Kerolite

Abstract: Mg-silicate minerals (e.g., stevensite, kerolite, talc, sepiolite) play an important role in the construction of facies models in lacustrine and peri-marine environments because they are sensitive to changes in solution chemistry. However, the response of Mg-silicate mineralogy to changing aqueous chemistry is only broadly understood because the mechanisms underpinning the co-precipitation of Mg 2+ and SiO 2 (aq) from surface water, and subsequent Mg-silicate crystallization, are unclear. Here we describe the results of experiments designed to systematically examine the effects of pH, Mg/Si and salinity of the parent solution on the nature of initially precipitated products. Structural interrogation of the products with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and thermal analysis (TGA/DTA) allow comparison of synthetic products with naturally occurring crystalline counterparts. In general, Mg 2+ and SiO 2 (aq) co-precipitation and nucleation of Mg-silicate layer structures first involves the rapid formation of 2:1 layers with trioctahedral occupancy and a mean coherent X-ray scattering domain between 1–2 unit cells with respect to the c axis. Well defined but diffuse hk reflections indicate two-dimensional growth, turbostratic stacking and highly variable interlayer hydration. Diffuse reflectance FTIR shows numerous structural similarities with stevensite, kerolite and sepiolite. However, TGA/DTA analysis indicates the presence of variable kerolite/stevensite interstratification not readily detectable through XRD analyses, as well as a significant degree of surface and interlayer hydration (e.g. 15–20 wt.%). We observe a number of clear trends in the products with respect to solution chemistry. For example, at low salinity, kerolite-like products dominate at high Mg/Si and high pH, whereas sepiolite-like products are formed at lower pH and lower Mg/Si. At high salinity and high Mg/Si, stevensite-like products are favoured at high pH and kerolite-like products dominate at lower pH, whereas a decrease in Mg/Si of the solution leads to sepiolite-like products at low pH and only stevensite-like products at high pH. Higher pH leads to an increase in octahedral vacancies which favour stevensite-like products; this may result from a higher rate of two-dimensional tetrahedral sheet expansion relative to the octahedral sheet, as inferred from studies of silica oligomerization and brucite growth kinetics. Together, our results indicate that the neoformation of Mg-rich silicates from solution may often begin with the rapid nucleation of hydrated 2:1 layers. Subsequent dehydration leads to progressive layer stacking order and could occur in response to wetting/drying cycles, prolonged exposure to high salinity solutions, or burial and heating. The surface and interlayer water associated with these products is undoubtedly an important source of diagenetic water in Mg-silicate-bearing successions, and the chemistry of this water upon later diagenesis should be a focus of future investigation.

Abstract: The mineralogy and crystal chemistry of “garnierites” in saprolitic ore from the Goro lateritic nickel deposit, New Caledonia, was investigated using optical and scanning electron microscopy, X-ray diffraction (XRD) and electron-microprobe analyses. These conspicuously, green-coloured phases occur either as sub-mm to cm-sized veins or as macroscopic (sub-cm sized) “booklets”. Veins comprised ~ 10 A (2:1) talc-like minerals identified as species of the Ni-kerolite/pimelite Ni for Mg solid solution series, with Ni contents ranging from 10 to 24 % NiO. Mineral nomenclature, defined by the inverse relationship between Ni and Mg content, varied continuously over the scale of several hundred micrometres. Pimelite, defined as containing > 1.5 Ni per formula unit (p.f.u.), was the main 10 A phase for one vein core whereas at the vein edges where the Ni content decreased to < 1.5 Ni p.f.u. (and Mg increased) kerolite was identified. “Booklets” comprised the ~ 7 A (1:1) serpentine-like phase, nepouite (Mg0.67Ni1.59Fe0.053+) (Si2.17Al0.22)O5(OH)4 with Ni contents averaging 30 % NiO and occurred as accordion-like structures supported in an undifferentiated matrix of mixed ~ 7 A and ~ 10 A phases.

Abstract: The composition, texture and genetic evolution of kerolite and related Mg clays belonging to the Intermediate Unit of the Miocene from the Madrid Basin have been studied. About 400 samples from the Esquivias deposit were analysed by several mineralogical and/or chemical techniques. Two genetic pathways for the development of Mg clays during early diagenesis have been observed: (1) mudflat environment: Al-smectite (beidellite) → Mg-smectite (saponite); and (2) palustrine environment: Si-Mg gel → kerolite → kerolite-stevensite → stevensite. In the mudflat deposit the transformation processes predominate, whilst in the palustrine environment, kerolite is neoformed, probably from a gel-like medium. Stevensite seems to have originated from the transformation of mixed-layer kerolite-Mg-smectite, but also through neoformation at a later stage. The textural features, isotopic data and sedimentary evolution within each lithofacies are indicative of shallowing-upward sequences with development of palaeosols. A post-sedimentary origin for sepiolite, calcite, authigenic quartz, zeolites and baryte is inferred.