Kaersutite

Kaersutite is a dark brown to black double-chain calcic titanium-bearing amphibole mineral with formula: NaCa₂(Mg₃Ti⁴⁺Al)(Si₆Al₂)O₂₂(O)₂.^[6]

Kaersutite
Kaersutite (black) in tuff. Locality: Suletice, Czech Republic
General
Category	Inosilicate
Formula	NaCa2(Mg3Ti4+Al)(Si6Al2)O22(O)2
IMA symbol	Krs[1]
Strunz classification	9.DE.10
Dana classification	66.01.03a.18
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Dark brown to black, yellow-brown, green-brown, or red-brown in thin section
Crystal habit	Prismatic phenocrysts, granular aggregates
Twinning	Simple or multiple twinning parallel to {100}
Cleavage	Perfect on {110}, intersecting at 56° and 124°
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Streak	Pale brownish-grey
Diaphaneity	Semitransparent
Specific gravity	3.20–3.28
Optical properties	Biaxial (−)
Refractive index	nα = 1.670–1.689 nβ = 1.690–1.741 nγ = 1.700–1.772
Birefringence	δ = 0.030–0.083
Pleochroism	Strong; X = yellow, yellow-brown; Y = red, red-brown; Z = deep brown, dark red-brown.
2V angle	66–82°
References	[2][3][4][5]

Ferro-kaersutite is the divalent iron-rich endmember of the kaersutite group, with the iron replacing magnesium in the structure.^[7]

Kaersutite occurs as phenocrysts in alkalic volcanic rocks, in nodules of peridotite and gabbro in alkalic basalts, and in syenites, monzonites, and carbonatite tuffs. Mineral associations include titanian augite, rhoenite, olivine, ilmenite, spinel, plagioclase, and titanian pargasite.^[2]

It was first described in 1884 and is named for Qaersut (formerly Kaersut), Umanq district in northern Greenland.^[3]