D. INOSILICATES:
Two of the very large and important families of rock-forming minerals-the pyroxenes and amphiboles-constitute the bulk of the inosilicate group. Members of this group are often called Chain silicates because the silicate tetrahedral are linked so as to form one-dimensionally "infinite' chains; that is the chains extend along one crystallographic direction without interruption throughout an entire crystal. The morphological result is that crystals tend to be elongated along the direction of the chains, which in all of them has been designated the c axis.

There are two types of chain silicates; these are single chains and double chains as shown in figure--. All of the double-chain silicates belong to the amphibole group and the single chain belong to the pyroxene group.

Pyroxenes BH-250-9, BH-250-6, BH-250-
Pyroxenes are divided into two groups based on the system in which they crystallize.
Pyroxenes belong to the chain-structure group of silicates. There are about 11 minerals that belong to the pyroxene group. These are divided in to two subgroups, the ortho-pyroxenes (OPX), and clino-pyroxenes (CPX). Those that are monoclinic are called Clinopyroxenes (CPX) and those that are orthorhombic, Orthopyroxenes (OPX). Orthopyroxenes (OPX) are a solid solution series of minerals with the general formula (Mg,Fe)SiO3. Clinopyroxenes (CPX) are more complex and are of the form XYZ2O6 where most commonly X= (Ca, Na), Y= (Mg, Fe, Al), and Z= (Si, Al). Although they can appear similar looking, the OPX's and CPX's have optical properties which makes them distinguishable.

Pyroxenes have two sets of cleavages that intersect at about 89 degrees you may not see these two cleavages but you will see at least one in most sections.
Pyroxenes are prominent constituents of igneous rocks and metamorphic rocks.
One important optical property in telling OPX from CPX is extinction angle.
CPX's have extinction angle that ranges from 6o to 50o on the (010).
OPX's have parallel extinction in most sections.



Comparison of Olivines and Pyroxenes
Examine these minerals in plane polarized light. Write down the significant differences in the properties of these two mineral groups. Pay attention to relief, cleavage, color, extinction angle, 2V, birefringence, texture and anything else you observe. Some slides contains both olivine and pyroxene for close comparison. Examine these slides carefully and list as many ways as possible in which you can tell these two minerals from each other.

Clinopyroxenes (CPX) vs Orthopyroxenes (OPX)
Distinguishing OPX and CPX is best accomplished by using 2V and extinction angle. Explain how you would choose a grain appropriate for measuring extinction angle (your book will help here).

There are many slides that show good euhedral pyroxenes, look at these minerals and become familiar with their appearance in thin sections. While looking at these slides be sure to note other types of minerals present, many of which you should be familiar with.

A. Find a slide that shows good Euhederal pyroxene (OPX or CPX) and draw the following sketche of the pyroxene you observe.

i. Perpendicular to the C-axis.
ii. Label the principal optic directions in each of the above views.
iii. Show the angle between cleavages.
iv. Show the angle between optic directions.
v. Give Miller indices for all cleavages and crystal faces shown.
The handout could be helpful. Refer your book for more information.