Iron-rich Talc-Opal-Minnesotaite Spherulites and Crystallochemical Relations of Talc and Minnesotaite

P. C. A. Kager and I. S. Oen
Geologisch Instituut, Universiteit van Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, The Netherlands

Synopsis: ZONED spherulites in greenalite-siderite-silica rocks of the Emilia-San Valentin lead-zinc deposit, Sierra de Cartagena, SE Spain, show a core of iron-rich talc, (Mgl.8Fel.2)Si4Ol0(OH)2, an intermediate zone of opal-chalcedony, and a rim of minnesotaite, (Fe2.7Mg0.3)Si4Ol0(OH)2. Crystalllization of the spherulites, presumably from undercooled silica sols carrying dissolved Mg and Fe, began with the non-equilibrium precipitation of metastable iron-rich talc; the residual fluids enveloping the growing spherulites became more iron-rich in composition until metastable equilibrium between iron-rich talc and iron-enriched residual liquid impeded the further growth of the iron-rich talc; the spherulites were then overgrown by colloidal silica flocculates and a rim of minnesotaite precipitated from the iron-enriched residual fluid. The proposed crystallization model implies that, if talc and minnesotaite represent an isomorphic series, under equilibrium conditions there is a broad immiscibility region in the series.

Mineralogical Magazine; June 1983 v. 47; no. 343; p. 229-231; DOI: 10.1180/minmag.1983.047.343.14
© 1983, The Mineralogical Society
Mineralogical Society (