Olivine + Quartz + Water ± HCl At Mid-Crustal Conditions: Controls On the Growth of Fibrous Talc As Determined From Hydrothermal Diamond Anvil Cell EXPERIMENTS

Abstract Many talc deposits are formed by metasomatism of dolostone or ultramafic rocks during metamorphism or hydrothermal alteration. In these deposits, talc commonly exhibits a platy habit; rarely, it forms fibers. In previous studies, the origin of fibrous talc has been explained by the secondary replacement of fibrous amphiboles during retrograde metamorphism. In this study, experiments have been performed in the MgO–FeO–SiO 2 –H 2 O–HCl system using a hydrothermal diamond anvil cell (HDAC). San Carlos olivine (Fo 90 ) + Brazilian quartz + aqueous fluid [deionized water or 0.1–10 m m (milli-molal) HCl] were used as starting materials and were reacted for approximately 2 to 6 hours at temperatures of 400–655 °C and pressures of 0.1–1.2 GPa. All experiments produced talc as the only new phase, but with both fibrous and platy habits. The talc fibers (length:width ≥ 25:1; average length ∼40 μm) grew on the surface of the olivine and on the gasket walls. Maximum talc growth rates in these experiments ranged from 10 –11 to 10 –13 moles per second or 10 –5 to 10 –6 moles per second per square meter. Experiments in this study demonstrate that the growth of fibrous talc does not require a fibrous precursor. Experimental results suggest that the mechanism for talc fiber growth is related to initial nucleation density of talc on the substrate and the degree of supersaturation. Conditions that appear to inhibit the growth of fibrous talc include a large surface area of olivine relative to quartz; an increase in the density of the aqueous fluid; and the chemical isolation of olivine by talc armoring. Decreases in aqueous fluid density and lower olivine surface area relative to quartz are consistent with the production of fibrous talc.