CHRYSOBERYL FROM THE BLACK HILLS, SOUTH DAKOTA

Science is fun. Science is curiosity. We all have natural curiosity. Science is a process of investigating. It's posing questions and coming up with a method. It's delving in.              Sally Ride

I was thinking, actually daydreaming, the other day about curiosity and my experiences as a 1-12 student.  Today I suppose it would read K-12 but my small community certainly did not have kindergarten in the system.  In fact, my friends and I heard rumors that kindergartners mostly took naps and drank milk with little learning.  At any rate, our science classes seemed to thrive with experiments and my curiosity increased each year.  Some of these experiments were repeated at home, along with several invented projects (some described in Posting August 18, 2018).  One project that I distinctly remember early in grade school (two grades in each room) was the teacher bringing out a prism and showing us white light was actually composed of several colors.  WOW. We examined these different colors and then brought out our box of crayons (mine was 16 colors and I was jealous of those students with 32 or 64) and duplicated the prism colors.  The class then learned about rainbows and waited for a Kansas rain!  That Christmas I asked Santa for a prism. 

Thanks to http://www.theozonehole.com/ for allowing use of this prism diagram.

In composing this post I was reminded of the prism as readers will notice below. I also thought about curiosity and how I still have that inner-gut feeling today.  This post started out to satisfy my curiosity about chrysoberyl, a gemstone I knew little about.  But it was like painting one wall of a room.  You soon decide that the entire room needs the paint, and then the ceiling, and then….!  Well, the same for this post---I described the gemstone, then found the sample from South Dakota, and then tried to explore how chrysoberyl formed in so many environments.   And so, this submission goes on and on and may begin to bore readers as I try to understand some complex (at least for me) geochemistry.  But, feel free to drop off the reading anytime!

The oxides are a group of minerals where oxygen (the negatively charged anion) combines with one, or more, cations (positively charged metals or semi-metals or things that act like metals).  We often think of simple oxides where the oxygen combines with a single cation—something like cuprite with copper and oxygen, Cu₂O, or maybe quartz with silicon and oxygen, SiO₂.  However, I suppose many/most mineralogists would argue that quartz is really a silicate rather than an oxide (another story for later)!  The compound oxides have two cations combined with an oxygen such as one of my favorite gemstones, spinel-MgAl₂O₄ with aluminum and magnesium plus oxygen.  Most classification schemes also include the hydroxides into the oxides.  Here a very tiny positive hydrogen (+) combines with a much larger negative oxygen (- -) and the resulting negative radical (OH^-) is about the same size as the oxygen and can fit into spaces previously occupied by just oxygen.  One of the better hydroxide examples is goethite [FeO(OH)], a hydrated iron oxide prized by Mr. Rockhounding the Rockies, the well-known amazonite hunter here in the Pikes Peak region.  Many of the oxides are quite hard (corundum), important ores (chromite; chrome, cassiterite; tin) and gemstones (ruby, sapphire, spinel).  Most oxides are accessory minerals in igneous and metamorphic rocks; however, hardness and durability allows many of these oxides to last through the weathering cycle and become detrital minerals in unconsolidated sediments (ruby is a good example) or in consolidated sedimentary rocks (such as magnetite, rutile and ilmenite). Some oxides are simply products of surficial weathering such as goethite (lateritic soils) or hematite precipitating from water (rust).

Oxygen readily combines with hydrogen (H₂O) and the result is water, a solution that helps in weathering and decomposition of minerals and rocks.  Water commonly takes on additional dissolved oxygen and it is this "secondary" oxygen (not the water oxygen) that helps create the oxide minerals.  In metallic ore deposits geologists hunt for the oxidized zone since minerals in this zone (commonly even metallic ores like azurite, copper carbonate hydroxide, are thrown together with all minerals in the "oxide zone")  are easier to mine and refine than the sulfides in the underlying primary zones.  This oxide zone, as compared to the primary sulfides, is situated closer to the surface since more oxygen is available.

But certainly not all oxide minerals are formed in the oxidized zones near the earth’s surface.  Corundum (Al₂O₃), for example (ruby and sapphire are gem varieties), is formed in high temperature igneous and metamorphic rocks where the magma (igneous) or parent rock (metamorphic) is deficient in, or under-saturated with, silica but is rich in aluminum. Seyenite (similar to granite but with a low quartz content, <5%) and nephaline seyenite (devoid of quartz with nephaline and feldspar) are igneous rocks that are under-saturated with silica and sometimes produce corundum.  If the sedimentary rock limestone is subjected to high heat and high pressure (metamorphism) corundum may be formed.  For example, I have seen (in museums) gorgeous red rubies perched on white marble (metamorphosed limestone).  Check the February 10, 2015 Posting for a less spectacular ruby! 

A gemmy chrysoberyl specimen acquired from Espirito Santo, Brazil. The crystal show several twins. Length of specimen ~1.6 cm.

One oxide mineral that is of interest to me is chrysoberyl, a beryllium, aluminum oxide [BeAl₂O₄] that is a relatively unknown mineral to most readers “but is a surprisingly widespread mineral, particularly in a historical context” (Cook, 2010).  If readers are familiar with the mineral it is likely due to the gem varieties known as alexandrite and cat’s-eye. I have a nice “normal” specimen of chrysoberyl (see photo above) collected from Espirito Santo, one of Brazil’s famous localities. It is a typical mostly yellow in color and transparent to translucent.  However, other crystals of chrysoberyl are different shades of green or yellow or brown or white and even blue, and some non-gem varieties are more opaque than translucent. 

The yellow color probably comes from trace amounts of iron while the green may be due to chromium and or even vanadium (Schmetzer and others, 2016).  Crystals are Orthorhombic; however, many/most specimens are twinned (sometimes multiple twinning) and those crystals seem blocky and/or distorted.  Other crystals are platy. Collectors like to focus on “trillings” where there are three twins each taking 120 degrees of space and producing a pseudohexagonal crystal. Chrysoberyl has a vitreous luster, especially the gemmy varieties whereas less-gemmy forms may be subvitreous (or at least “duller”).  The mineral has a fairly high refractive index or brilliance (more light is bent and reflected back up through the crown of a faceted stone rather than passing through) ranking slightly below ruby and emerald.  However, one of the major distinguishing properties of chrysoberyl is its hardness, coming in at ~8.5 (Mohs) and third in hardness among gemstones behind diamond and tourmaline.  Therefore, gemmy specimens are valued for use as gemstones, especially rings. This hardness and durability also allows chrysoberyl to weather out of the host rock and show up in sands and gravels---Sri Lanka produces a number of gems gathered as rounded pebbles from local streams and then worked into jewelry stones. 

A twinned chrysoberyl crystal known as a trilling (7.33 mm).  Collected from Minas Gerais, Brazil.  The file is posted on Wikimedia Commons and has been approved for free distribution and use by the author, Matteo Chinellato.  Originally published at: http://www.mindat.org/photo-282796.html

A v-twinned chrysoberyl floater from Espirito Santo, Brazil (2.7 x 2.3 x 1.0 cm).  Compare with my specimen shown above.  The file is posted on Wikimedia Commons and has been approved for free distribution and use by the author, Rob Lavinsky/www.irocks.com.  Image published at: http://www.mindat.org/photo-266069.html

The rarest and most expensive chrysoberyl gems are a variety known as alexandrite.  When this stone is viewed under normal daylight it appears green while under incandescent tungsten light the gem appears red.  Why?  Here is what I understand about the situation, and remember I am not a physicist!  The easiest way to understand this property is to think of light bulbs as their ratings are color correlated!  There is a major difference between color temperatures of light emitted from bulbs and these colors are measured on the Kelvin Scale.  Natural sunlight is composed of most or all colors of the spectrum and is measured at ~5250+ Kelvin and is rated as a "cool light" on the bulbs.  For a confusing lesson on lights go to the hardware store, check out the multitude of light bulbs, and read the labels.  The higher the Kelvin rating, the brighter the emitted light seems.  I use a variety of sunlight-rated bulbs (high Kelvin) in my basement office (devoid of natural sunlight).  Some people prefer a "warm light" that generally is low on the Kelvin scale.  For example, halogen bulbs are ~ 3200K (mid-range) while incandescent bulbs (the ones being phased out) are ~ 2600K.  These incandescent bulbs are closer to the low end of the scale and tend to turn out a yellow light hue, the cozy and inviting warm lights preferred by many vs. the “harsh” 5250K “sunlight” bulbs.  As a kid I always wondered why my Kodak slides and photos had a yellow tint when taken in a home under “normal” (incandescent) light.  Flash bulbs (in the old days) or electronic strobe lights emitted a much brighter white light; hence, the yellow tint was gone.  What all this means is that different light sources have different contributions of light wavelengths.  If you want to see the visible colors get out a prism and stick it in the sun! Light is measured in wavelengths defined by nanometers (one billionth of meter) and visible light ranges from ~380 nanometers (nm) to 740 nm.

 I appreciate Georgia State University (www.hyperphysics.phy-astr.gsu.edu/hbase/vision/specol.html) allowing the use of this color spectrum. As for the space below, Blogger will not allow it to disappear.

Alexandrite seems to have chromium (according to most authors) as the chromophore (less than 1% replacing part of the aluminum) and in sunlight, or high Kelvin light, atoms of that element absorb the blue (~400 nm) and yellow portions (~600 nm) of the light leaving behind the blue-green and green (470-520 nm) and red (650 nm and above) portions of the spectrum.  However, natural sunlight produces a higher concentration of green light than red so the gem appears green when viewed in such light.  Incandescent bulbs produce more red light of the color spectrum and so chrysoberyl appears red under many household lights.  But, with all of the new LED, halogen and CFL bulbs, often intermixed in a building, it is hard telling what color the gem might show.  With the price of color-change alexandrite gems, I suppose this question will not be answered by my sleuthing! That is about all I can do to explain color change in alexandrite, something that is way above my pay grade! 

Step Cut Alexandrite Cushion, 26.75 cts. Bluish-green in daylight, red in incandescent light.  Thanks to photographer David Weinberg for posting on Alexandrite.net and releasing to the public domain.

 By-the-way, natural color-change alexandrite is rare.  It was first “discovered” in the Ural Mountains of Russia and named for Czar Alexander II (an excellent way to curry favor with the boss).  However, it is my understanding that the Russian mines have played out and most stones today come from Brazil with minor sources in Zimbabwe, Tanzania, Sri Lanka and perhaps a few other localities.  However, the major source of alexandrite is Russia, not from the mines but from laboratories.  Most alexandrite gems on the market today are synthetic, especially stones seen on jewelry television and internet sites.  If you are looking to purchase a natural stone, “know your dealer.”

Another, completely different, gemstone cut from chrysoberyl is cat’s-eye.  In the gemstone trade there are a number of different minerals that exhibit the cat’s-eye effect such as tiger eye, moonstone, corundum, spinel, scapolite, apatite, aquamarine, sillimanite and quartz.  However, only the stones cut from chrysoberyl are technically termed cat’s-eye while all others must have mineral or varietal names as modifiers, such as scapolite cat’s-eye.  Whatever the mineral, the gems are mostly cut as cabochons for the cat’s-eye to appear.  Evidently the cat's-eye minerals contain a very dense network of parallel fibers such as rutile, maybe titanium, or very thin tubes running parallel to the C-Axis. Light entering the stone then reflects from the dome of the cabochon at right angles to the parallel fibers or tubes.  If cut correctly the milky white line of the cat’s-eye will be at the center of the cabochon dome appearing to pass through the entire crystal as a single ray of light. The best cat’s-eyes also are chatoyant having a haziness derived from the scattering of inner light rays. The “old” name for this type of gem is cymophane.  Most cat’s-eye rough stones seem to come from Brazil or Sri Lanka; the latter stones are usually found as rounded pebbles screened from alluvial gravels.

Chrysoberyl cut en cabochon showing the “cat’s--eye line as well as chatoyancy. Thanks to photographer David Weinberg for posting on Alexandrite.net and releasing to the public domain.

As I understand the situation, chrysoberyl is found in pegmatites (Brazil) or high grade metamorphic rocks (Sri Lanka) or perhaps volcanic rocks (Australia).  In pegmatites chrysoberyl forms directly from the magma, or in a zone where the magma is penetrating aluminum-rich rocks.  At any rate, the magma or the metamorphic host rocks must be enriched with beryllium and to a lesser degree, aluminum. It also is interesting that magma rich in beryllium could produce either beryl [Be₃Al₂(Si₆O₁₈) or chrysoberyl. In some localities either beryl or chrysoberyl is dominant; however, in other cases both are common constituents.   Why?  That is one of those persistent questions of life!  I do know that beryl has a high ratio of beryllium to aluminum whereas chrysoberyl has a low ratio. For the quite rare variety alexandrite to form, the magma must intrude surrounding rocks with available chromium.  Franz and Morteani (1982) studied metamorphosed pegmatites in Sweden and the Czech Republic and concluded that “chrysoberyl is always accompanied by quartz, and is a breakdown product of primary pegmatitic beryl… and that the formation of Al-rich minerals like chrysoberyl and sillimanite in pegmatites is due to a post-pegmatitic event at high P[ressure]—T[emperature] conditions.”  Is this the case for all metamorphic, chrysoberyl?  I don’t know but I would like to find the answer.

Specimen of chrysoberyl, in a quartz matrix along with accessory muscovite, collected from Scott’s Rose Quartz Mine in the Black Hills of South Dakota.  Width of specimen 5.4. cm.

The second half of this story concerns a small rock picked up decades ago (in the late 1960s) from a mine in the Black Hills of South Dakota—the Scott Rose Quartz Mine near Custer. It has been stuck away all these years in my South Dakota collection labeled “unknown, maybe quartz but too hard.”  Recently I was sorting through my Black Hills collection and shelving and shuffling specimens around.  At the same time, I was writing this little post on chrysoberyl and was examining mineral photos on MinDat when whoa, that photo looks like my specimen labeled unknown.  So, I pulled it out and was able to identify the specimen as chrysoberyl.

The reason, well maybe my excuse, as to why it was never identified in the past decades is that the South Dakota specimen does not appear as expected (to me).  The only chrysoberyl that I have seen is the more typical cat’s-eye and the faceted green or yellow-green variety.  And, I never expected chrysoberyl to appear in the South Dakota pegmatites.  In retrospect, I missed reading about the mineral in Roberts and Rapp (1965) although I have perused the volume many times.  My mind simply had established a picture of typical chrysoberyl: cat’s-eye, green or yellow-green faceted stones or twinned crystals.

 

Photomicrograph of vitreous, and hard, mass of chrysoberyl crystals from South Dakota. I presume twinning is present but the jumbled mass sort of defies that identification.  Width of photo ~ 8 mm.

Chrysoberyl crystals of a very light yellow-green color, the upper center crystals are more white due to light reflection from a crystal face.  Q is the quartz matrix while M are tiny flakes of muscovite.  Width of photo ~ 2.2 cm.

180-degree rotation of above photo.  Width ~ 2.9 cm.

Roberts and Rapp (1965) described the Scott’s Quarry chrysoberyl as “intergrown crystal masses weighing several hundred pound, associated with beryl, muscovite and quartz…greenish gray twinned crystals…and fine striated yellowish to yellow-green crystals have been collected…”  MinDat described their single photographed specimen as “radiating clusters of elongate chrysoberyl crystals…partially hidden in the iron-stained quartz matrix…”  My specimen is a mixture of sort-of-yellow massive chrysoberyl crystals, white to clear muscovite, and quartz.  Again, the most distinguishing property is the extreme hardness of the chrysoberyl. MinDat also noted other minor occurrences of chrysoberyl collected in the following mines in the Black Hills:

Custer County

         Elk No. 1 Mine

Rock Ridge Pegmatite

Roosevelt Group (Rough Rider)

Victory Mine

Rocky Ridge Mine

Mountain Rose Mine

Pennington County

          Peerless Mine

In conclusion, the more I delved into chrysoberyl, the more questions popped up in my mind about its formation.  I believe/think that: 1) chrysoberyl is a fairly uncommon mineral, especially the gem varieties; 2) primary chrysoberyl is formed in pegmatites crystallizing from the magmatic melt and “is usually restricted to high-T[emperature] pegmatites with low to moderate degree of fractionation” (Cerny, 2002); 3) other chrysoberyl crystallizes in some sort of a reaction zone when the magma invades host rocks rich in aluminum; 4) some chrysoberyl is formed during very high grade metamorphism; 5) in metamorphosed pegmatites chrysoberyl may be “a breakdown product of primary pegmatitic beryl… and that the formation of Al-rich minerals like chrysoberyl and sillimanite in pegmatites is due to a post-pegmatitic event at high P[ressure]—T[emperature] conditions” (Franz and Morteani, 1982); 6) some chrysoberyl may form in volcanic rocks (Schmetzer and others, 2016); 7) beryl occasionally is associated with chrysoberyl in beryllium-rich pegmatites (Sardi and others, 2016), something that seems in contrast with the South Dakota chrysoberyl; 8) in trying to answer my question about crystallization of beryl or chrysoberyl, Sardi and others (2016) noted  that “calculated crystallization temperatures for the beryl-bearing sample are somewhat higher than those obtained for some other beryl-bearing granitoids that contain more complex mineral assemblages, including chrysoberyl”; and 9) Merino and others, (2013) found that “the interplay between the silica and alumina activities likely controls the stabilization and the preferential crystallization of gahnite [zinc oxide] + chrysoberyl or beryl + chrysoberyl assemblages…”

Stamp issued by USPS honoring George Washington Carver.

As for science and curiosity, as a kid I was always fascinated by the work of George Washington Carver and was curious about how he succeeded.  Why?  Well remember that I grew up in the 1950s in a very small “white” community where the nearest large city had a segregated grade school in the early part of the decade.  But, one week we had a grade school history and science lesson about the famous African American plant scientist George Washington Carver (born into slavery) who spent part of his teen years after the War in Minneapolis, Kansas (our county seat).  As a science experiment we then grew some peanuts. As a history lesson we learned how cotton had destroyed much of the fertile land in many Southern states and how Carver had suggested farmers rotate crops and plant peanuts.  He also developed many useful products from peanuts.  I thought something like “well, if an African American person from the county seat could be a scientist so could a small town white kid” and suggested to my parents that we plant some peanuts in the garden this summer so I could do some experiments.  My father thought the better idea was squash, tomatoes, beans, peas, etc., the standard fare for gardens in central Kansas.  Later as an adult I did plant peanuts but was fairly unsuccessful in roasting them.  However, I did maintain my curiosity about science. 

REFERENCES CITED

Černý, P., 2002, Mineralogy of beryllium in granitic pegmatites, in E.S. Grew, Ed., Beryllium: Mineralogy, Petrology, and Geochemistry: Reviews in Mineralogy, v. 50.

Cook, R.B., 1999, Connoisseur's Choice: Chrysoberyl, Minas Gerais and Bahia, Brazil, and Takovaya, Russia: Rocks & Minerals, v.74, no. 5.

Franz,G., and G. Morteani, 1984, The formation of chrysoberyl in metamorphosed pegmatites; Journal of petrology, v. 25, issue 1.

Merino, E., C. Villaseca, D. Orejana, and T. Jeffries, 2013, Gahnite, chrysoberyl and beryl co-occurrence as accessory minerals in a highly evolved peraluminous pluton: The Belvís de Monroy leucogranite (Cáceres, Spain): Lithos, v. 179.

Roberts, W.L. and Rapp, G. Jr., 1965, Mineralogy of the Black Hills; South Dakota School of Mines and Technology, Bull. 18.

Sardi, F., A. Heimann, and P. Grosse, 2016, Non-pegmatitic beryl related to Carboniferous granitic magmatism, Velasco Range, Pampean Province, NW Argentina: Andean Geology, v.43, no. 1. 

Schmetzer, K., F. Caucia, H.A. Gilg, and T.S. Coldham, 2016, Chrysoberyl from the New England Placer Deposits, New South Wales, Australia: Gems & Gemology, v. 52, no. 1.