Hardystonite

Hardystonite is a rare calcium zinc silicate mineral first described from the Franklin, New Jersey, U.S. zinc deposits.[1] It often contains lead, which was detrimental to the zinc smelting process, so it was not a useful ore mineral.[4] Like many of the famous Franklin minerals, hardystonite responds to short wave ultraviolet (254 nm wavelength) light, emitting a fluorescence from dark purple to bright violet blue. In daylight, it is white to gray to light pink in color, sometimes with a vitreous or greasy luster. It is very rarely found as well formed crystals, and these are usually rectangular in appearance and rock-locked.[5]

Hardystonite in plain light, same sample as in fluorescent light image above right

Hardystonite
Hardystonite is fluorescing blue in this Franklin Furnace specimen. Red is calcite, and green is willemite (size: 7.0 x 6.0 x 3.2 cm)
General
Category	Sorosilicates
Formula (repeating unit)	Ca2ZnSi2O7
Strunz classification	9.BB.10
Dana classification	55.4.2.2
Crystal system	Tetragonal
Crystal class	Scalenohedral (42m) H-M symbol: (4 2m)
Space group	P421m
Unit cell	a = 7.8287(16) Å c = 5.0140(2) Å; Z = 2
Identification
Color	light brownish white, pale greyish-white, very pale pink
Crystal habit	Massive granular
Cleavage	[001] good, [100] and [110] fair
Tenacity	Brittle
Mohs scale hardness	3-4
Luster	vitreous, resinous, greasy, dull
Diaphaneity	Transparent to translucent
Specific gravity	3.396–3.443
Optical properties	Uniaxial (-)
Refractive index	nω = 1.672 nε = 1.661
Ultraviolet fluorescence	purple to violet blue in short wave ultraviolet light
Alters to	hydrothermal alteration to clinohedrite
References	[1][2][3]

Hardystonite has a chemical composition of Ca₂ZnSi₂O₇. It is frequently found with willemite (fluoresces green), calcite (fluoresces red), and clinohedrite (fluoresces orange). Hardystonite can be found altered to clinohedrite CaZn(SiO₄)·H₂O through direct hydrothermal alteration.[5] Other minerals often associated with hardystonite are franklinite, diopside, andradite garnet, and esperite (fluoresces yellow).

It was first described in 1899 by J.E. Wolff, when the New Jersey Zinc Company mines were located in what was called Franklin Furnace, in Hardyston Township, New Jersey.[5]