Thin Section Textures Quiz

IDIOMORPHIC GRANULAR

Having a majority of euhedral grains. Rare. ▪ Also known as panidiomorphic.

HYPIDIOMORPHIC GRANULAR

Consisting predominantly of subhedral grains. Common in many granitic rocks. ▪ Also occurs in gabbroic rocks.

ALLOTRIOMORPHIC GRANULAR

Having a majority of anhedral grains (common)

PORPHYRITIC

Approximately bimodal size distribution. ▪ Microporphyritic: having a porphyritic texture that is visible only under the microscope

GLOMEROPORPHYRITIC

A porphyritic texture in which phenocrysts are clustered into aggregates called glomerocrysts or crystal clots. ▪ The reason for the adhesion is probably the low free energy of specific faces on which the minerals collide and attach, for example (010) twin interfaces in plagioclase

POIKILITIC

The state of a host phenocryst (oikocryst) containing many inclusions of other minerals.

OPHITIC

Large pyroxene grains enclosing small, random plagioclase laths.

SUBOPHITIC

Plagioclase laths that are larger and only partially enclosed by the pyroxene

INTERGRANULAR

Small, discrete grains of pyroxene, olivine, etc., filling the interstitial space in a random network of larger plagioclase laths

INTERSERTAL

Glass, cryptocrystalline material, or alteration products occupying the spaces between plagioclase laths

HYALOPILITIC

A large amount of glass, with plagioclase occurring only as tiny, random microlites. ▪ Microlite is a minute crystal that is visible only under the microscope

GRAPHIC

An intergrowth in which the guest shows angular wedge like forms. ▪ A long-favored origin involves simultaneous crystallization of quartz and feldspar from a melt; this process has been confirmed experimentally by Fenn (1986) and MacLellan and Trembath (1991). ▪ Micrographic: graphic texture that is visible only under the microscope

MYRMEKITE

Intergrowth of vermicular (“wormy”) quartz in a sodium-rich plagioclase host typically in contact with K-rich alkali feldspar. ▪ Origin is not certain, but we know it is not primary. ▪ May represent deformation induced replacement of K feldspar by quartz and sodium-rich plagioclase.

PERTHITIC

Irregular veins, patches, lenses, etc., of sodic plagioclase in an alkali feldspar host. Usually results from exsolution

SERICITIZATION

Sericitization is the process by which felsic minerals (usually feldspars of feldspathoids in igneous rocks) are hydrated to produce sericite. ▪ Sericite is a term applied to any very fine-grained white mica

CHLORITIZATION

Chloritization is the alteration of any mafic mineral to chlorite. Chlorite is a very hydrous phyllosilicate and typically replaces the less hydrous mafics at low temperature when water is available. ▪ Pyroxenes, hornblendes, and biotites are commonly observed in thin section in various stages of alteration to chlorite

VESICULAR

Igneous rocks with a vesicular texture contain air bubbles, called vesicles. These can be round or distorted. ▪ In thin-section, vesicles are filled with the mounting medium and consequently appear clear and colorless with low relief under plain polarized light

TRACHYTIC

Consisting of (feldspar) microlites aligned due to flow

CORONA

A mantle of mineral grains surrounding a core of another partially consumed mineral. The rim is the product of an incomplete reaction between the core and neighboring phases

SIEVE

Crystals filled with channelways due to dissolution.

Specific to Mafic Igenous Rocks

Ophitic, subophitic, intergranular, intersertal, hyalopilitic,

Specific to Intergrowths

Graphic, Myrmekite, Perthitic

Specific to Replacement

Sericitization, Chloritization

Idiomorphic Granular- specific image is dunite

Hypidiomorphic Granular- specific image is gabbro

Allotriomorphic Granular- specific image is granite

Porphyritic- specific image is andesite

Glomeroporphyritic- specific image is pyroxene and plag in basalt

Poikilitic- specific image is olvine inclosed by augite

Ophitic

Subophitic

Intergranular

Intersertal

Hyalopilitc

Graphic- usually occurs with quartz and alkali feldspar

Myrmekite

Perthitic

Sericitization

Chloritization

Vesicular

Trachytic

Corona

Sieve

Order of Crystallization (Bowen)