Chemical Variation in Minerals of the Astrophyllite Group

R. Macdonald and M. J. Saunders
Department of Environmental Sciences, University of Lancaster
Grant Institute of Geology, University of Edinburgh

Summary: Eight new and fourteen published analyses of astrophyllites are used to examine chemical variation within the group. Al, Ca, Cs, F, Mg, Mn, Nb, Rb, Sn, Zn, and Zr may all substitute in moderate to large amounts in the unit cell, whose basic formula is (K,Na)3Fe7Ti2Si8(O,OH)31; the natural Mn- and Nb- analogues of astrophyllite have been recorded. Ba, Cl, Cu, Li, P, Pb, Sr, and Ta are present, but in smaller amounts. The majority of specimens show appreciable cation deficiencies, up to 5 % of sites remaining vacant. Astrophyllites from undersaturated igneous rocks are characterized by higher Al, Ca, Mg, Mn, (OH+F), (K+Na) and Zr, and lower Li, Pb, and Zn contents than those from oversaturated igneous rocks. A ‘high-alkali’ type of astrophyllite is distinguished. A new occurrence of nigerite is noted.

Mineralogical Magazine; March 1973 v. 39; no. 301; p. 97-111; DOI: 10.1180/minmag.1973.039.301.15
© 1973, The Mineralogical Society
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