Khomyakovite

Khomyakovite is an exceedingly rare mineral of the eudialyte group, with formula Na₁₂Sr₃Ca₆Fe₃Zr₃W(Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl)₂.^[2][3] 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.^[4] Some niobium substitutes for tungsten in khomyakovite. Khomyakovite is an iron-analogue of manganokhomyakovite, the second mineral being a bit more common.^[3] 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).^[2]

Khomyakovite
General
Category	Silicate mineral, cyclosilicate
Formula	Na12Sr3Ca6Fe3Zr3W(Si25O73)(O,OH,H2O)3(OH,Cl)2 (original form)
IMA symbol	Kmy[1]
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	[2][3]

Occurrence and association

Khomyakovite, manganokhomyakovite, johnsenite-(Ce) and oneillite are four eudialyte-group minerals with type locality in Mont Saint-Hilaire, Quebec, Canada.^[5] Khomyakovite itself is associated with analcime, annite, natrolite, titanite, calcite, and pyrite.^[3]

Notes on chemistry

Impurities in khomyakovite include niobium, potassium and manganese, with minor rare earth elements, magnesium, titanium, hafnium and aluminium.^[3]