Original paper

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

S. Grew, Edward; G. Yates, Martin; M. Romanenko, Igor; G. Christy, Andrew; H. Swihart, George

European Journal of Mineralogy Volume 4 Number 3 (1992), p. 475 - 486

30 references

published: Jun 11, 1992
manuscript accepted: Sep 10, 1991
manuscript received: Apr 15, 1991

DOI: 10.1127/ejm/4/3/0475

BibTeX file

ArtNo. ESP147050403005, Price: 29.00 €

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Abstract 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 indicate that Ca is concentrated in irregular patches several tens of micrometers across. The variation in MgO, AI2O3 and SiO2 contents with CaO can be fitted with a line joining the compositions Mg3.34Fe2+0.06Al9.19Si1.40O20 and Ca2.08Mg2.80Fe2+0.07Al5.07B1.60Si2.53O20. 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 with sapphirine. B2O3 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 B2O3. 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, borian 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 CO2