Charoite

Charoite (/tʃæroʊ.aɪt/ CHAR-ow-ait) is a rare silicate mineral with the chemical composition K(Ca,Na)
₂Si
₄O
₁₀(OH,F)•H
₂O, first described in 1978. It is named after the Chara 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
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) (same H-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
Ultraviolet fluorescence	Weakly fluorescent
Common impurities	Al, Fe, Mn, Sr, Ba
Other characteristics	Radioactive 10.69% (K)
References	[2][3][4][5]

Properties

Charoite is translucent lavender to purple in color with a pearly luster. Charoite is strictly massive in nature, and fractures are conchoidal. It has an unusual swirling, fibrous appearance, sometimes chatoyant, 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 per Gamma Ray American Petroleum Institute Units.^[3]

Inclusions mainly come in the swirly patterns of the mineral.^[8] The black spots on some specimens are either augite or aegirine, the latter occurring in almost all charoites since they commonly grow together. Larger round greenish spots are feldspar. Lighter colored stones or ones with translucent areas are likely due to canasite. The yellowish brown spots are tinaksite, 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]

Occurrences

It has been reported only from the Aldan Shield, Sakha Republic, Siberia, Russia. It is found where a syenite of the Murun Massif has intruded into and altered limestone deposits producing a potassium feldspar metasomatite,^[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 include aluminium, iron, manganese, strontium and barium.^[7] Charoitite is a rock like lapis lazuli, but unlike the latter one, charoitite mostly consists of the mineral charoite.^[9]

Gallery

• 
  Charoitite, a charoite-dominated potassic metasomatite rock from the type area. Photo is 5.7 cm wide.
• 
  Charoite postage stamp, 2000, from a series commemorating "300 Years of Mining and Geological Service in Russia."