There is considerable variation in the occupancy of the octahedral sites and some chlorites are extremely Al-rich. The optical orientation diagram at left shows that chlorites can be both monoclinic positive and monoclinic negative. Mg-rich chlorites are optically positive, have low relief and very low birefringence, and are length fast. Al-rich chlorites are optically negative, have higher relief and birefringence and are length slow. When the c-axis lies in the plane of the microscope stage, chlorites change from colorless to various shades of green during stage rotation when observed in plane polarized light.
Chlorites occur in a wide variety of low and medium grade metamorphic rocks.
Chlorite is also a common low temperature alteration product of biotite in igneous rocks. It is common to see biotites in granites with rims of green chlorite . As shown in the diagram at the left, talc has a tri- octahedral mica structure with no interlayer cations. Talc is colorless in thin section and, due to its softness and perfect cleavage, is generally very fine grained. The serpentine group of minerals includes the three phases lizardite , chrysotile and antigorite .