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 (repeating unit)	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]

It occurs as phenocrysts in alkalic volcanic rocks; in nodules of peridotite and gabbro in alkalic basalts; in syenites, monzonites and carbonatite tuffs. Mineral association includes 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]