Post subject: Does alexandrite's formation is an extremely rare phenomenon
Posted: Sun Mar 20, 2022 1:59 am
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Joined: Sat Apr 11, 2020 12:15 am Posts: 13
This article :https://agta.org/education/gemstones/alexandrite/ says that :
Quote:
the geographic conditions that produce alexandrite are so unusual, experts generally agree that future discoveries are unlikely.
I think all Al2BeO4 and Cr and V are common elements in earth. I know that Cr usually cause gems to contain a lot of imperfection, but all element in alexandrite is not a rare earth element. Ruby compose of Al2O3 and Cr, the chemical composition is almost the same as alexandrite. What kind of geographical condition is required to produce alexandrite? Is there any specific reason why experts think otherwise?
A beryllium-based gem, distinct from the beryls, is chrysoberyl, which has the general formula BeAl2O4. It is generally a golden-yellow, to a greenish- or brownish-yellow color. The color is due to iron cations (Fe3+) substituting for aluminum in the crystal structure. The bulk of gem chrysoberyl available in recent years has come from alluvial deposits in the Bahia, Espírito Santo and Minas Gerais states of Brazil.
The two most valued varieties of chrysoberyl are alexandrite and cymophane. Alexandrite ranges in color from greenish blue in natural light to deep red under incandescent light. The effect is from small amounts of chromium substituting for aluminum in the crystal structure. Cymophane is translucent chatoyant or “cat’s eye” chrysoberyl. In this variety, needlelike inclusions of rutile produce an effect visible as a bright band of light that moves across the stone as it is rotated. This effect is best seen in gemstones cut in cabochon form. Cat’s eye material is found as a small percentage of the overall chrysoberyl production.
There has been much debate about the origin of chrysoberyl deposits. Most are associated in some way with pegmatites, but in many cases the stone is associated with aluminum-rich minerals absent in most pegmatites. Some studies have concluded that under conditions of high temperature and pressure, the assemblage of beryl and aluminum-silicate is unstable and decomposes to the assemblage of chrysoberyl and quartz.
Chrysoberyl was discovered in association with emerald and phenakite (another beryllium-silica mineral) at Franqueira in northwestern Spain in 1968–1969. The chrysoberyl is often cyclic twinned (nonparallel crystals share some of their crystal-lattice points), and many crystals show the alexandrite effect of color shifting under different types of light. The beryllium minerals occur in zones rich in phlogopite (a silicate mineral) in a dunite (an ultramafic rock) intruded by a pegmatite associated with a type of granite that is high in aluminum oxide and potassium. The chrysoberyl could have formed by the buildup of thin layers on existing olivine crystals (a magnesium-iron silicate mineral), because the two minerals share features in their crystal structures.
Study of additional samples resulted in the conclusion that chrysoberyl formed from the breakdown of another mineral called sapphirine (a silicate of magnesium and aluminum with a high beryllium content, named for its color being similar to sapphire) during metamorphism after it was formed.
Textural and compositional evidence suggests that chrysoberyl formed during the regional metamorphism of granulite facies (medium or coarse-grained metamorphic rock bodies that underwent intense pressure and temperature changes), a process that could have released beryllium from the host sedimentary rock.
Post subject: Re: Does alexandrite's formation is an extremely rare phenom
Posted: Sun Mar 20, 2022 4:59 pm
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Joined: Mon Apr 16, 2018 7:25 am Posts: 38
Very basically, it's because beryllium is found in very different environments (granites, pegmatites) to chromium (ultrabasic rocks). You need the right set of circumstances to bring the two together, and those circumstances don't happen often.
Post subject: Re: Does alexandrite's formation is an extremely rare phenom
Posted: Thu Apr 14, 2022 9:43 am
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Joined: Fri Jun 25, 2010 12:04 am Posts: 257 Location: Idaho
Depends on who you ask - I don't remember the term "ultrabasic" even coming up in my geological education... Maybe I wasn't listening those times.
Designator for ultrabasic (now) is the silica content (under 45%), while ultramafic is a subset that has low silica plus high in iron and magnesium, leading to a dark color.
Post subject: Re: Does alexandrite's formation is an extremely rare phenom
Posted: Thu Apr 14, 2022 2:11 pm
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Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Mindat wrote:
ULTRABASIC Said of an igneous rock having a silica content lower than that of a basic rock. Percentage limitations are arbitrary; the upper limit was originally set at 44%. The term is frequently used interchangeably with ultramafic. Although most ultrabasic rocks are also ultramafic, there are some exceptions; e.g., monomineralic rocks composed of pyroxenes are ultramafic but are not ultrabasic because of their high silica content. A monomineralic rock composed of anorthite would be considered ultrabasic (SiO2 = 43.2%) but not ultramafic. Ultrabasic is one subdivision of a widely used system for classifying igneous rocks on the basis of silica content; the other subdivisions are acidic, basic, and intermediate.
Post subject: Re: Does alexandrite's formation is an extremely rare phenom
Posted: Mon May 02, 2022 6:49 am
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Joined: Sat Feb 19, 2022 9:52 am Posts: 17
Nick_G wrote:
Very basically, it's because beryllium is found in very different environments (granites, pegmatites) to chromium (ultrabasic rocks). You need the right set of circumstances to bring the two together, and those circumstances don't happen often.
Now I am curious on this topic. Emerald contains both beryllium and chromium right? and we have an abundant sources and reserve of emerald in the ground. At the very least, emerald is a lot more abundant than alexandrite do I miss something here?
You're correct, it is one part of the puzzle but not all of it. A further wrinkle is that chrysoberyl in general does not form as commonly as beryl--large beryls form in countless locales after all, while chrysoberyl of any kind is much more restricted.
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