Weathering of hornblende, plagioclase and chlorite in meta-dolerite, Australia

Geology and Characterization of Two Hydrothermal Nontronites from Weathered Metamorphic Rocks at the Uley Graphite Mine, South Australia

“…Remnants of hornblende grains typically show highly etched but clean surfaces, essentially free of smectite (Figure 7). Similar observations of amphibole dissolution have been reported by Berner and Schott (1982), Anand and Gilkes (1984), and Velbel (1989) and were interpreted as reflecting a relatively open hydrologic regime, A more closed system might produce smectite replacement of amphibole or pyroxene (e.g., Eggleton, 1975;Banfield and Barker, 1993). At Uley, the pattern of NAu-2 distribution is consistent with brittle fracturing of the amphibolite and infiltration of fluid that preferentially dissolved hornblende (and pyroxene?)…”

“…Ca-rich amphibole, of composition approximating alumino-ferro-hornblende (EDX analyses), was usually restricted to the central portion of rock fragments, away from areas of high nontronite concentration. Most grains examined showed extensive dissolution via crystallographically controlled etch pits (Figure 7) resulting in cavernous voids lined with sharp dentate projections, a common characteristic of advanced amphibole weathering (e.g., Berner and Schott, 1982; A n a n d and Gilkes, 1984;Velbel, 1989). Although clay products were observed as thin coatings on the surface of some amphibole grains, voids formed by dissolution were typically clean and free of any obvious secondary crystallization products (Figure 7).…”

Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing

“…In contrast to the hornblende crystals in the corestone and in the rindlet zone, hornblende crystals in the protosaprolite zone are significantly smaller and dramatically etched ( Figure 6). Similar sawtoothed etching along weathered hornblende cleavage planes has been reported by others (e.g., Berner and Schott, 1982;Anand and Gilkes, 1984;Velbel, 1989). Hornblende grains are extremely difficult to find under SEM above the rindlet-saprolite interface, although XRD patterns indicate that hornblende persists in the first 2 cm of the saprolite, but not beyond.…”

“…Hornblende is the most abundant Fe-silicate in the bedrock, but the mechanism and location of hornblende weathering has not been previously identified. Pseudomorphic replacement of hornblende by clay minerals such as chlorite, chlorite-saponite, or saponite has been documented in soil and saprolite (Wilson and Farmer, 1970;Anand and Gilkes, 1984), but no widespread evidence for such phase changes was observed in the present samples. Similarly, dissolution of hornblende and re-precipitation of iron (oxy)hydroxides, gibbsite, or kaolinite can occur in soil, saprolite, and weathering rinds ( e.g., Velbel, 1989), but was not observed in the rindlet samples: i.e., we observed no etching of the hornblende grains, no boxwork texture or precipitates associated with (Figure 4d).…”

Weathering of Hornblende to Ferruginous Products by a Dissolution-Reprecipitation Mechanism: Petrography and Stoichiometry

“…Dissolution-reprecipitation mechanisms have been invoked on petrographic grounds to explain several occurrences of pyribole weathering (e.g., Berner and Schott, 1982;Glasmann, 1982), serpentinization (e.g., Cressey, 1979), feldspar weathering (e.g., Grant, 1963Grant, , 1964Koppi and Williams, 1980;Anand and Gilkes, 1984;Anand et aL, 1985), and pseudomorphous clay replacements after plagioclase feldspar, which appear to preserve twinning in clay pseudomorphs (Velbel, 1983). Transport of ostensibly "immobile" elements in dissolved form is also required to account for the composition of pseudomorphous weathering products of ferromagnesian silicates.…”