Greifensteinite

Greifensteinite is beryllium phosphate mineral with formula: Ca₂Fe²⁺₅Be₄(PO₄)₆(OH)₄·6H₂O. It is the Fe²⁺ dominant member of the roscherite group.^[5] It crystallizes in the monoclinic crystal system and typically forms prismatic dark olive green crystals.^[2]

Greifensteinite
Greifensteinite (picture size: 3 mm)
General
Category	Phosphate mineral Roscherite group
Formula	Ca2Fe2+5Be4(PO4)6(OH)4·6H2O
IMA symbol	Gfs[1]
Strunz classification	8.DA.10
Dana classification	42.7.7.4
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 15.903, b = 11.885 c = 6.677 [Å]; β = 94.68°; Z = 2
Identification
Color	Yellow green, olive green, light brown
Crystal habit	Prismatic
Cleavage	Good on {100} (or parting on {100})[2]
Fracture	Uneven
Tenacity	brittle
Mohs scale hardness	4.5
Luster	Vitreous
Streak	White, greenish
Diaphaneity	Transparent to translucent
Specific gravity	2.93
Optical properties	Biaxial (-)
Refractive index	nα: 1.624 nβ: 1.634 nγ: 1.638
Birefringence	0.014
2V angle	80°
References	[2][3][4]

It was first described in Germany at Greifenstein Rocks, Ehrenfriedersdorf, and was named for the location. At the type locality, it occurs within a lithium-rich pegmatite in miarolitic cavities. It was approved by the International Mineralogical Association in 2002.^[2]