Biotite dissolution processes and mechanisms in the laboratory and in nature: Early stage weathering environment and vermiculitization

Abstract

Despite the relatively small abundance of biotite [7.6% of the exposed continental crust surface (Nesbitt and Young 1984)], biotite weathering is important because it is a source of K for plant nutrition and a major source of Fe and Mg in ground water (e.g., Berner and Berner 1996).The dissolution of minerals is generally explained by protonation, deprotonation, and the subsequent detachment of metal species (e.g., Stumm 1992) based on laboratory experiments under "far from equilibrium" conditions. Surface complexation and its dependence on pH describe the dissolution of sheet silicates [see review by Nagy (1995)]. The crystal structure char-* ABSTRACT Biotite dissolution in the laboratory and in nature was examined and compared to elucidate certain aspects of the weathering processes. Batch dissolution experiments of fresh biotite [(K 0.91 Na 0.01 )(Mg 0.40 Fe 2.07 Mn 0.05 Al 0.14 Ti 0.19 )(Si 2.82 Al 1.18 )O 10 (OH) 2 ] in granite were carried out at 150 ∞C for 1 to 56 days. Examination by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) revealed that, in the early stage, dissolution proceeds from the edges of crystals inward and secondary minerals such as Fe oxide are precipitated mostly at the edges, with only a few secondary minerals found on the basal surfaces. A dissolution experiment using a mixture of biotite and muscovite, done at 150 ∞C for 7 days, indicated that hematite crystals formed mostly at the edges of biotite but not on muscovite. This observation suggests that released Fe is precipitated before it diffuses into the bulk solution. Because a dissolution rate at the edge is larger by two orders of magnitude than at the basal surface (Turpault and Trotignon 1994), precipitation before diffusion of dissolved elements to bulk solution well explains the preferential secondary mineralization at the edges in the laboratory experiments. SEM-EDS of fresh to slightly weathered biotites revealed that early stage weathering proceeds in the same way as in the laboratory with the edges being preferentially weathered and secondary minerals being precipitated mostly at the edges. Because of the similarity between the occurrence of secondary minerals in the laboratory experiments and in nature, the laboratory results elucidate the early stage weathering conditions, namely that (1) supersaturation with respect to secondary minerals in a solution occurs around biotite, i.e., the solution is poorly connected to a main flow pathway of water, and (2) once supersaturation is achieved, secondary minerals are precipitated mainly at the edge before some released elements diffuse into the solution. The immobility of water immediately adjacent to primary minerals partly explains the large difference in dissolution rate between the laboratory and natural samples.A phlogopite dissolution experiment was done at 150 ∞C for 5.5 days to examine the effect of biotite composition on vermiculitization. High-resolution transmission electron microscopy revealed that vermiculite layers ar...

Influence of pH and temperature on the early stage of mica alteration

“…Pits have circular form and appear randomly distributed over the surface. This confirms previous work on silicate alteration (Zuddas and Michard, 1993;Murakami et al, 2002;Aldushin et al, 2006), where dissolution effects on the surface state were found from the very early stages of the process. However, the relationship between the measured dissolution fluxes and surface changes is unclear.…”

Analytical Investigations of Disintegrated Granite Surface from the Un-finished Obelisk in Aswan

“…Th ese symptoms are aff ected by several factors such as time; rate and degree of hydrolysis cycles, ageing conditions dominated pH and chemical environments, as reported by (Taylor, 1987), where all Fe oxides can form under humid conditions in all climatic zones as reviewed before (Segalen, 1971), or may be present as an essential component of kaolin group (Newman and Brown, 1987). In our case, Fe oxides are precipitated mostly at the edges of stone surfaces as a direct result of natural weathering and dissolution processes aff ecting some iron bearing compounds such as biotite grains especially with the presence of some of severe oxidation and hydration cycles, where air and water break down ferrous silicate as biotite and convert their ferrous iron to ferric oxide Red spots, thus changing the original colors to red or brown ones as discussed before by (Murakami et al, 2003;Murakami, et al, 2004). Furthermore, iron which was expelled from the biotite lattice was not removed from the grain in early stages of weathering and interlamellar crystallization of iron oxyhydroxide could take place.…”

Diavik Waste Rock Project: Evolution of Mineral Weathering, Element Release, and Acid Generation and Neutralization during a Five-Year Humidity Cell Experiment

“…The weathering of pyrrhotite and biotite in the schist produced a visible secondary mineral phase (ocherous precipitate) in the Type III cells after the pH dropped below 4.5, although this precipitate dissolved at a later date. The precipitate likely was jarosite ([KFe 3 (SO 4 ) 2 (OH) 6 ]; Equation (6)), which is indicative of the dissolution of biotite (K and additional Fe source), Fe 2+ to Fe 3+ oxidation, and an acidic environment [6,[51][52][53][54][55]. Formation of jarosite is an indication of incongruent dissolution of biotite (release of more easily removed elements such K).…”