Cleavage, the breaking of a mineral along crystallographically controlled planes, is observable in thin section as well as hand specimen.  In fact, in the process of thin section preparation, cleavage may be enhanced.   In A, the characteristic {110} cleavage of pyroxene (orthopyroxene in this sample) intersects at approximately right angles in the two large grains (with their c axes approximately perpendicular to the plane of the stage) at the bottom of the image.  The trace of the {110} cleavage appears as parallel lines in the grains at the top of the image, which have their c axes approximately parallel to the stage.  In B, the {110} cleavage of amphibole intersects at about 56 and 124 degrees, viewed down the c axis.  The perfect {001} basal cleavage of the phyllosilicates (e.g., micas) is shown in C, in which the cleavage appears as closely spaced parallel lines in grains of magnesian chlorite.  In D, the {001} cleavage of plagioclase feldspar (this cleavage produces the “record groove” effect in hand specimens) appears as fine, sub-horizontal lines that deflect across the {110} Albite Law twin planes.