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 (repeating unit)	(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]