Railsback's Some Fundamentals of Mineralogy and Geochemistry
Hardness of minerals IVb: variation with bond length and anion size
Halite Villiaumite Frohbergite Ferroselite Diamond One control on the NaCl NaF C hardness of a mineral is the Pyrite FeS2 FeTe2 FeSe2 length of bonds within that Iodargyrite Chlorargyrite mineral. To illustrate this Troilite AgI AgCl point, this diagram shows AgBr Wüstite pairs of chemically analogous Bromargyrite FeS FeO Corundum minerals that differ only in their anions. In most of the Millerite Bunsenite Al2O3 examples shown on this NiO Topaz diagram (the ones joined by NiS Telluro- Al2SiO4(F,OH)2 light gray lines), minerals with bismuthite nerals larger anions, and thus Bismuthinite Bismite Mi greater bond lengths, are
a mattera graphical of convenience. Quartz softer than their counterparts Bi Te Bi S Bi O 2 3 2 3 2 3 SiO2 with smaller anions and thus Ortho- shorter bonds. Galena Clausthalite clase However, in two cases Altaite shown here (the ones joined Massicot & KAlSi O Lithargite PbTe 3 8 by red lines), larger anions PbO PbS PbSe lead to harder minerals. Index These are the examples Naumannite where soft cations,Ag+ and Hessite Apatite Pb2+, bond to 2- anions. In Acanthite Ca (PO ) (F,Cl,OH) rtical position of minerals is only AgTe 5 4 3 these cases, the bonds of AgS AgSe Larger anion Smaller anion soft cations to increasingly Fluorite (and thus greater (and thus lesser large and thus increasingly CaF length of bond) length of bond) soft anions may lead to Mohs 2 sufficiently covalent bonds to Calcite cause greater hardness of CaCO3 minerals. Hardness data are from Nickel, Gypsum E.H., and Nichols, M.C., 1991, Hauerite Pyrolusite* Mineral Reference Manual: New CaSO4•H O 2 MnO York, Van Nostrand Reinhold, 250 p. is diagramvertical has scaleve no - 2 MnS2
Th * The hardness of pyrolusite is Alabandite Manganosite commonly listed as 2, but that is Talc the value for earthy masses of MnO minute crystals. The hardness of Mg3Si4O10(OH)2 MnS a single crystal of pyrolusite is 6.5 (Gaines et al., 1997, Dana's New 1 234 5 6 7 8910 Mineralogy: New York, John Mohs Hardness Wiley & Sons, 1819 p.). LBR HardnessTrends29 1/2008