so it’s best to stay within 10 feet or so to prevent expensive mishaps.

So now you want to cut one of those prize rocks you found last weekend, gotta know what’s inside right ! Your saw uses oil as a lubricant or should, using water is insufficent to lubricate and keep the blade cool during cutting, I recommend NOT using water. What I do recommend is using an appropriate oil available through slab saw dealers, which will give you many options to choose from. I use a Shell Oil product, Amber Neutral Oil 100 – available in 5 gallon containers. So how much oil will I need? Just enough to cover the diamonds on the rim of the blade, on my 18” that’s 3/8 of an inch. Adding more than what covers the rim will cause unnecessary drag making the motor work harder.

My saw is a 1960’s Great Western 18” which uses about 4 gallons of oil to cover the diamonds on the rim 3/8” and a combination hydraulic feed and 15 lb weight, to pull the rock through the blade. The speed at which the rock moves through the blade is important, slow is your friend, giving a smooth cut, too fast a feed and you get hop marks making more work for your finished piece.

Depending on what type of material you are cutting, mud build-up in the pan will signal when it’s time to drain the oil. Your motor may run slower, overheat and shut off due to mud building up in the bottom of the pan. This can happen to such an extent that the saw blades’ diamond rim are in the mud. Soft materials like Onyx, Serpentine, and Howlite are ground away as the rock passes through the saw blade, creating a lot of mud.

When it’s time for an oil change, look forward to getting filthy. At this time the entire machine interior should be cleaned. My ritual is to let the machine sit idle for at least 3 days after my last cut to let the mud settle out. I next use a 5 gallon bucket, with a wire strainer I made to fit inside, and a paper grocery bag resting on that. I then drain the oil into the bag. So now there is a layer of mud at the bottom of the pan and oil above that. I pull the drain plug and let the oil drain out, the mud is heavy enough that the oil will drain right over it – now I have reclaimed 75% of the oil leaving the mud and some oil. The fun begins when it’s time to push the mud through that drain hole and do the same process with the 5 gallon bucket strainer and grocery bag. It takes 3 separate buckets and strainers on my machine, as the oil remaining suspended in the mud will gravitate out into the bucket over several weeks time, retrieving a little more oil.

Opal as a Gemstone

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

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

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

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

Source of Color in Preciouse Opal

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

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

Sources of color in Fire Opal

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

Opals in Biology

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

How Opal Protects the Cell Walls of Grasses and Diatoms

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

In grasses

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

In diatoms

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

HOW GEMSTONE COLORS ARE PERCEIVED

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

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

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

Almandine Garnets

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

[metaslider id=”1476″]

Pyrope Garnets

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

[metaslider id=”1490″]

Spessartine Garnets

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

[metaslider id=”1502″]

Andradite Garnets

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

[metaslider id=1516]

Grossular Garnets

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

[metaslider id=”1533″]

Uvarovite Garnet

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

[metaslider id=”1534″]

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.