porphyroblast: a grain that has grown larger than others (typical examples are garnet, staurolite, chloritoid, andalusite). The following image shows chloritoid (a hydrous Mg-Fe-Al silicate typical of low-grade pelites)--notice the characteristic i) tabular shape, ii) sector zoning, and iii) growth in a fine-grained (thus low-grade) rock

Garnet porphyroblasts

garnet from the Tobacco Root Mountains of Montana

poikiloblast: a grain with inclusions of other minerals--also called ''sieve'' texture (typical examples are garnet, staurolite, cordierite, andalusite). The image shows staurolite (a hydrous Mg-Fe-Al silicate typical of medium-pressure metapelites)-- the characteristic - yellow color

Poikiloblasts:cordierite (a hydrous Mg-Fe-Al silicate typical of low-pressure metapelites), with its abundant inclusions, low birefringence, and polysynthetic

porphyroclast: a relict grain that is larger than others surviving in a deformed rock. The key difference between porphyroblast and porphyroclast is that porphyroblasts are bigger than surrounding grains because of growth, whereas porphyroclasts are bigger because surrounding grains have undergone grainsize reduction during deformation. The image show K-feldspar grains

Porhyroclasts: K-feldspar grains

Porhyroclasts: K-feldspar grains

Garnet Gneiss

Sigma clasts can be used to determine sense of shear. This sigma clast from Norway shows sinistral (left-lateral) shear

This is one of the most unusual sigma clasts known (it is an enormous pyrite grain)

limestone �> marble (the characteristic - high birefringence and twinning)

Textural Changes with increasing metamorphism, the protolith undergoes increasing recrystallization and fabric change. quartz arenite �> quartzite (notice the characteristic low birefringence)

a metaquartzite

a metaconglomerate

ultramafic rock �> serpentinite

shale �> slate �> phyllite �> schist �> gneiss �> melt. Shale is a sedimentary term. Slate refers to a fine-grained, low-grade metasedimentary rock in which bedding is typically still visible

Schist refers to a medium- to high-grade, medium- to coarse-grained metamorphic rock of any composition that has a fabric

Gneiss refers to a high-grade, coarse-grained metamorphic rock of any composition that has a fabric

Complex and blobby plutonic complexes can become extremely layered as a result of deformation

lineation: a linear fabric; typically defined by either linear minerals, such as hornblende, or stretched grains, such as quartz. Rocks with purely linear fabrics are called L tectonites and form by a constrictional deformation (prolate strain ellipsoid). Here is a mylonitic granite from Norway, with clearly stretched K-feldspar grains in a fine-grained matrix of quartz, plagioclase, and biotite

up close

The cleavage can be very planar, like in this slate from the Sierra Nevada foothills - Mariposa slate

The cleavage can be wavy, like in this Tibetan rock

In favorable circumstances one can determine sense of shear

the micas that form foliations and cleavages are clearly visible -foliation, shist

crenulation cleavage: a special kind of cleavage that forms as a result of shortening at a low angle to a pre-existing cleavage

gneissic: foliation of equant minerals such as feldspar and quartz. Most commonly used to describe high-grade metaplutonic rocks such as granulite gneisses.A Rapikivi granite being turned into a gneiss (Norway).

pseudomorph: replacement of one phase by another, generally preserving shape. This Norwegian rock shows chlorite pseudomorphs after garnet -pseudogarnet

idioblastic: sharp crystal form (euhedral in igneous terminology). The mineral shown is sphene (CaTiSiO5), with its characteristic high relief and extreme birefringence -sphene

hypidioblastic: intermediate form (subhedral in igneous terminology). The mineral shown is garnet ((Mg,Fe,Ca,Mn)3Al2Si3O12), with its characteristic high relief and optical isotropy -porphyroblast

Porphyroblast

xenoblastic: no clear crystal form (anhedral in igneous terminology). The image on the left shows calcite (CaCO3), with high birefringence and twinning, and the image on the right is of plagioclase ((NaAlSi3O8-CaAlSi2O8), with low birefringence and twinning

Granoblastics