Metamorphic Textyres

Relict

A remnant texture inherited from the parent rock and not yet overprinted or obscured by metamorphism.

Blasto-

A prefix indicating a relict texture (e.g., "blastophitic").

-blast

A suffix indicating a texture of true metamorphic origin.

Idioblastic

Euhedral crystals with well-formed external faces.

Hypidioblastic (subidioblastic)

Subhedral crystals with partially developed external faces.

Xenoblastic

Anhedral crystals with no well-formed external faces.

Porphyroblast

A larger metamorphic mineral in a matrix of smaller ones.

Poikiloblast

A porphyroblast with numerous inclusions.

Skeletal web

An extreme variation of a poikiloblast in which the including mineral occurs as a thin, optically continuous film surrounding the inclusions.

Nodular

Ovoid porphyroblasts.

Spotted

Used to describe the appearance of small (commonly ovoid) porphyroblasts in hand specimen.

Depletion haloes

Zones surrounding porphyroblasts that have had one or more minerals removed by reaction and diffusion to the growing porphyroblast.

Granoblastic

Equidimensional generally anhedral crystals of approximately equal size with regular polygonal shapes (usually 5- or 6-sided) and fairly straight boundaries meeting at triple junctions.

Decussate

Randomly oriented interlocking subhedral prisms.

Symplectite

An intimate intergrowth of two or more minerals that grew simultaneously.

Corona

A rim of one or more minerals surrounding another mineral.

Moat

A coronitic rim that completely surrounds a mineral.

Cataclastic

A general texture resulting from the predominantly brittle deformation of minerals (it typically involves grain-size reduction).

Mylonitic

Foliated cataclastic/plastic texture.

Pseudotachylite

A dynamically metamorphosed rock containing glass generated by localized shear melting.

Undulose extinction

Irregular extinction resulting from deformation of the lattice of a crystal.

Cracked

Crystals showing obvious cracks.

Crushed

Crystals are ground by shear and exhibit fine-grained margins derived from the original larger crystals.

Deformation band

Dimensionally elongated bands within a crystal resulting from cracking or slip.

Porphyroclast

A large, relict crystal in a fine crush matrix.

Mortar

Rounded porphyroclasts rimmed by a crush matrix.

Ribbon

Very elongate crystals (usually quartz) resulting from cataclasis and intense plastic deformation.

Shredded

The intense breakup of minerals (usually phyllosilicates) along cleavages to produce very fine locally crushed margins.

Kink band

Zones bounded by parallel planes in which some feature, usually cleavages, have a different orientation.

Deformation twin

Twinning or lamellae produced by deformation.

Polygonized

Incipient recrystallization where larger deformed crystals break down into smaller, undeformed subgrains.

Sutured

Incipient recrystallization in which the larger crystals differentially incorporate the marginal crush matrix to produce an interdigitation of grain boundaries.

Oblique foliation

Foliations that cut across the foliation developed due to shear offsets.

Shear band cleavage

A texture that has shear bands (or C foliations), which are spaced cleavages that transect or S-C texture well-developed mineral foliation (S foliation) at a small angle.

Mantled porphyroclast

Resistant porphyroclast with a rim that has the same mineralogy as the porphyroclast.

Augen, flaser

Eye-shaped mantled porphyroclasts.

Quarter structure

A structure in which the four quadrants defined by the foliation and its normal are not symmetric.

Quarter fold

Small fold in the foliation around porphyroclasts due to drag.

Quarter mat

Concentration of mica resulting from dissolution of quartz in the shortened quadrants of porphyroclast.

Foliation

Any planar fabric element.

Lineation

Any linear fabric element.

Spaced foliation

A foliation developed in zones separated by non-foliated "microlithons."

Continuous foliation

A foliation that is not spaced, but occurs continuously.

Cleavage

Any type of foliation in which the aligned platy phyllosilicates are too fine to see with the unaided eye.

Slaty cleavage

Fine-grained continuous cleavage.

Crenulation cleavage

A cleavage or schistosity that becomes microfolded.

Schistosity

A planar orientation of elongated mineral grains or grain aggregates produced by metamorphic processes.

Gneissose structure

Either layered by metamorphic processes or a poorly developed schistosity.

Layering, banding

A foliation consisting of alternating layers of different composition.

Prekinematic

Refers to crystals which were present prior to deformation/metamorphism and were deformed.

Synkinematic

Refers to crystals which grew during deformation/metamorphism.

Postkinematic

Refers to crystals which grew after deformation.

Inter-kinematic

Mineral growth that is post-kinematic to one event and pre-kinematic to another.

S-surface

A foliation or planar fabric in a deformed rock.

Pressure shadow

Concentration of a matrix mineral in low-stress areas adjacent to a porphyroblast due to pressure solution (solution transfer) and re-precipitation.

Wrapped

A foliation compressed about a pre-kinematic porphyroblast (or possibly bowed out by the growing porphyroblast).

Helicitic

Folded internal S, indicates a post-kinematic porphyroblast.

Spiral (rotational, snowball)

Showing a texture, such as a spiraled inclusion trail, that indicates rotation.

Polygonal arc

A post-kinematic recrystallization of bent crystals into an arcuate pattern of smaller straight crystals. A form of polygonization.