The sample is a rock fragment (7 cm × 4 cm × 3 cm), composed mainly of intergrowths and radial-radiant aggregates of dark green, almost black potassic-hastingsi — Photo: Kaneva, Ekaterina and Radomskaya, Tatiana and Shendrik, Roman and Chubarov, Victor and Danilovsky, Victoria · CC BY 4.0
Hastingsite
Hastingsite is an IMA-recognized mineral species in the silicate class, with the chemical formula NaCa₂(Fe²⁺₄Fe³⁺)(Si₆Al₂)O₂₂(OH)₂.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Amphibole structural group.
Key properties: Mohs hardness 5–6; specific gravity 3.35–3.5; typically green or black or brown; white streak; [Vitreous.] luster; optically biaxial.
IMA symbol Hst, IMA status: approved, named in 1896.
Found in nepheline syenite and granite; in schists, gneisses, tactites, and amphibolites.
It is found associated with Scapolite, apatite, magnetite, spinel (gneiss); garnet, epidote, hedenbergite, quartz (tactite).
The name refers to a locality in Hastings Co., Ontario, Canada.
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