BIOTITE

Biotite is a silicate which may be transparent, translucent or opaque. It may be black, dark green, dark brown or red brown. It is a mica mineral and is fairly soft (2.5-3 on the mohs scale).

It is very widely distributed in igneous and metamorphic rocks and is very common in granite and syenite rock formations. It forms where magma has touched another non-magmatic rock. This is known as contact metamorphism. Biotite is known as an index mineral because it's presence provides scientists with information about the make up the rocks that surround it. If rocks contain biotite then it can be concluded that they have undergone a higher degree of metamorphosis than those containing chlorite and no biotite.

It can be found in Brazil, Greenland, Italy, Japan, Norway and California and Colorado (US).

Some of the best samples of biotite may be found on the slopes of Mount Vesuvius.

A form of silica. It is member of the oxide group and is principally made up of SiO2. Pure quartz is colourless or white. Blue quartz obtains it's colour from inclusions (these are not part of the chemical make up). The blue colouring may be due to the presence of dumortiorite (this usually has a uniform colour), sodalite, rutile (especially samples from India) or magnetite (particularly samples from the Apennine mountains in Italy)

It is fairly hard (6.5-7 on the mohs scale) and is also known as sapphire quartz. An important source is at Golling near Salzburg (Austria).

It may occur in all rock types and may also be found in Brazil, India and Arizona, California, Montana and Washington (US).