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Phyllite Photomicrograph of thin section of phyllite (in cross polarised light) Fractured Duke stone showing phyllitic texture Phyllite. Phyllite (/ ˈ f ɪ l aɪ t / FIL-yte) is a type of foliated metamorphic rock formed from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation. [1]
Pyrophyllite occurs in phyllite and schistose rocks, often associated with kyanite, of which it is an alteration product.It also occurs as hydrothermal deposits. Typical associated minerals include: kyanite, andalusite, topaz, mica and quartz.
A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them.
The slatey cleavage typical of slate is due to the preferred orientation of microscopic phyllosilicate crystals. In gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as S-tectonite in sheared rock masses.
Phyllic alteration typically forms in the base-metal zone of a porphyry system. [2] Alteration assemblages vary with depth and with degree of fluid interaction. In deep environments, the most highly altered areas are veins and thin selvages, or halos, that surround them.
It is this crystal habit that gives astrophyllite its name, from the Greek words astron meaning "star" and phyllon meaning "leaf". Its great submetallic gleam and darkness contrast sharply with the light matrix the mineral is regularly found within. Astrophyllite is usually opaque to translucent, but may be transparent in thin specimens.
It is often found in minute micaceous aggregates (with individual plates barely visible), as a major component of chromium rich phyllitic or schistose metamorphic rocks of the greenschist facies. Verdite is a type of metamorphic rock made mostly of an impure, often multicolored, variety of fuchsite. It is used for ornamental carvings.
The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt , andesite , and rhyolite . Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur.
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