Homilite

Homilite is a borosilicate mineral belonging to the gadolinite group of minerals with formula Ca₂(Fe,Mg)B₂Si₂O₁₀.

Homilite
General
Category	Nesosilicate
Formula (repeating unit)	Ca2(Fe2+,Mg)B2Si2O10
IMA symbol	Hom[1]
Strunz classification	8/B.29-30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m)
Space group	P21/b
Unit cell	a = 9.78 Å, b = 7.61 Å, c = 4.78 Å; β = 90.56°; Z = ?
Identification
Color	Greenish to brownish black
Mohs scale hardness	5–5+1⁄2
Luster	Vitreous, resinous
Diaphaneity	Opaque
Specific gravity	3.34
Optical properties	Biaxial (+)
Refractive index	nα = 1.715 nβ = 1.725 nγ = 1.738
Birefringence	δ =
2V angle	Measured: 80°
References	[2] [3] [4]

It occurs as brown monoclinic crystals (space group P2₁/a^[5]) within feldspar masses in pegmatite and was discovered in 1876 in Stoko island, Langesundfiord, Norway. The name is from the Greek for to occur together, in allusion to its association with meliphanite and allanite.

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. Mineral Society of America Handbook (PDF)
3. Mindat with locations
4. Webmineral data
5. Miyawaki, R.; Nakai, I.; Nagashima, K. (15 January 1985). "Structure of homilite, Ca2.00(Fe0.90Mn0.03)B2.00Si2.00O9.86(OH)0.14". Acta Crystallographica Section C Crystal Structure Communications. 41 (1): 13–15. doi:10.1107/S010827018500261X.