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Chrysoberyl

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Chrysoberyl is a member of oxide mineral or gemstone with the formula: BeAl2O4.In spite of the similarity in their names, chrysoberyl and beryl are absolutely extraordinary stones, despite the fact that both include beryllium. Chrysoberyl is the third most commonplace herbal stone and is found at 8.5 at the Mohs mineral hardness scale among corundum and topaz. Chrysoberyl is typically yellow, green, or brown in color. It forms tabular or short prismatic crystals and heart-shaped or pseudohexagonal twinned crystals. Alexandrite, one of its gemstone varieties, is one of the rarest and most expensive gems. Another variety, cat’s eye, is also prized as a gemstone. It contains parallel fibrous crystals of other minerals that reflect light across the surface of a polished gemstone an effect known as chatoyancy. Chrysoberyl occurs in some granite pegmatites, gneisses, mica schists, and marbles. Crystals that weather out of the parent rock are often found in streams and gravel beds.

Name: From the Greek for golden, in allusion to the mineral’s color, and beryl

Association: Quartz, muscovite, albite, beryl, columbite, tourmaline, topaz, kyanite, staurolite (granite pegmatites); phenakite, apatite, tourmaline, fluorite (reaction zone pegmatites).

Crystallography: Orthorhombic; dipyramidal. Usually in crystals tabular parallel to macropinacoid, the faces of which are vertically striated. Commonly twinned, giving pseudohexagonal appearance

Chrysoberyl Composition: Beryllium aluminate, BeAhCh. BeO = 19.8 per cent, AI2O3 = 80.2 per cent. Be = 7.1 per cent.

Diagnostic Features: Characterized by its extreme hardness, its yellowish to emerald-green color, and its twin crystals.

Major varieties: Alexandrite, Cymophane

Chrysoberyl Uses: Serves as a gem stone. The ordinary yellowish green stones are inexpensive; the varieties alexandrite and cat’s-eye are of considerable value.

Alexandrite

Photographed by David Weinberg for Alexandrite.net and released to the public domain. Alexandrite.net contributors. Step Cut Alexandrite Cushion, 26.75 cts. In Alexandrite.net, Tsarstone collectors guide. December 07, 2006, 16:42 UTC. Available at: http://www.alexandrite.net/viewpage.html?id=ALX-001-00001. Accessed February 26, 2007.
Alexandrite 

The variety of Alexandrite shows a color trade (alexandrite impact) depending on the character of ambient lighting. The Alexandrite impact is the trade of color observed from a change in supply illumination to a reddish colour. Alexandrite is because of small-scale change of chromium ions inside the crystal shape, which causes extreme absorption of mild in a slim wavelength range within the yellow location of the visible mild spectrum (580 nm). Due to the fact human vision is the most touchy to inexperienced mild and least sensitive to red, alexandrite appears greenish in sunlight hours with full spectrum of visible mild and reddish in incandescent mild emitting less inexperienced and blue spectrum. This shade change is unbiased of any color exchange with a crystal point of view because of pleochroism.

Cymophane

Cat's Eye Chrysoberyl
Cat’s Eye Chrysoberyl

Translucent yellowish chatoyant chrysoberyl is called cymophane or cat’s eye. Cymophane has its derivation also from the Greek phrases that means ‘wave’ and ‘appearance’, in connection with the haziness that visually distorts what might normally be regarded as a nicely defined surface of a cabochon. This impact can be mixed with a cat eye effect. In this variety, microscopic tubelike cavities or needle-like inclusions of rutile arise in an orientation parallel to the c-axis, generating a chatoyant effect seen as a unmarried ray of mild passing throughout the crystal. This effect is quality visible in gems cut in cabochon form perpendicular to the c-axis. The colour in yellow chrysoberyl is because of Fe3+ impurities.

Even though other minerals together with tourmaline, scapolite, corundum, spinel and quartz can form “cat’s eye” stones comparable in appearance to cymophane, the jewellery enterprise designates these stones as “quartz cat’s eyes”, or “ruby cat’s eyes” and handiest chrysoberyl can be called “cat’s eye” without a different designation.

Gemstones missing the silky inclusions required to produce the cat’s eye effect are generally faceted. An alexandrite cat’s eye is a changes colour. “Milk and honey” is a time period typically used to explain the coloration of the best cat’s eyes. The impact refers to the pointy milky ray of white light generally crossing the cabochon as a center line alongside its duration and overlying the honey-colored background. The honey coloration is taken into consideration to be pinnacle-grade with the aid of many gemologists but the lemon yellow colorings also are popular and appealing. Cat’s eye material is determined as a small percentage of the overall chrysoberyl manufacturing anyplace chrysoberyl is observed.

Chemical Properties

Chemical Classification Oxide minerals
Formula BeAl2O4
Common Impurities Fe,Cr,Ti

Chrysoberyl Physical Properties

Crystal habit Crystals tabular or short prismatic, prominently striated
Color Green shades, emerald-green, greenish white, yellowish green, greenish brown, yellow, blue
Streak White
Luster Vitreous
Cleavage Distinct/Good Distinct on {110}, imperfect on {010}, poor on (001)
Diaphaneity Transparent, Translucent
Mohs Hardness 8,5
Crystal System Orthorhombic
Tenacity Brittle
Density 3.75(1) g/cm3 (Measured)    3.69 g/cm3 (Calculated)
Fracture Irregular/Uneven, Conchoidal

Chrysoberyl Optical Properties

Color / Pleochroism Visible X = c = columbine-red Y = b = orange-yellow Z = a = emerald-green
2V: Measured: 70° , Calculated: 72°
RI values: nα = 1.746 nβ = 1.748 nγ = 1.756
Twinning Contact and penetration twins common, often repeated forming rosette structures
Optic Sign Biaxial (+)
Birefringence δ = 0.010
Relief High
Dispersion: r > v

Occurrence of Chrysoberyl

Chrysoberyl shapes due to pegmatitic processes. Liquefying within the Earth’s outdoor layer offers fairly low-thickness liquid magma that can upward thrust upwards toward the surface. As the precept magma frame cools, water first of all present in low fixations turned out to be gradually moved in the liquid shake since it could not be fused into the crystallization of sturdy minerals. The remainder magma along these lines ends up greater extravagant in water, and furthermore in uncommon additives that comparably do not healthy inside the precious stone structures of widespread shake shaping minerals. The water expands the temperature enlarge downwards earlier than the magma seems to be completely sturdy, permitting centralization of unusual additives to hold so far that they produce their very personal unmistakable minerals. the following shake is molten in appearance but framed at a low temperature from a water-wealthy liquefy, with big gemstones of the regular minerals, as an example, quartz and feldspar, but in addition with raised centralizations of unusual additives, as an instance, beryllium, lithium, or niobium, regularly shaping their very own minerals; this is referred to as a pegmatite. The high water substance of the magma made it feasible for the gem stones to expand swiftly, so pegmatite treasured stones are often very large, which improves the opportunity of diamond examples shaping.

Chrysoberyl can likewise develop in the state shakes near pegmatites, while Be-and Al-rich beverages from the pegmatite reply with encompassing minerals. Therefore, it tends to be found in mica schists and in contact with transformative shops of dolomitic marble. On the grounds that it is a tough, thick mineral this is impervious to artificial trade, it very well may be persevered out of rocks and kept in movement sands and rock in alluvial shops with other pearl minerals, as an instance, precious stone, corundum, topaz, spinel, garnet, and tourmaline. at the factor whilst located in such placers, it will have adjusted edges as opposed to sharp, wedge-shape frames. A massive a part of the chrysoberyl mined in Brazil and Sri Lanka is recuperated from placers, because the host rocks were severely persisted and disintegrated.

In the occasion that the pegmatite liquid is wealthy in beryllium, gems of beryl or chrysoberyl may want to shape. Beryl has a high percentage of beryllium to aluminum, even as the inverse is legitimate for chrysoberyl. Each are consistent with the everyday mineral quartz. For alexandrite to form, a few chromium would likewise have need to be available. Be that as it can, beryllium and chromium do not will in widespread take place in comparable sorts of shake. Chromium is commonest in mafic and ultramafic shakes in which beryllium may be very unusual. Beryllium winds up accumulated in felsic pegmatites in which chromium is practically lacking. Thus, the principle circumstance wherein an alexandrite can expand is when Be-wealthy pegmatitic beverages respond with Cr-wealthy nation shake. This odd prerequisite clarifies the uncommonness of this chrysoberyl collection.

Distribution

Widespread, however fine crystals are uncommon. Notable localities include:

  • From many places in Brazil, with exceptional crystals from Tancredo, Itagua¸cu, and Colatina, Espirito Santo; from Faria Lemos, Santa Luzia de Carangola, and Americana, Te´ofilo Otoni, Minas Gerais; at Campo Formoso, Teixeira de Freitas, and Cachoeira, Bahia.
  • From Marsıkov, Czech Republic.
  • At the Izumrudnye mines, Tokovaya River, near Yekaterinburg (Sverdlovsk), and Mursinka, Ural Mountains, Russia.
  • From Miakanjovato, near Lake Alaotra, northeast of Ambatosoratra, Madagascar.
  • Near Masvingo (Ft. Victoria), Zimbabwe. Abundant in gem gravel placers in the Ratnapura district, Sri Lanka.
  • In the USA, from near Golden, Jefferson Co., Colorado; in Maine, at Topsham, Sagadahoc Co., from Paris, Norway, and Hartford, Oxford Co., and elsewhere.
Cite this article as: Mahmut Mat, "Chrysoberyl", in Geology Science, [online] Accessed 18th July 2019, Available at: http://geologyscience.com/minerals/chrysoberyl/

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Dana, J. D. (1864). Manual of Mineralogy… Wiley.
  • Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
  • Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
  • Wikipedia contributors. (2018, October 12). Chrysoberyl. In Wikipedia, The Free Encyclopedia. Retrieved 02:05, July 8, 2019, from https://en.wikipedia.org/w/index.php?title=Chrysoberyl&oldid=863702417
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