First find of ferropseudobrookite in quartz from Napier Complex, East Antarctica

Ferropseudobrookite exsolution rods, approximately Mg²⁺_0.11Fe²⁺_0.70Al³⁺_0.04Fe³⁺_0.34Ti⁴⁺_1.81O₅, space group Cmcm with a about 0.98 nm, b about 0.37 nm, c about 1.00 nm, are first reported in quartz from a garnet-orthopyroxene-bearing quartzofeldspathic gneiss, Napier Complex, East Antarctica. The elongated direction of ferropseudobrookite rods is the b axis and is parallel to either the <101> or <111> of low-quartz. Moreover these rods have almost six-fold symmetry along the c axis of quartz. The formation of the exsolution must result from the cooling of the gneiss and the exsolution took place in the high-quartz field. The present exsolution phenomenon suggests that quartz can dissolve not only Ti but also Fe²⁺ and Fe³⁺ and that the gneiss contains Fe²⁺ ion under the ultrahigh-temperature peak metamorphic conditions.