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

Hypidioblastic (subidioblastic) Subhedral

Xenoblastic Anhedral

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 Polygonal (polygonal mosaic) 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. Best displayed in monomineralic rocks (marble, quartzite). Poymineralic rocks approach this, but are modified by the differences in degree of idioblastic form.

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

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. The proportion of crush material can vary considerably and even comprise 90% of the rock.

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. Common in carbonates and recognized in feldspars when the twins/lamellae are bent or dominantly wedge shaped.

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

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. The mantles are assumed to be derived from the porphyroclast by grinding.

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. Aligned minerals are coarse enough to see with the unaided eye.

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. 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.

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.