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

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... wing 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.