Lab 6. Petrology of Basaltic Rocks

Sideromelane

Transparent yellow-brown basaltic volcanic glass.

Glomerocryst

A cluster or clot of phenocrysts that grew together.

Skeletal

A crystal habit in which the outline or framework of a mineral is incompletely filled in. Evidence of rapid cooling.

Intersertal

A magmatic texture that resembles intergranular texture, except brown glass is interspersed among the microcrystalline grains.

Diabase

Basaltic rock in which the grain size is marginally phaneritic; typically occurs in dikes and sills.

Palagonite

Chemically altered basaltic glass that consists of a complex mixture of clay and zeolite minerals and hydrated ferric oxides. It is variably orange to brown and is isotropic to weakly birefringent in thin section.

Intergranular

Holocrystalline, microcrystalline texture of magmatic rocks in which randomly oriented crystals of plagioclase, pyroxene, and Fe-Ti oxides form a tight, interlocking mosaic.

Porphyritic

Inequigranular magmatic texture made of two grain sizes; more or less euhedral larger crystals, called phenocrysts, are embedded in a finer grained or glassy matrix.

Vesicular

Magmatic fabric with abundant smooth-walled, more or less spherical cavities, which form by entrapment of a volatile fluid bubble during solidification of the melt.

Trachytic

Magmatic texture in an aphanitic rock expressed by a planar preferred orientation of tabular feldspars.

Ophitic

Magmatic texture in which large clinopyroxenes partially to completely enclose smaller euhedral plagioclases.

Poikilitic

Magmatic texture in which large oikocrysts enclose smaller, randomly-oriented mineral inclusions.

Aphyric

Nonporphyritic aphanitic texture (a fine-grained aphanitic groundmass without larger crystals).

Scoriaceous

Texture of highly vesicular mafic glass.

Aphanitic

Texture that consists of a mosaic of crystals too small to be seen without magnification.

Amygdaloidal

Texture where vesicles are filled with low-T secondary minerals (especially calcite, quartz, and chalcedony) that precipitate from fluid solutions percolating through the rock.