BEARTHITE, CA2AL[PO4]2(OH), A NEW MINERAL FROM HIGH-PRESSURE TERRANESOF THE WESTERN ALPS

Bearthite occurs associated with lazulite in synmetamorphic quartz seg regations of the Monte Rosa massif, where it has been partly described as a probably new Ca-phosphate by Peter Bearth (1952); it also occurs as an accessory rock-forming mineral in metapelite and magnesian schi st of the coesite-bearing terrane, Dora Maira massif. Textural relatio nships suggest that it formed during the early, high-pressure stages o f the Alpine metamorphic evolution; it was later replaced by lazulite and then apatite in Monte Rosa. by apatite + monazite(?) in Dora Maira . Bearthite is close to ideal Ca2Al[PO4]2(OH) in Monte Rosa, with some Sr reversible Ca substitution toward its Sr-homologue, goedkenite. In the Dora Maira material, up to one third Al may be replaced by Mg thr ough the coupled substitution Ca Al reversible RE Mg. Bearthite is opt ically biaxial positive, n(x) 1.662(1), n(y) 1.671(1), n, 1.696(1), 2 Vm 65.0(3)-degrees, dispersion r < nu, X//b. It is monoclinic, space g roup P2(1)/m, a 7.231(3), b 5.734(2), c 8.263(4) angstrom, beta 112.57 (8)-degrees, Z = 2, D(calc) 3.28. The structure, refined to a conventi onal R of 5.8% from 1499 independent reflections, is that of the brack ebushite group, with infinite chains of edge-sharing AlO5(OH) octahedr a extending parallel to b. In this group bearthite has the smallest ca tion in each structural position, which may account for its stability at high pressure. It has been synthesized at 4 kbar water pressure, 50 0-600-degrees-C, and 24 kbar, 820-degrees-C.