Geikielite

Geikielite is a magnesium titanium oxide mineral with formula: MgTiO₃. It is a member of the ilmenite group. It crystallizes in the trigonal system forming typically opaque, black to reddish black crystals.

Geikielite
Crystals of geikielite from the Maxwell quarry, Chelsea, Outaouais, Québec, Canada
General
Category	Oxide mineral
Formula	MgTiO3
IMA symbol	Gk[1]
Strunz classification	4.CB.05
Crystal system	Trigonal
Crystal class	Rhombohedral (3) (same H-M symbol)
Space group	R3
Unit cell	a = 5.05478(26) Å c = 13.8992(7) Å; Z = 6
Identification
Color	Black, ruby red uncommon; red internal reflections
Crystal habit	Tabular prismatic crystals, also as finely granular masses
Cleavage	Good on {1011}
Mohs scale hardness	5 - 6
Luster	Sub-metallic
Streak	Purplish brown
Diaphaneity	Opaque to translucent
Specific gravity	3.79 - 4.2
Optical properties	Uniaxial (-)
Refractive index	nω = 2.310 - 2.350 nε = 1.950 - 1.980
Birefringence	δ = 0.360 - 0.370
Pleochroism	Weak, O = pinkish red, E = brownish to purplish red
References	[2][3][4]

It was first described in 1892^[5] for an occurrence in the Ceylonese gem bearing gravel placers. It was named for Scottish geologist Sir Archibald Geikie (1835–1924).^[4] It occurs in metamorphosed impure magnesian limestones, in serpentinite derived from ultramafic rocks, in kimberlites and carbonatites. Associated minerals include rutile, spinel, clinohumite, perovskite, diopside, serpentine, forsterite, brucite, hydrotalcite, chlorite and calcite.^[2]