Wollastonite

Wollastonite
General
CategoryInosilicate mineral
Formula
(repeating unit)
Calcium metasilicate, CaSiO3
IMA symbolWo[1]
Strunz classification9.DG.05
Crystal systemTriclinic
Monoclinic polytype exists
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1 (Triclinic)
P21/a (Monoclinic)
Unit cella = 7.925 Å, b = 7.32 Å,
c = 7.065 Å; α = 90.055°,
β = 95.217°, γ = 103.42°; Z = 6
Identification
Formula mass116.159 g/mol
ColorWhite, colorless or gray
Crystal habitRare as tabular crystals—commonly massive in lamellar, radiating, compact and fibrous aggregates.
TwinningCommon
CleavagePerfect in two directions at near 90°
FractureSplintery to uneven
Mohs scale hardness4.5 to 5.0
LusterVitreous or dull to pearly on cleavage surfaces
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.86–3.09
Optical propertiesBiaxial (−)
Refractive indexnα = 1.616–1.640
nβ = 1.628–1.650
nγ = 1.631–1.653
Birefringenceδ = 0.015 max
2V angleMeasured: 36° to 60°
Melting point1540 °C
SolubilitySoluble in HCl, insoluble in water
Other characteristicsHeat of formation (@298): −89.61kJ
Gibbs free energy: 41.78kJ
References[2][3][4][5][6]

Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolomite is subjected to high temperature and pressure, which sometimes occurs in the presence of silica-bearing fluids as in skarns[7] or in contact with metamorphic rocks. Associated minerals include garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Despite its chemical similarity to the compositional spectrum of the pyroxene group of minerals—where magnesium (Mg) and iron (Fe) substitution for calcium ends with diopside and hedenbergite respectively—it is structurally very different, with a third SiO4−4 tetrahedron[8] in the linked chain (as opposed to two in the pyroxenes).

  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. ^ Wollastonite, Mindat
  3. ^ Wollastonite, Webmineral
  4. ^ Wollastonite, Handbook of Mineralogy
  5. ^ American Mineralogist, V. 79, pp. 134-144, 1994
  6. ^ Virta, Robert; Van Gosen, Brad (January 27, 2015). "Mineral Resource of the Month: Wollastonite". Earth Magazine. American Geosciences Institute.
  7. ^ Whitley, Sean; Halama, Ralf; Gertisser, Ralf; Preece, Katie; Deegan, Frances M.; Troll, Valentin R. (2020-10-18). "Magmatic and Metasomatic Effects of Magma–Carbonate Interaction Recorded in Calc-silicate Xenoliths from Merapi Volcano (Indonesia)". Journal of Petrology. 61 (4). doi:10.1093/petrology/egaa048. ISSN 0022-3530.
  8. ^ William Alexander Deer; Robert Andrew Howie; J. Zussman (1992). An introduction to the rock-forming minerals. Longman Scientific & Technical. ISBN 978-0-470-21809-9.