HRTEM STUDY COMPARING NATURALLY AND EXPERIMENTALLY WEATHERED PYROXENOIDS

The mineralogy and chemistry of both naturally and experimentally weat hered MnSiO3 chain silicate minerals (rhodonite and pyroxmangite) were compared. In natural MnSiO3, high-resolution transmission-electron mi croscope observations reveal that alteration begins at grain boundarie s and planar defects parallel to the silicate chains that represent ju nctions between regions with different chain periodicities. Dissolutio n along these defects results in elongate etch pits that may be partly filled by smectite. Smectite (Ca0.3Mn2.2Zn0.4Al0.1Si4O10(OH)2) also d evelops in larger etches at grain boundaries. The Zn apparently releas ed by weathering of coexisting sphalerite, may facilitate crystallizat ion of manganese-smectite; rhodochrosite is also an initial product. X -ray diffraction patterns from highly altered materials reveal only rh odochrosite and quartz. Simplified reactions are H2CO3(aq) + 4 MnSiO3( s) = Mn3SiO10(OH)2(s) + MnCO3(s) accompanied by 3H2CO3(aq) + Mn3Si4O10 (OH)2(s) = 3 MnCO3(s) + 4SiO2(s) + 4H2O(1). Pyroxenoid dissolution is incongruent under experimental conditions. A 3-7 nm-thick layer of amo rphous silica is present at the mineral surface after approximately 20 00 h of reaction in acidic and near-neutral pH solutions that were und ersaturated with respect to bulk amorphous silica. This thin layer of polymeric silica, which is absent on unreacted grains, is interpreted to have formed largely by incongruent dissolution at the mineral surfa ce as protons in solution rapidly exchange for near-surface Mn. The la yer may also contain silica readsorbed back onto the surface from solu tion. The net result is that silica from the pyroxenoid is redistribut ed directly into reaction products. Upon aging in air for a year, leac hed layers partially recrystallize. Both natural and experimental reac tions produce secondary products by direct modification of the pyroxen oid surface. Manganese does not change oxidation state in the early st ages of weathering in either setting. Unlike orthosilicates, compositi onal variations exert only a secondary control on chain silicate disso lution rates. For all chain silicate minerals, depolymerization of the silicate anion probably limits overall dissolution rates. As the thic kness of the modified layer increases, rates may be further suppressed by diffusion (through the leached surface in the case of experimental reactions, and through secondary minerals in the case of natural weat hering). The rates for wollastonite are exceptional in that the minera l dissolves more rapidly than other chain silicates and because leachi ng reactions are more pronounced. Natural surface modification reactio ns appear to be distinctive in that they occur in the presence of high er concentrations of metal cations. Clay mineral formation may be prom oted by periodic drying.