Calcium-free pumpellyite, a new synthetic hydrous Mg–Al-silicate formed at high pressures

Abstract

The mineral pumpellyite, W₄X₂Y₄Z₆O_(20+t) (OH)_(8−t) where W = Ca, Mn; X = (Mg, Fe²⁺, Mn)_2−t (Fe³⁺, Al)_t; Y = Fe³⁺, Al; Z = Si (ref. 1) is a well-known constituent of very low-grade metamorphic rocks, especially of metabasic compositions. Schiffmann and Liou² have synthesized the ideal Fe-free endmember ‘Mg–-Al’ pumpellyite', Ca₄MgAl₅Si₆O₂₁(OH)₇, and found its upper thermal stability to lie near 375–400 °C in the range of 5–8 kbar water pressure. Here we report the high-pressure synthesis of a pumpellyite-type phase in which the position W of the above formula is solely occupied by magnesium. This ‘Mg–Mg–Al pumpellyite’, which is chemically nearly equivalent to pyrope+H₂O, crystallizes at 40–55 kbar, 700–850 °C. Therefore, it qualifies as a potential hydrous mantle phase in a subduction zone regime.