Metamorphism kaolinites

Because of its simple chemical composition, the role of kaolinite in clay mineral assemblages is peripheral to or a limiting case for chemiographic representation of clay mineral systems which contain free silica forms as a compositional pole. Most often during epi-metamorphism kaolinite is incorporated into other phases due to a displacement of the bulk... 

For various reasons, gibbsite and kaolinite are really the only minerals of the "neutral lattice" type which occur with any frequency in non-metamorphic rocks. These minerals are formed in soils, most noticeably from granitic rocks but are also commonly found forming from basic rocks during the weathering process (Millot, 1964 Tardy, 1969). The... 

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

Koons PO, Zeitler PK, Chamberlain CP, Craw D, Meltzer AS (2002) Mechanical links between erosion and metamorphism in Nanga parbat, Pakistan Himalaya. Am J Sci 302 749-773 Kutzbach JE, Guetter PJ, Ruddiman WF, Prell WL (1989) Sensitivity of climate to Late Cenozoic uplift in southern Asia and the American West Numerical Experiments. J Geophys Res 94 18,393 — 18,407 Lawrence JR, Rashkes Meaux J (1993) The stable isotope composition of ancient kaolinites of North America. In Climate Change in Continental Isotopic Records. Swart, PK, Lohmann KC, McKenzie J, Savin S (eds) Geophysical Monograph 78 249-261... 

In an exploration area in northern Peru, igneous rocks cover an area of several square km within a metamorphosed series of quartzites and limestones. The younger sequence consists of ignimbrites, tuffs and tuffites of approximately 1000 m thickness. The metamorphism produced skams, which are connected with the ore body. A porphyry Cu deposit occurs in the metamorphosed rocks and is characterised by intense hydrothermal alteration (quartz-kaolinite-sericite) which makes determination and classification of magmatic and sedimentary rocks at the surface very difficult. A zone of propylitic alteration can locally be followed up to 500 m into the andesitic-dioritic wall rocks. 

Fig. 14.8. The system AI2O3 — S1O2 — H2O and some of the phases stable under low grade metamorphic conditions. If water exists as a phase, only corundum+kaolinite or quartz+kaolinite can coexist. Three-mineral assemblages preclude the existence of pure water as a phase. Chemical potentials and mole fractions of water along the section A-B are shown in Figure 14.10.

The features of the adsorbed complex of the Cambrian argillites were intensely transformed by diagenetic reactions, making them unsuitable for any interpretations. The complex composition of the clays which consist of kaolinite, illite, chlorite and mixed-layer minerals, together with the absence of organic matter leads to the conclusion that the areas of denudation must have been of a local nature. At the same time, we may reconstruct a temperate to humid climate interrupted at times by periods of relative aridity. This explains the lack of laterites on the eruptive and metamorphic rocks. 

An important difference between the two subgroups is that kaolinites (and related minerals) are secondary minerals, known to form rather easily from weathering of feldspars, and are stable in ambient conditions (which explain its abundance). On the other hand, serpentines are not so stable, found in rocks as metamorphic alteration products, and usually have not been observed to form under low-temperature conditions, thus are more akin to primary minerals. 

The feldspar minerals have similar physical properties and often occur as prismatic or tabular crystals in igneous rocks, or as more anhedral grains in metamorphic and sedimentary rocks. They are colourless when fresh but are more commonly white due to incipient alteration impurities or inclusions result in coloured varieties, with green-brown alkali feldspars found in some metamorphic rocks, and orthoclase commonly found as pink. The surfaces of feldspar crystals are often iridescent due to twinning on a microscopic scale, with labradorite characterised by blue surface iridescence. Feldspars readily alter under hydrothermal action or chemical weathering to form members of the clay minerals group (. v.). Sodium-rich feldspars commonly decompose to form montmorillonite, in the presence of limited water, or to kaolinite with excess water alkali feldspars typically form illite or kaolinite sub-group (qq.v.) clay minerals (Deer et al, 1992 Rutley, 1988). 

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