The nature of crystalline silica from the TAG submarine hydrothermal mound, 26 degrees N Mid Atlantic Ridge

Silica occurs in abundance in a variety of hydrothermal samples from the Tr ans-Atlantic Geotraverse (TAG) hydrothermal mound, 26 degrees N Mid-Atlanti c Ridge. The water content, trace element chemistry, and mineralogy of crys talline silica from 15 different samples have been examined by vibrational spectroscopy and probe microanalysis. The samples are from: shallow subsurf ace ferric iron oxyhydroxide silica deposits (n = 4), a fragment of an acti ve white smoker chimney (n = 1), anhydrite bearing hydrothermal breccias (n = 2), pyrite silica breccias (n = 3), and silicified wall rock breccias (n = 5). Length-fast chalcedony occurs in association with variable quantitie s of ferric iron oxyhydroxides in hydrothermal breccias from the mound flan ks, within shallower subsurface chert samples, and within white smoker chim ney walls. Samples from the anhydrite zone contain textures which are sugge stive of an origin involving replacement of anhydrite, Samples taken from T AG I and 5 from below the anhydrite zone contain no chalcedony. Instead the y contain subhedral quartz crystals which show oscillatory zoning in alumin ium. Two types of crystalline silica namely, type A and type B quartz, are defined on the basis of the infrared spectra in the OH region from 3200 cm( -1) to 3600 cm(-1). The type A quartz occurs beneath the anhydrite zone at TAG 1 and 5. We propose a model that relates specific varieties of crystall ine silica to different thermal and chemical environments within the mound interior, Length-fast chalcedony occurs in an outer low temperature envelop e across the top and sides of the mound. The common association between len gth-fast chalcedony and ferric iron oxyhydroxides suggests that chalcedony crystallization is favoured where catalysis by ferric iron can occur, The a pparent suppression of fibrous silica at the expense of single quartz cryst als with increasing depth is attributed to differing growth rates and degre es of supersaturation of silica-bearing solutions with increasing temperatu re within the mound. The transition from type A to type B single crystal gr owth is interpreted to occur at temperatures approaching similar to 360 deg rees C due to decreasing solubility of aluminium in quartz, so that alumini um is rendered unavailable for type A valence compensation.