Golyshevite

Golyshevite is a rare mineral of the eudialyte group,^[2] with the formula Na₁₀Ca₃Ca₆Zr₃Fe₂SiNb(Si₃O₉)₂(Si₉O₂₇)₂CO₃(OH)₃•H₂O.^[3] The original formula was extended to show both the presence of cyclic silicate groups and silicon at the M4 site, according to the nomenclature of the eudialyte group.^[4] The characteristic feature of golyshevite is calcium-rich composition, with calcium at two main sites instead of one site.^[3] Together with feklichevite, fengchengite, ikranite and mogovidite it is a ferric-iron-dominant representative of the group.^[2] It is chemically similar to mogovidite.^[2][3] Golyshevite was named after Russian crystallographer Vladimir Mikhailovich Golyshev.^[3]

Golyshevite
Golyshevite crystals in matrix, some of them showing a pseudohexagonal section, with the typical yellowish-brown zircon-like colour. Locality: Kovdor Phlogopite Mine, Kovdor Massif, Northern Region, Russian Federation Size: 2.3 × 2.1 × 1.5 cm
General
Category	Silicate mineral, cyclosilicate
Formula	(Na,Ca)10Ca9(Fe3+,Fe2+)2Zr3NbSi25O72(CO3)(OH)3·H2O (original form)
IMA symbol	Gsv[1]
Strunz classification	9.CO.10
Dana classification	64.1.6
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) (same H-M symbol)
Space group	R3m
Unit cell	a = 14.23 c = 29.98 [Å] (approximated); Z = 3
Identification
Color	Brown to reddish-brown
Crystal habit	Grains and crystals
Cleavage	No
Tenacity	Brittle
Specific gravity	2.89 (measured)
Optical properties	Uniaxial (−)
Refractive index	nω = 1.62, nε = 1.61 (approximated)
Pleochroism	Green to pale yellow
References	[2][3]

Occurrence and association

Golyshevite and mogovidite were found in calcium-bearing peralkaline pegmatites of the Kovdor massif, Kola Peninsula, Russia. Minerals associated with golyshevite are aegirine-augite, calcite, cancrinite, hedenbergite, orthoclase, pectolite, tacharanite, and thomsonite-Ca.^[3]

Notes on chemistry

Impurities in golyshevite include chlorine, potassium, manganese, aluminium, cerium and lanthanum.^[3]

Notes on crystal structure

Calcium in golyshevite is present at M(1) and N(4) sites.^[3]