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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)
(repeating unit)
IMA symbolHdy[1]
Strunz classification9.BB.10
Dana classification55.4.2.2
Crystal systemTetragonal
Crystal classScalenohedral (42m)
H-M symbol: (4 2m)
Space groupP421m
Unit cella = 7.8287(16) Å
c = 5.0140(2) Å; Z = 2
Colorlight brownish white, pale greyish-white, very pale pink
Crystal habitMassive granular
Cleavage[001] good, [100] and [110] fair
Mohs scale hardness3-4
Lustervitreous, resinous, greasy, dull
DiaphaneityTransparent to translucent
Specific gravity3.396–3.443
Optical propertiesUniaxial (-)
Refractive indexnω = 1.672 nε = 1.661
Ultraviolet fluorescencepurple to violet blue in short wave ultraviolet light
Alters tohydrothermal alteration to clinohedrite

Hardystonite is a rare calcium zinc silicate mineral first described from the Franklin, New Jersey, U.S. zinc deposits.[2] It often contains lead, which was detrimental to the zinc smelting process, so it was not a useful ore mineral.[5] 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.[6]

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

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.[6] 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.[6]


  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. ^ a b http://www.mindat.org/min-1818.html Mindat
  3. ^ http://www.webmineral.com/data/Hardystonite.shtml Webmineral data
  4. ^ Handbook of Mineralogy
  5. ^ Palache, Charles (1935). "The minerals of Franklin and Sterling Hill, Sussex County, New Jersey" (PDF). Geological Survey Professional Paper. 180: 122–123. doi:10.3133/PP180. ISSN 0096-0446. Wikidata Q58144929.
  6. ^ a b c Dunn, Pete J. Franklin and Sterling Hill, New Jersey: the world's most magnificent mineral deposits. Dr. Pete J. Dunn, 2004.