Peridotite - Morphology and Texture

Morphology and Texture

Some peridotites are layered or are themselves layers; others are massive. Many layered peridotites occur near the base of bodies of stratified gabbroic complexes. Other layered peridotites occur isolated, but possibly once composed part of major gabbroic complexes. Both layered and massive peridotites can have any of three principal textures: (1) rather well formed crystals of olivine separated by other minerals. This probably reflects the original deposition of olivine sediment from magma. (2) Equigranular crystals with straight grain boundaries intersecting at about 120°. This may result from slow cooling whereby recrystallization leads to a minimization of surface energy. (3) Long crystals with ragged curvilinear boundaries. This probably results from internal deformation.

Many peridotite occurrences have characteristic textures. For example, peridotites with well-formed olivine crystals occur mainly as layers in gabbroic complexes. "Alpine" peridotites generally have irregular crystals that occur as more or less serpentinized lenses bounded by faults in belts of folded mountains such as the Alpines, the Pacific coast ranges, and in the Appalachian piedmont. Peridotite nodules with irregular equigranular textures are often found in alkaline basalts and in kimberlite pipes. Some peridotites rich in amphibole have a concentric layered structure and form parts of plutons called Alaskan-type zoned ultramafic complexes.