| Chrysoberyl | |
|---|---|
 Cyclic trilling of chrysoberyl | |
| General | |
| Category | Oxide minerals |
| Formula (repeating unit) | BeAl2O4 |
| IMA symbol | Cbrl[1] |
| Strunz classification | 4.BA.05 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pbnm |
| Unit cell | a = 5.481 Å, b = 9.415 Å, c = 4.428 Å; Z = 4 |
| Identification | |
| Color | Various shades of green, emerald-green yellow, blue, brownish to greenish black, may be raspberry-red under incandescent light when chromian; colorless, pale shades of yellow, green, or red in transmitted light |
| Crystal habit | Crystals tabular or short prismatic, prominently striated |
| Twinning | Contact and penetration twins common, often repeated forming rosette structures |
| Cleavage | Distinct on {110}, imperfect on {010}, poor on {001} |
| Fracture | Conchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 8.5 |
| Luster | Vitreous |
| Streak | White |
| Specific gravity | 3.5–3.84 |
| Optical properties | Biaxial (+) |
| Refractive index | nα=1.745 nβ=1.748 nγ=1.754 |
| Pleochroism | X = red; Y = yellow-orange; Z = emerald-green |
| 2V angle | Measured: 70° |
| References | [2][3][4][5] |
| Major varieties | |
| Alexandrite | Color change; green to red |
| Cymophane | Chatoyant |
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4.[5][6] The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones, although they both contain beryllium. Chrysoberyl is the third-hardest frequently encountered natural gemstone and lies at 8.5 on the Mohs scale of mineral hardness, between corundum (9) and topaz (8).[7]
An interesting feature of its crystals are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" oriented at 120° to its neighbors and taking up 120° of the cyclic trilling. If only two of the three possible twin orientations are present, a V-shaped twin results.
Ordinary chrysoberyl is yellowish-green and transparent to translucent. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. The three main varieties of chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane, and alexandrite. Yellow-green chrysoberyl was referred to as "chrysolite" during the Victorian and Edwardian eras, which caused confusion since that name has also been used for the mineral olivine ("peridot" as a gemstone); that name is no longer used in the gemological nomenclature.
Alexandrite, a strongly pleochroic (trichroic) gem, will exhibit emerald green, red and orange-yellow colors depending on viewing direction in partially polarised light. However, its most distinctive property is that it also changes color in artificial (tungsten/halogen) light compared to daylight. The color change from red to green is due to strong absorption of light in a narrow yellow portion of the spectrum, while allowing large bands of more blue-green and red wavelengths to be transmitted. Which of these prevails to give the perceived hue depends on the spectral balance of the illumination. Fine-quality alexandrite has a green to bluish-green color in daylight (relatively blue illumination of high color temperature), changing to a red to purplish-red color in incandescent light (relatively yellow illumination).[8] However, fine-color material is extremely rare. Less-desirable stones may have daylight colors of yellowish-green and incandescent colors of brownish red.[8]
Cymophane is popularly known as "cat's eye". This variety exhibits pleasing chatoyancy or opalescence that reminds one of the eye of a cat.[9] When cut to produce a cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ "Mineralienatlas – Fossilienatlas". Archived from the original on 2 February 2017. Retrieved 20 January 2017.
- ^ Handbook of Mineralogy Archived 2011-12-15 at the Wayback Machine
- ^ "Chrysoberyl: Chrysoberyl mineral information and data". Archived from the original on 13 January 2017. Retrieved 20 January 2017.
- ^ a b Barthelmy, Dave. "Chrysoberyl Mineral Data". Archived from the original on 19 February 2017. Retrieved 20 January 2017.
- ^ Rudler, Frederick William (1911). . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 6 (11th ed.). Cambridge University Press. p. 320.
- ^ Klein, Cornelis; Cornelius S. Hurlbut Jr. (1985). Manual of Mineralogy (20th ed.). New York: Wiley. ISBN 0-471-80580-7.
- ^ a b "Alexandrite Quality Factors". Archived from the original on 18 December 2016. Retrieved 20 January 2017.
- ^ Rudler, Frederick William (1911). . Encyclopædia Britannica. Vol. 5 (11th ed.). p. 537.