| Charoite | |
|---|---|
 | |
| General | |
| Category | Silicate mineral |
| Formula | K(Ca,Na) 2Si 4O 10(OH,F)•H 2O |
| IMA symbol | Cha[1] |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | P21/m |
| Identification | |
| Color | Violet, lilac, light brown |
| Crystal habit | Fibrous, massive |
| Cleavage | Good in three directions |
| Fracture | Conchoidal |
| Mohs scale hardness | 5 - 6 |
| Luster | Vitreous to pearly |
| Streak | White |
| Diaphaneity | Translucent |
| Specific gravity | 2.54 - 2.58 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.550 nβ = 1.553 nγ = 1.559 |
| Birefringence | 0.009 |
| Ultravioletfluorescence | Weakly fluorescent |
| Common impurities | Al, Fe, Mn, Sr, Ba |
| Other characteristics |  Radioactive 10.69% (K) |
| References | [2][3][4][5] |
Charoite (/tʃæroʊ.aɪt/CHAR-ow-ait) is a raresilicate mineral with thechemical compositionK(Ca,Na)
2Si
4O
10(OH,F)•H
2O, first described in 1978. It is named after theChara River, despite its being 70 kilometres (43 mi) away from the discovery place.[6][3] When it was discovered, it was thought to be a fake, dyed purple to give it its striking appearance.[7]
Charoite is translucent lavender to purple in color with a pearly luster. Charoite is strictly massive in nature, and fractures areconchoidal. It has an unusual swirling, fibrous appearance, sometimeschatoyant, and that, along with its intense color, can lead many to believe at first that it is synthetic or enhanced artificially. Though reportedly discovered in the 1940s, it was not known to most of the world until its description in 1978. It is said to be opaque and unattractive when found in the field; a fact that may have contributed to its late recognition.[5] Charoite consists of oxygen (43.75%), silicon (27.65%) and calcium (17.53%) mainly, but its composition includes potassium (10.69%) - which gives it its radioactive properties - and hydrogen (0.39%) as well. It has a barely detectable, 0.65% radioactivity concentration perGamma Ray American Petroleum Institute Units.[3]
Inclusions mainly come in the swirly patterns of the mineral.[8] The black spots on some specimens are eitheraugite oraegirine, the latter occurring in almost all charoites since they commonly grow together. Larger round greenish spots arefeldspar. Lighter colored stones or ones with translucent areas are likely due tocanasite. The yellowish brown spots aretinaksite, which was discovered at the same time as charoite was. Charoite's silky, fibrous structure results in a shimmery looking polished stone. Sometimes, the very white flashes are tiny white feldspar.[7]Sugilites and charoites can be confused, as both are purple, and sugilite can have black and white inclusions, however sugilite lacks the swirling pattern that are present in charoite stones, and it also lacks the chatoyant effect.[8]
It has been reported only from theAldan Shield,Sakha Republic, Siberia, Russia. It is found where asyenite of theMurun Massif has intruded into and alteredlimestone deposits producing apotassium feldsparmetasomatite,[5] and forms between 200 - 250 °C.[3] Tinaksite, canasite and charoite are associated and black aegirine is also common where these three minerals grow. Common impurities includealuminium,iron,manganese,strontium andbarium.[7] Charoitite is a rock like lapis lazuli, but unlike the latter one, charoitite mostly consists of the mineral charoite.[9]
Gemmological classifications by E. Ya. Kievlenko (1980), updated | |||||||||
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