HRTEM study comparing naturally and experimentally weathered pyroxenoids

Abstract

The mineralogy and chemistry of both naturally and experimentally weathered MnSiO 3 chain silicate minerals (rhodonite and pyroxmangite) were compared. In natural MnSiO 3 , high-resolution transmission-electron microscope observations reveal that alteration begins at grain boundaries and planar defects parallel to the silicate chains that represent junctions between regions with different chain periodicities. Dissolution along these defects results in elongate etch pits that may be partly filled by smectite. Smectite (Ca 0.3 Mn 2.2 Zn 0.4 Al 0.1 Si 4 O 10 (OH) 2 ) also develops in larger etches at grain boundaries. The Zn apparently released by weathering of coexisting sphalerite, may facilitate crystallization of manganesesmectite; rhodochrosite is also an initial product. X-ray diffraction patterns from highly altered materials reveal only rhodochrosite and quartz. Simplified reactions are H 2 C O 3 ( a q ) + 4 M n S i O 3 ( s ) = M n 3 S i 4 O 10 ( O H ) 2 ( s ) + M n C O 3 ( s ) accompanied by 3 H 2 C O 3 ( a q ) + M n 3 S i 4 O 10 ( O H ) 2 ( s ) = 3 M n C O 3 ( s ) + 4 S i O 2 ( s ) + 4 H 2 O ( 1 ) Pyroxenoid dissolution is incongruent under experimental conditions. A 3–7 nm-thic layer of amorphous silica is present at the mineral surface after ∼ 2000 h of reaction in acidic and near-neutral pH solutions that were undersaturated 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 surface as protons in solution rapidly exchange for near-surface Mn. The layer may also contain silica readsorbed back onto the surface from solution. The net result is that silica from the pyroxenoid is redistributed directly into reaction products. Upon aging in air for a year, leached layers partially recrystallize. Both natural and experimental reactions produce secondary products by direct modification of the pyroxenoid surface. Manganese does not change oxidation state in the early stages of weathering in either setting. Unlike orthosilicates, compositional variations exert only a secondary control on chain silicate dissolution rates. For all chain silicate minerals, depolymerization of the silicate anion probably limits overall dissolution rates. As the thickness 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 weathering). The rates for wollastonite are exceptional in that the mineral dissolves more rapidly than other chain silicates and because leaching reactions are more pronounced. Natural surface modification reactions appear to be distinctive in that they occur in the presence of higher concentrations of metal cations. Clay mineral formation may be promoted by periodic drying.