Pyroxene
The pyroxenes are the most important group of rock-forming ferromagnesian silicates. They are a group of minerals that are closely related structurally, in physical properties, and in chemical composition, even though they crystallize in two different systems; orthorhombic and monoclinic. In all species of the group, the fundamental and common form is the prism. There are good cleavages parallel to the prism faces.
The chemical composition of the pyroxenes can be expressed by the general formula (W,X,Y)2Z2O6, in which W, X, Y, and Z indicate elements having similar ionic radii and capable of replacing each other within the structure. In the pyroxenes, these elements may be:
The proportion of W atoms is generally close to 1 or 0. Of the X group, manganese is generally present in minor amounts, and Li occurs as a major constituent only in spodumene (LiAlSi2O6). Of the Y group, Ti is present only in minor amounts, replacing Al and Fe+3. Z is generally Si; in natural pyroxenes.
The following table gives the names that have been applied to the common members of the group:
Orthorhombic
Monoclinic