Khomyakovite

Khomyakovite
General
Category	Silicate mineral, Cyclosilicate
Formula; (repeating unit)	Na12Sr3Ca6Fe3Zr3W(Si25O73)(O,OH,H2O)3(OH,Cl)2 (original form)
Strunz classification	9.CO.10 (10 ed) ; 8/E.23-30 (8 ed)
Dana classification	64.1.2.5
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) ; H-M symbol: (3m)
Space group	R3m
Unit cell	a = 14.30, c = 30.08 [Å] (approximated); Z = 3
Identification
Color	Orange to orange-red
Crystal habit	aggregates (anhedral to subhedral)
Cleavage	None
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5-6
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Density	3.14 (calculated)
Optical properties	Uniaxial (-)
Refractive index	nω=1.63, nε=1.63 (approximated)
Pleochroism	None
References	[1][2]

Khomyakovite is an exceedingly rare mineral of the eudialyte group, with formula Na₁₂Sr₃Ca₆Fe₃Zr₃WSi(Si₉O₂₇)₂(Si₃O₉)₂O(O,OH,H₂O)₃(OH,Cl)₂.[2][1] The original formula was extended to show the presence of both the cyclic silicate groups and M4-site silicon, according to the nomenclature of the eudialyte group.[3] Some niobium substitutes for tungsten in khomyakovite. Khomyakovite is an iron-analogue of manganokhomyakovite, the second mineral being a bit more common.[2] The two minerals are the only group representatives, beside taseqite, with species-defining strontium, although many other members display strontium diadochy. Khomyakovite is the third eudialyte-group mineral with essential tungsten (after johnsenite-(Ce) and manganokhomyakovite).[1]

Occurrence and association

Khomyakovite, manganokhomyakovite, johnsenite-(Ce) and oneillite are four eudialyte-group minerals with type locality in Mont Saint-Hilaire, Quebec, Canada.[4] Khomyakovite itself is associated with analcime, annite, natrolite, titanite, calcite, and pyrite.[2]

Notes on chemistry

Impurities in khomyakovite include niobium, potassium and manganese, with minor rare earth elements, magnesium, titanium, hafnium and aluminium.[2]

References

Mindat, Khomyakovite, http://www.mindat.org/min-7137.html
Johnsen, O., Gault, R.A., Grice, J.D., and Ercit, T.S., 1999: Khomyakovite and manganokhomyakovite, two new members of the eudialyte group from Mont Saint-Hilaire, Quebec, Canada. The Canadian Mineralogist 37, 893-899
Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785-794
"Poudrette quarry (Demix quarry; Uni-Mix quarry; Desourdy quarry; Carrière Mont Saint-Hilaire), Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada - Mindat.org". Mindat.org. Retrieved 2016-03-11.

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[Mindat-1] Mindat, Khomyakovite, http://www.mindat.org/min-7137.html

[CanMin1-2] Johnsen, O., Gault, R.A., Grice, J.D., and Ercit, T.S., 1999: Khomyakovite and manganokhomyakovite, two new members of the eudialyte group from Mont Saint-Hilaire, Quebec, Canada. The Canadian Mineralogist 37, 893-899

[CanMin-3] Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785-794

[4] "Poudrette quarry (Demix quarry; Uni-Mix quarry; Desourdy quarry; Carrière Mont Saint-Hilaire), Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada - Mindat.org". Mindat.org. Retrieved 2016-03-11.