Original paper

Crystal chemistry of synthetic Mn-bearing anorthite: Incorporation of MnAl2Si2O8 end-member into feldspar

Matsui, Tomoaki; Kimata, Mitsuyoshi

European Journal of Mineralogy Volume 9 Number 2 (1997), p. 333 - 344

44 references

published: Jun 26, 1997
manuscript accepted: Nov 19, 1996
manuscript received: Apr 9, 1996

DOI: 10.1127/ejm/9/2/0333

BibTeX file

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Abstract A representative series of feldspars with intermediate compositions along the join CaAl2Si2O8 (anorthite)-MnAl2Si2O8 has been synthesized by heating stoichiometric mixtures of chemical reagents under an N2 atmosphere. Morphological comparison with synthetic Mn-free anorthites suggests that Mn may play a role as a mineralizer for megacrystallization of anorthite. The reciprocal relationship between the Mn and Ca contents of these anorthites and the variation of the unit-cell parameters provides strong evidence that the Mn is incorporated into the extra-framework M site of the feldspar structure. The maximum limit of solubility of MnAl2Si2O8 is about 25 mol% in CaAl2Si2O8. Gradual decreases in the unit-cell dimensions and unit-cell volume with Mn content are similar to those observed in the CaAl2Si2O8-SrAl2Si2O8 series. The crystal structure of synthetic Mn-bearing anorthite, Ca0.715Mn0.196Na0.045□0.044Al1.867 Si2.133O8 (□ represents an M site vacancy) with cell parameters a = 8.131(2) Å, b= 12.847(3) Å, c = 7.069(1) Å, α = 94.03(1)°, ß = 115.89(1)°, γ= 90.72(1)°, V= 661.8(2) Å3, space group Cl, Z= 4, has been refined to an fl-index of 3.9% using 2109 X-ray reflection intensities collected with MoKa radiation. The structure is isotypic with high albite NaAlSi3O8, but has the M site (containing Ca, Mn and Na) split into three sites (M1, M2, M3) with unequal occupancies: Mn/Ca/Na = 0.144/0.358/0.021 (Ml), 0.0/0.179/00 (M2), 0.052/0.179/0.024 (M3). The Al/Si distributions calculated from the T-O bond lengths are as follows: Al/Si = 0.557/0.443 (Tlo site) 0.434/0.566 (Tlm), 0.455/0.545 (T2o) and 0.465/0.535 (T2m). The distribution of the Mn2+ and Ca2+ over the three split M sites, together with the disordered Al/Si distribution can give rise to many different locally ordered configurations. Spatial disorder of these configurations accounts for the reduction of the c axis to 7 A in Mnbearing anorthite. In addition, solid solution of the □ Si4O8 end-member (the excess silica component) contributes to the shrinkage of the M site making it more favourable for Mn to enter the anorthite structure. The crystal-chemical conditions promoting incorporation of Mn into anorthite are therefore (1) a highly disordered Al/Si distribution, (2) solid solution of □ Si4O8 end-member, (3) crystallization from anhydrous melt at high temperature and (4) rapid cooling after crystallization. The rarity of Mn feldspars in nature cannot therefore be attributed to an intrinsic exclusion of Mn from the feldspar structure, but to the unusual combination of temperature, pressure and chemical conditions required for its crystallization


anorthiteMnAl2Si2O8synthesissolid solutioncrystal structure