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.
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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!
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| 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.
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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
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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
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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!
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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180-degree rotation of above photo. Width ~ 2.9 cm.
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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…”
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Stamp issued by USPS honoring George Washington Carver.
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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.
I was at the estate sale right on time and was able to
purchase several micromounts, especially perky boxes filled with gold, silver
and diamond crystals. I also acquired
Willards’s binocular microscope, one macro specimen and 36 specimen
labels printed by Lazard Cahn! For my perspective on Lazard Cahn see the Blog
Posting September 17, 2015. For
additional information on Willard’s life see his autobiography printed in March
2016 edition of the CSMS Newsletter (www.csms.us).
