Abstract--The oxidation of As(III) to As(V) by K-birnessite was examined at different temperatures, pHs, and birnessite/As(III) ratios. Experiments ranged in duration from 5 to 64 hr, and solution and solid products were determined at several intervals. All experiments showed that the reaction produced large amounts of K + to solution and very little Mn 2+. As(V) was released to solution and incorporated into the K-birnessite. The oxidation was initially rapid and then slowed. The oxidation of As(III) was probably facilitated initially by autocatalytic Mn-As(V) reactions occurring mostly in the interlayer, in which large amounts of As(V) and K + could be easily released to solution. The reaction also slowed when interlayer Mn was exhausted by forming Mn-As(V) complexes. Mn(IV) could only be acquired from the octahedral sheets of the birnessite. The two-stage reaction process proposed here depended on the layered structure of birnessite, the specific surface, and presence of exchangeable cations in K-birnessite.
Abstract--Samples of cuttings from the Borehard A-2 well, Imperial Valley, California, were collected over a measured borehole temperature interval 135* to 275*(2. The < 0.5-/~m (e.s.d.) fraction was separated using high-gradient magnetic separation (HGMS) to create a nonmagnetic fraction rich in illite and a magnetic fraction rich in chlorite. Chlorite was less easily separated from illite in lower temperature samples (< 200"C), presumably due to the presence of polymineralic grains of chlorite and illite. Grains in higher temperature samples were more nearly monomineralic and more easily separated.The chlorite is the IIb polytype. The thickness of coherent scattering domains of chlorite increased until 220*(2 and then remained constant. The amount of 7-/~ interstratified material increased downhole until 195"C and then decreased. Over the same temperature interval, the illite polytypes varied systematically from 1Ma (135" to 175"C) to 1M + 2M~ (230* to 275"C) and coherent scattering domains in the mineral became thicker to about 200"C and then remained constant in thickness. The percentage of iUite in mixed-layer illite/smectite (US) increased from 40% at 135"C to 100% at temperature > 205*(2; ordering in the I/S changes from R0 to R1 between 135" and 155"C, and from R1 to R -> 3 at temperatures >155"C.The concurrent structural changes in chlorite and illite indicate a general improvement in the overall structural order of the clay minerals with increasing temperature. Differences between chlorite and illite suggest that the minerals may have reacted differently to changing conditions or that they may have formed by different mechanisms. The exclusive occurrence of IIb chlorite at temperatures as low as 135"C extends the limit of IIb chlorite stability to temperatures lower than previous estimates.
Abstract--The structure of a highly disordered synthetic birnessite was studied by comparing powder X-ray diffraction (XRD) data with calculated patterns generated by BIRNDIF and WILDFIRE 9 in an attempt to describe the nature of disorder and to estimate the size of the coherent diffracting domains. The material has a turbostratic stacking sequence and coherent diffracting domains that are 25 to 30 ]k on a side in the ab plane (N1 = 5 unit cells, N2 = 10 unit cells) and which average 2.5 unit cells thick parallel to c. Turbostratic stacking probably results because there are few constraints on the relationship between adjacent layers.
Abstract--Since the initial studies of chlorite polytypes, it has been suggested that the stability of the various polytypes may be a function of the temperature at which the mineral formed; however, few studies have been performed in which polytypes of chlorite in a specific suite of samples have been determined and correlated with temperature. A review of the reported occurrences of type I chlorite indicates that other factors, including grain size of the host rock, may be at least as important as temperature in controlling the stability of these polytypes. Results of systematic studies in areas of diagenesis and very low-grade metamorphism suggest that type-It chlorite is stable at temperatures well below 200~ and that it can form as the initial chlorite phase without passing through any intermediate polytypic stages. The conditions under which type-I polytypes occur are somewhat restricted, and cognizance of those restrictions will help to direct future studies of chlorite polytype transformations. These studies should focus on the structural details of polytype transformations; on the relationship of polytype stability to pressure, composition and kinetics; and on experimental calibration of the transformations.
Abstract--The structure of a disordered lib Mg-chamosite was studied using Rietveld refinement techniques and powder X-ray diffraction (CuKa, 18-120" 20 in 0.02* 20 increments). The refinement in space group Ci yielded high precision lattice parameters (a = 5.36982(5)A,, b = 9.3031(9)A, c = 14.2610(6)/~ a = 90.315(5)*, ~/= 97.234(7) ~ 3' = 90.022(9) ~ and atomic coordinates very similar to previous studies. However, the presence of semi-random stacking in this specimen created a situation in which not all atoms could be precisely located: the positions of the octahedral cations and anions which repeat at intervals of +_b/3 could be uniquely determined in three dimensions whereas only the z parameter of the other atoms could be refined. The reasonable appearance of the final model, despite the fact that many of the atom positions could be located in only one dimension, may have resulted because all of the atoms in this structure except 0(5) repeat at intervals which are very nearly +b/3.
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