The Stability of Brucite in the Weathering Zone of the New Idria Serpentinite

Abstract

AbstractPetrographic studies have shown that brucite is a major constituent of the New Idria serpentinite and of the short-fiber asbestos deposit associated with it. Acid-leaching data suggest that the serpentinite averages 7–8 weight per cent brucite, which contains approximately 15 mole per cent “Fe(OH)2”. Unit cell parameters and electron probe analysis suggest an empirical formula close to (Mg10Fe2)(OH)24 for this phase. Brucite formed during the initial serpentinization of an olivine-rich parent and is concentrated today in the hard, dense serpentinite fragments scattered throughout a highly sheared matrix of soft, friable asbestos. Although brueite is abundant in the fresh serpentinite it is almost absent from the surface weathering zone, which persists to a depth of 20–30 ft across the entire body. Here, serpentinite fragments have oxidized and brueite has transformed in situ into pyroaurite [Mg6Fe2CO3(OH)16.4H2O] and a new mineral, coalingite [Mg10Fe2CO3(OH)24.2H2O]. Brueite in the matrix material has dissolved in the CO2-rich ground waters, yielding soluble magnesium ions and amorphous iron oxides which discolor the surface asbestos.In the laboratory, samples of fresh serpentinite oxidized and disintegrated completely when exposed to the atmosphere for a few months, due to the brucite-coalingite transformation. In the presence of O2 and CO2, brueite dissolved completely from a water slurry of the serpentinite, yielding a dark brown residue and a clear filtrate which later precipitated hydromagnesite [Mg4(OH)2(CO3)3.3H2O]. These data indicate that in the relatively impervious environment of the residual serpentinite “boulders”, iron-rich brueite oxidizes in air, picking up CO2 and H2O to form coalingite. In the presence of excess ground waters, brueite in the friable matrix dissolves, leaving behind a residue of amorphous iron oxides. Dissolved magnesium ions later precipitate as hydromagnesite, which is also abundant in the surface weathering zone of the serpentinite.