Calcian, borian sapphirine from the serendibite deposit at Johnsburg, N.Y., USA

A calcian, borian sapphirine occurs in grains up to 1.5 mm across with calcite, tourmaline, phlopopite, and spinel as a breakdown product of serendibite. The sapphirine is heterogeneous in composition; back-scattered electron and CaKα images indcate that Ca is concentrated in irregular patches several tens of micrometers acros. The variation in MgO, Al 2 O 3 and SiO 2 contents with CaO can be fitted with a line joining the compositions Mg 3.34 Fe 2+ 0.06 Al 9.19 Si 1.40 O 20 and Ca 2.08 Mg 2.80 Mg 2.80 Fe 2+ 0.07 Al 5.07 B 1.60 Si 2.53 O 20 . The first composition is ideal for sapphirine; the second is approximate for serendibite. CaO contents of the Johnsburg sapphirine range 0.02-5.4 wt%, equivalent to 0-35% of this serendibite composition in solid solution wit sapphirine. B 2 O 3 contents (ion microprobe analyses) are 1-2 wt% and exceed those calculated for a sapphirine-serendibite solid solution, implying that Ca-poor patches in the Johnsburg sapphirine also contain B 2 O 3 . TEM examination of the sapphirine did not reveal any sharp interfaces between discrete Ca-poor and Ca-rich regions. Electron diffraction patterns suggest that the ITc polytype is dominant. Therefore, we hypothesize that the chemical inhomogeneity is a consequence of partial exsolution in a single calcian, borain triclinic sapphirine phase, rather than an intergrowth of sapphirine and serendibite. This sapphirine phase formed under conditions whereby the unusual assemblage sapphirine+calcite was stable, possibly at temperatures near 600 ° C or somewhat less for pressures of 8 kbar or less, at high activities of CO 2