Structural sericite

The interpretation of lithogeochemical data from basaltic structural domes is complicated by lithological changes associated with the transition from basalt to overlying siliciclastic rocks, as well as by the polydeformed nature of the host sequence. Ferroan carbonate alteration is well developed, and low-level Au enrichment extends for a considerable distance away from zones of economic interest. Arsenic and Sb/AI anomalies are restricted to within approximately 10 m of mineralized zones. Sericite alteration is indicated by Na depletion and K enrichment in basalt within 20 to 40 m of mineralized zones. A number of other elements, including Mn, P, S, Zn, Mo, Cu, Se and Ba, are variably enriched within the rocks hosting Au mineralization, but it is not clear whether elevated concentrations of these elements are a product of syn-sedimentary exhalative activity or result from later hydrothermal alteration. 

Eberl, D.D., Srodon, J., Lee, M., Nadeau, P.H., Northrop, H.R. 1987. Sericite from the Silverton caldera, Colorado Correlation among structure, composition, origin and particle thickness. American Mineralogist, 72, 914-934. 

In drill hole GP07-92, a strong VIRS response to sericite and chlorite occurs in host rocks up to 15 m structurally above a quartz vein intercept, whereas a sample of lithic greywacke, 5 m below a ca. 3 m wide quartz veined zone (22.2-25.9 m) yields... 

Clay minerals Kaolinite, illite, sericite, clay minerals with mixed-layer structure, Tonstein — Illite, chlorite... 

The type phengite suggested by Foster (1956) has a composition almost identical to that of the Belt illite (No.8). When the H20 content is appreciably higher and the K20 content lower than muscovite, the minerals have been called hydromicas or hydromuscovites. The excess H20 in some instances is present as interlayer water, particularly in the trioctahedral hydrobiotites. Table XII contains a selection of sericite and hydromuscovite analyses and Table XIII the structural formulas. The H20 and K20 values of these minerals are similar to those reported for the illite minerals however, the MgO content of the sericites and hydromuscovites is lower and the NazO contents higher than for the illites (Table XIV). 

There exists a group of minerals that is structurally similar to the micas, but contain less potash and more combined water than the latter. These materials are called illites, but have also been called hydrous micas or sericites. 

Soil micas exhibit a wide variety of compositional, structural, and morphological features. This is also reflected by the nomenclature used in soil mineralogy. Hydrous mica, micaceous clay, sericite, illite, and degraded illite are some of the terms used, partly synonymously for the general description of fine-grained micas in soils, and partly with special reference to particular deviations from the ideal mica structure. 

Several studies of the synthesis of mixed-layer minerals have been made. Tomita and SuDO [1968] have used HCl and H2SO4 treatment on a preheated 2M sericite and obtained a regularly interstratified allevarditelike structure, which on MgCl treatment, gives mica-montmorillonite. Ross and Kodoma [1970] have studied the release of K " from interstratified mica clays treated with tetraphenyl boron and 0.1 N BaCl2, finding that the rate was lower when the Si/AF ratio of the layer approached that of mucsovite. 

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