The formation of a clinopyroxene corona; mass balance considerations

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doi: 10.1016/0024-4937(92)90023-R
Authors:Zingg, A. J.
Author Affiliations:Primary:
Mining Univ., Inst. Miner. Petrol., Leoben, Austria
Volume Title:Lithos (Oslo)
Source:Lithos (Oslo), 28(1), p.55-68. Publisher: Elsevier, Amsterdam, International. ISSN: 0024-4937
Publication Date:1992
Note:In English; illus. incl. 4 tables
Summary:The formation of a clinopyroxene corona is discussed from the viewpoint of continuous reaction techniques. Two cases are distinguished: a closed and an open system. The first case is based on the equation eOlw + aPlu = bPlv + Opxx + cCpxy + dSplt/z. Plagioclase is assumed to be a solid solution between anorthite and albite. The other minerals are solutions of their Fe and Mg end members. Spinel forms a ternary system of hercynite-spinel-magnetite. Stoichiometric coefficients are expressed in terms of the exchange parameters t, u, v, w, x, y and z. According to stoichiometries, the net molar quantity of plagioclase consumed equals the number of moles of clinopyroxene and spinel produced. For each mole of olivine consumed there is one mole of orthopyroxene produced. A closed system is suggested by the ability to predict one of the mineral compositions in terms of the others using one of the six mass-balances (Na, Ca, Mg, Fe, Al, Si) left from the calculation of five stoichiometric coefficients. This model allows the calculation of mass transfer between different shells and explains coronas from the Adirondacks, Scandinavia and the Niquelandia layered complex (Brazil). In the second case the system is open and is described by the equationOpxx + aPlu + kCa2+ + hSi4+ + function signFe2+ = cCpxy + iNa+ + lAl3+ + gMg2+. The compositional variation of orthopyroxene and clinopyroxene allows the determination of c(Cpx), f(Fe2+) and g(Mg2+) which are interdependent. With a known value of c and assuming a constant volume of reaction the value of a may be determined. The final equation, for plagioclase composition of XAn = 0.75, is 1 Opx + 0.35 Pl + 0.74 Ca2+ + 0.21 Si4+ + 0.10 Mg2+ = 1 Cpx + 0.61 Al3+ + 0.10 Fe2+ + 0.09 Na+. This is the reaction responsible for the formation of Cpx-coronas in the Bushveld complex. [G.R.]
Subjects:Chain silicates; Clinopyroxene; Closed systems; Mass balance; Open systems; Pyroxene group; Reaction rims; Silicates
Abstract Numbers:93M/4605
Record ID:1993036072
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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