If the vibration directions of a mineral are oriented parallel to the polarizer (and analyser), no light passes the analyser and the mineral appears black, or extinct, viewed under crossed nicols.  This occurs four times as an anisotropic mineral is rotated through 360 degrees (if an optic axis is not vertical, in which case the mineral appears black continuously during rotation).  The angle at which extinction occurs, relative to cleavage or crystal habit, can be useful in identifying some minerals.  Certain minerals go extinct when oriented with their cleavage or crystal faces parallel to the polarizer or analyser directions, such as the muscovite mica shown in A and B.  In A (ppl), a prominent grain of muscovite at the centre of the image has its {001}edges oriented north-south, parallel to the analyser and in B (x-nicols), this grain is extinct and the grains with their {001} planes at other angles are not extinct.  This is referred to a parallel extinction (parallel to the cross hairs, which should indicate the directions of the polarizer and analyser).  In C and D, the mineral augite is oriented with its {110} cleavage north-south, and it is not extinct (D).  Augite has inclined extinction.  In some minerals, such as calcite and lawsonite, extinction occurs in positions in which intersecting  cleavage surfaces, or crystal outlines, are bisected by the cross hairs.  This  property is known as symmetric extinction.  Extinction angle is not considered in minerals that do not have appropriate features, such as outline or cleavage, relative to which extinction can be measured.  Quartz, for example, has no cleavage and rarely has crystal faces, so its extinction angle is not considered an important property.  Uniaxial minerals (those in the hexagonal and tetragonal systems) and biaxial  minerals in the orthorhombic system can exhibit parallel extinction.   All fields of view are 2.2 mm across.  A and C ppl, B and D x-nicols.