Augen (from German "eyes") are large, lenticular eye-shapedmineral grains or mineralaggregates visible in somefoliatedmetamorphic rocks. In cross section they have the shape of an eye.[1]
Feldspar,quartz, andgarnet are common minerals which form augen.[2]
Augen form in rocks which have undergone metamorphism andshearing. The core of the augen is aporphyroblast orporphyroclast of a hard, resilient mineral such as garnet. The augen grows by crystallisation of amantle of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows.[citation needed]
During shearing, the porphyroblast may rotate, to form a characteristic augen texture of asymmetric shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information.[citation needed]
A metamorphic rock which is clotted with augen is often called anaugen gneiss.[1] A long wall of this augen gneiss can be felt at theMineral and Lapidary Museum ofWestern North Carolina.[citation needed]
