ABSTRACT
An inexpensive biological microscope can be obtained much more readily than a polarizing microscope and by incorporating two pieces of polaroid in the light path such a microscope may be used for the study of thin sections of rocks provided. Many minerals, although colored in hand specimen, may be nearly colorless in thin section. Some minerals are opaque in thin section and their properties can only be studied with a reflected light microscope. Many minerals break or cleave along certain planes, the positions of which are controlled by the atomic structure of the minerals. The greater the difference between the refractive index of a mineral and its surrounding material the greater its relief. Most common minerals are covered by the range of birefringence shown, except for the carbonates in which the birefringence is nearly 0.18. Whilst not an optical property of the mineral, the grain size of the minerals is crucial to the classification of most rocks.
