Diallage

Diallage is a textural feature observed in some igneous rocks, particularly in gabbros and norites. It refers to a type of pyroxene, specifically augite or sometimes bronzite, that exhibits a distinctive schiller effect and parting. The schiller effect is an iridescent, metallic luster caused by microscopic inclusions aligned along crystallographic planes within the pyroxene. Parting refers to a planar weakness within the crystal structure, allowing it to break along specific planes that are not true cleavage planes.

In diallage, these features are typically well-developed, leading to large, irregularly shaped pyroxene grains that may appear almost platy or lamellar due to the prominent parting. The parting planes are often oriented parallel to the {100} crystallographic face. The term "diallage" is sometimes used to refer to the pyroxene mineral itself, when it exhibits these characteristic features, rather than just the textural feature.

The schiller and parting in diallage are caused by the presence of fine-grained exsolution lamellae of other minerals, such as ilmenite, magnetite, or orthopyroxene. These lamellae are typically aligned parallel to the crystallographic planes of the host pyroxene. The interference of light with these lamellae produces the characteristic schiller effect. The presence of these exsolution lamellae also contributes to the planar weaknesses responsible for the parting.

Diallage is an important petrographic feature that can provide insights into the cooling history and crystallization conditions of the host igneous rock. The presence and characteristics of the exsolution lamellae within diallage can be used to estimate the temperature and pressure at which the rock formed.