Hodgkinsonite

Hodgkinsonite is a rare zinc manganese silicate mineral Zn₂MnSiO₄(OH)₂. It crystallizes in the monoclinic system and typically forms radiating to acicular prismatic crystals with variable color from pink, yellow-red to deep red. Hodgkinsonite was discovered in 1913 by H. H. Hodgkinson, for whom it is named in Franklin, New Jersey, and it is only found in that area.^[2]

Hodgkinsonite
Hodgkinsonite from Franklin Mine, Franklin, New Jersey USA. The translucent and lustrous crystals reach 8 mm and the main crystal here is doubly terminated.
General
Category	Nesosilicate
Formula	Zn2MnSiO4(OH)2
IMA symbol	Hgk[1]
Strunz classification	9.AE.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Unit cell	a = 11.76, b = 5.31 c = 8.18 [Å]; β = 95.42°, Z = 4
Identification
Mohs scale hardness	4+1⁄2 - 5
Optical properties	Biaxial (-)
Refractive index	nα = 1.720 nβ = 1.741 nγ = 1.746
Birefringence	δ = 0.026
Dispersion	r < v moderate