Manganokhomyakovite

Manganokhomyakovite is a very rare mineral of the eudialyte group,^[2] with the chemical formula Na₁₂Sr₃Ca₆Mn₃Zr₃WSi(Si₉O₂₇)₂(Si₃O₉)₂O(O,OH,H₂O)₃(OH,Cl)₂.^[3] This formula is in extended form (based on the original one), to show the presence of cyclic silicate groups and domination of silicon at the M4 site, basing on the nomenclature of the eudialyte group.^[4] Some niobium substitutes for tungsten in khomyakovite. As suggested by its name, manganokhomyakovite is a manganese-analogue of khomyakovite, the latter being more rare.^[3] The two minerals are the only group representatives, beside taseqite, with species-defining strontium, although many other members display strontium diadochy. Manganokhomyakovite is the third eudialyte-group mineral with essential tungsten (after johnsenite-(Ce) and khomyakovite).^[2]

Manganokhomyakovite
Manganokhomyakovite
General
Category	Cyclosilicates
Formula	Na12Sr3Ca6Mn3Zr3W(Si25O73)(O,OH,H2O)3(OH,Cl)2 (original form)
IMA symbol	Mkmy[1]
Strunz classification	9.CO.10 (10 ed) 8/E.23-35 (8 ed)
Dana classification	64.1.2.6
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) (same H-M symbol)
Space group	R3m
Unit cell	a = 14.28, c = 30.12 [Å] (approximated); Z = 3
Identification
Color	Orange to orange-red
Crystal habit	Pseudo-octahedra
Cleavage	None
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Density	3.13 (measured), 3.17 (calculated)
Optical properties	Uniaxial (−)
Refractive index	nω = 1.63, nε = 1.63 (approximated)
Pleochroism	None
References	[2][3]

Occurrence and association

Manganokhomyakovite, khomyakovite, johnsenite-(Ce) and oneillite are four eudialyte-group minerals discovered in alkaline rocks of Mont Saint-Hilaire, Quebec, Canada.^[5] Association of manganokhomyakovite is rich and includes: aegirine, albite, analcime, annite, kupletskite, microcline, natrolite, sodalite, titanite, wöhlerite, zircon, cerussite, galena, molybdenite, pyrite, pyrrhotite, and sphalerite.^[3]

Notes on chemistry

Impurities in manganokhomyakovite include niobium and iron, with traces of rare earth elements, hafnium, titanium, tantalum, and aluminium.^[3]