Textures and Fabrics in thin section

Reading 11

texture

The size, shape, and arrangement of the grains in a rock (also used to refer to specific sizes/shapes/arrangements of minerals)

Fabric

The geometric configuration of minerals, e.g., alignments of minerals to form lineations or schistosities

Porphyroblastic

One large grain in a sea of smaller grains. Garnet is common.

Porphyroblast

a large mineral crystal that grew during the metamorphic event. 

Porphyroclast

a large mineral crystal that was a piece of the original rock that got metamorphosed

Poikioblastic

Porphyroblast shows original rock beneath it

Schistosity

All mineral aligned, usually defined by mica and/or quartz.

Crenulation Cleavage

Two directions of alignment, again usually defined by mica and/or quartz. Superimposed foliations from two or more stress events.

Pressure Shadows

Minerals growing along the “shadows” of a large mineral in a deformed rock. The large minerals are usually things like feldspar or garnet – something sturdy that resists strain. The newly-grown minerals are often mica or quartz.

Augen texture

Feldspar porphyroblasts. Look like “eyes”

Retrograde minerals

One mineral replaces another mineral after metamorphic event ends. Looks like one mineral is “eating” another. Very common to see mica and/or chlorite eating garnet, for example.

Exsolution Lamellae

Two minerals separate during formation, are physically trapped together (usually CPX/OPX or plag/alkali feldspar)

Perthitic Feldspar

Exsolution in plagioclase / alkali feldspar system

Sericitic Feldspar

Tiny fuzzy mica mats that form from hydrothermally altered feldspar

Zoning

 happens when the chemical composition of a magma changes as the mineral crystallize —> concentric rings of color