Jeremejevite

Jeremejevite is a rare aluminium borate mineral with variable fluoride and hydroxide ions. Its chemical formula is Al₆B₅O₁₅(F,OH)₃.

Jeremejevite
General
Category	Borate mineral
Formula (repeating unit)	Al6B5O15(F,OH)3
Strunz classification	6.AB.15
Crystal system	Hexagonal
Crystal class	Dipyramidal class (6/m) H-M symbol: (6/m)
Space group	P63/m
Unit cell	a = 8.5591(3), c = 8.1814(6) [Å]; Z = 2
Identification
Color	Colorless, white, yellowish, blue, light yellow brown, aquamarine blue; colourless in transmitted light
Crystal habit	Prismatic
Cleavage	None observed
Fracture	Conchoidal
Mohs scale hardness	6.5 - 7.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	3.28 - 3.31
Optical properties	Uniaxial (-)
Refractive index	nω = 1.653 nε = 1.640
Birefringence	0.0130
Pleochroism	Colorless - light blue-violet
Other characteristics	Piezoelectric
References	[1][2][3]

It was first described in 1883[4] for an occurrence on Mt. Soktui, Nerschinsk district, Adun-Chilon Mountains, Siberia. It was named after Russian mineralogist Pavel Vladimirovich Eremeev (Jeremejev, German) (1830–1899).[3]

It occurs as a late hydrothermal phase in granitic pegmatites in association with albite, tourmaline, quartz and rarely gypsum. It has also been reported in the Pamir Mountains of Tajikistan, Namibia and the Eifel district, Germany.[3]

Blue jeremejevite in rare, clear crystal form. Image width: 1.5 mm.