Chrysothallite

Chrysothallite is a rare thallium-bearing chloride mineral with the formula K₆Cu₆Tl³⁺Cl₁₇(OH)₄•H₂O.^[3][4] Chrysothallite is unique in being only the second mineral with essential trivalent thallium,^[3] a feature shared with natural thallium(III) oxide, avicennite.^[5] Another examples of natural thallium chlorides are steropesite, Tl₃BiCl₆, and lafossaite, TlCl.^[6][7] Chrysothallite is one of numerous fumarolic minerals discovered among fumarolic sites of the Tolbachik volcano, Kamchatka, Russia^[8] The mineral is named in allusion to its colour and thallium content.^[3]

Chrysothallite
General
Category	Halide mineral
Formula	K6Cu6Tl3+Cl17(OH)4•H2O
IMA symbol	Cry[1]
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	I4/mmm
Unit cell	a = 11.37 Å, b = 26.21 Å (approximated); Z = 4
Identification
Color	Golden-yellow to light yellow
Crystal habit	tabular
Cleavage	None
Fracture	Uneven
Tenacity	Brittle
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Density	2.95 (measured)
Optical properties	Uniaxial (+)
Refractive index	ω=1.72, ε=1.73 (approximated)
References	[2][3][4]

Notes on chemistry

Chrysothallite contains a relative high amount of zinc admixture. Zinc is substituting for copper.^[3]

Association and origin

Chrysothallite may be associated with many other minerals:^[3]

• chlorides: atacamite, avdoninite, belloite, eriochalcite, mitscherlichite, sanguite, carnallite, halite, sylvite;
• sulfates: antlerite, chlorothionite, kröhnkite, natrochalcite, gypsum, kainite

Crystal structure

The crystal structure of chrysothallite is unique. Its building elements are:^[3]

• layer of distorted CuCl₄(OH)₂ octahedra, in which the octahedra share edges
• isolated Tl-centered TlCl₆ octahedra
• isolated Tl-centered TlCl₄(H₂O)₂ octahedra
• KCl₆ and KCl₉ polyhedra, that connect all the above elements

Origin

Chrysothallite is supposed to be a product of interaction of relatively high-temperature fumarolic minerals with fumarolic gas and atmospheric water, that takes place in temperatures up to 150 °C (302 °F).