Metamorphic Textures [MORE]

"-blast"

A suffix indicating a texture of true metamorphic origin

"Blasto-"

A prefix indicating a relict texture

Augen, flaser

Eye-shaped mantled porphyroclasts

Cataclastic

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

Cleavage

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

Continuous foliation

Foliation that is not spaced, but occurs continuously

Corona

An layered rim of one or more minerals surrounding another mineral

Cracked

Crystals showing obvious cracks

Crenulation cleavage

Cleavage or schistosity that becomes microfolded

Crushed

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

Decussate

Randomly oriented interlocking subhedral prisms; tends to form in minerals with strong anisotropy of surface energy, such as mica, pyroxene, or hornblende

Deformation band

Dimensionally elongated bands within a crystal resulting from cracking or slip

Deformation twin

Twinning or lamellae produced by deformation; common in carbonates and recognized in feldspars when the twins/lamellae are bent or dominantly wedge-shaped

Depletion haloes

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

Foliation

Any planar fabric element

Gneissose structure

Layering by metamorphic processes or a poorly developed schistosity

Granoblastic

Equidimensional generally anhedral crystals of approximately equal size with regular polygonal shapes and fairly straight boundaries meeting at triple junctions

Helicitic

Folded internal S, indicates a post-kinematic porphyroblast

Hypidioblastic (subidioblastic)

Subhedral

Idioblastic

Euhedral fabric

Interkinematic

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

Kelyphite

Fine-grained, fibrous intergrowth pyroxene, spinel and amphibole around garnet or olivine

Kink band

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

Layering, banding

Foliation consisting of alternating layers of different composition

Lineation

Any linear fabric element

Mantled porphyroclast

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

Moat

A coronitic rim that completely surrounds a mineral

Mortar

Rounded porphyroclasts rimmed by a crush matrix

Mylonitic

Weakly foliated plastic texture

Nodular

Ovoid porphyroblasts

Oblique foliation

Foliations that cut across the foliation developed due to shear offsets

Poikiloblast

A porphyroblast with numerous inclusions

Polygonal arc

Post-kinematic recrystallization of bent crystals into an arcuate pattern of smaller straight crystals

Polygonized

Incipient recrystallization where larger deformed crystals break down into smaller, undeformed subgrains; the outlines of the larger crystals are still distinguishable

Porphyroblast

A larger metamorphic mineral in a matrix of smaller different ones

Porphyroclast

A large, relict crystal in a fine crush matrix

Postkinematic

Crystals which grew after deformation

Prekinematic

Crystals which were present prior to deformation/metamorphism and were deformed

Pressure shadow

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

Pseudotachylite

Dynamically metamorphosed rock containing glass generated by localized shear melting

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

Quarter structure

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

Relict/Palimpsest

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

Ribbon

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

S-surface

"Foliation or planar fabric in a deformed rock. Commonly numbered (S1, S2, S3, etc.) if there are successively developed foliations;

S0 is reserved for pre-metamorphic foliations, such as bedding, flow banding, cumulate layering, etc"

Schistosity

Planar orientation of elongated mineral grains or grain aggregates produced by metamorphic processes; aligned minerals are coarse enough to see with the unaided eye

Shear band cleavage or S-C texture

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

Shredded

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

Skeletal, web

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

Slaty cleavage

Fine-grained continuous cleavage

Spaced foliation

Foliation developed in zones separated by non-foliated "microlithons"

Spiral (rotational, snowball)

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

Spotted

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

Sutured

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

Symplectite

An intimate intergrowth of two or more minerals that grew simultaneously

Synkinematic

Crystals which grew during deformation/metamorphism

Undulose extinction

Irregular extinction resulting from deformation of the lattice of a crystal

Wrapped

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

Xenoblastic

Anhedral