F. MICAS AND OTHERS

Introduction
Micas are a diverse mineral group of wide geologic occurrence, but they are unified by certain common structure characteristics. They are sheet structures, in which each silica tetrahedron shares the oxygen ions of one of its triangular bases with other tetrahedra.

Muscovite, biotite, talc, chlorite, and serpentine are representative of the large and complex group of sheet silicates, with muscovite and biotite belonging to the mica subgroup of the sheet silicates. This gives them their excellent and characteristic cleavage. Muscovite is characteristic of metamorphic rocks but is found in all igneous and detrital sedimentary rocks as well. Biotite is common in all the above rock types. Talc, chlorite, and serpentine are common in metamorphic rocks (low grade) and may be found as alteration products in igneous rocks. I will give you a handout that discusses the different micas.

Muscovite BH-250-14 (x) and BH250-20 ( x)
Biotite BH-250-26 (x)
Talc BH250-5 (x)

Epidote BH250-28 (x)
Minerals of the epidote group of which epidote is the most common member, are complex Ca- Al silicates. Epidote is a common primary constituent of low grade, regional metamorphic rocks and of metamorphosed carbonate rocks. In igneous rocks it occurs in granular aggregates calcium-rich plagioclase or calcic mafic minerals. It also occur as veins or segregation of almost pure epidote in metamorphic or plutonic rocks.

Sphene: CaTiSiO4 (OH, F)
Sphene is a relatively easy mineral to identify because it commonly forms diamond-shaped crystals of very high relief having a brown or red-brown color. The crystals are pleochroic and simple twinning is common

Sillimanite, BH-250-1 and BH250-10
Andalusite, BH250-40
Kyanite BH250-30
Sillimanite, andalusite, and kyanite are the aluminosilicate polymorphs. Kyanite and sillimanite occur in high grade gneiss and schist. Andalusite is characteristically found in contact metamorphosed shale and low grade schist.

Staurolite BH-250-26, BH250-20, BH-250
Staruolite is an iron rich alumina silicate. It is a mineral of metamorphic rocks and occur in schist, slates and gneiss where it can be associated with garnet, kyanite, and tourmaline.

Rutile
Rutile is a widely distributed accessory mineral in various metamorphic and igneous rocks. It also occurs as a detrital mineral. Commonly assoicted with sphen.

Some good questions that will help you learn minerals that might look alike:
When you look at a new mineral you should be thinking of ways you can differentiate it from other minerals you know that may have somewhat similar optical properties. To help you thinking along this here are a few questions to answer.

1. What would be a possible way to distinguish muscovite from talc in thin section? Consider characteristic growth habits as well as optical properties.

2. If the talc and/or muscovite in your slide was too fine grained to determine optical properties, would it be helpful to have the hand specimen? Why.

3 Biotite can sometimes show green rather brown colors. How would you differentiate such biotite from chlorite?

4. Both chlorite and serpentine can occur as fine grained aggregates replacing pre-existing minerals. What might be the best way to distinguish between the two in such a situation? Be sure to look at the appropriate Tables (or better yet, the slides to refresh your memory about these two similar minerals).

5. At first glance epidote might resemble a clinopyroxene. What are the three ways to distinguish between these minerals?

6. Stauralite is a fairly distinctive mineral with its golden yellow to colorless pleochroism, straight extinction, high relief, and moderate birefringence. It does, however, bear some resemblance to iron-rich olivine. What are two good ways to differentiate the two minerals.

7. In the slides of andalusite most of the crystals have their Z- axis perpendicular to the slide and are suggestive of enstatite. In such an orientation how can you convince yourself that this is andalusite? In what rock type is andalusite growing?

8. How can you differentiate kyanite from andalusite? and from sillimanite?

9. List all the other minerals that you can identify in the thin-sections of Kyanite.

10. List all the other minerals that you can identify in the thin-sections of Sillimanite.

11. Examine the slides of Coridierite. How can you distinguish cordierite from quartz (1,2,3,4,5,6) ?

12. Examine the slides of Chondrodite and Humite. How can you tell these minerals from Olivines?