1.2 Examine the Rocks

It can be difficult to identify minerals in hand samples of metamorphosed ultramafic rocks, particularly at low grades, because of the fine grain size and lack of color contrast among the minerals. Even the relatively small percentage of iron in these rocks makes some of the minerals green to black in color. Nevetheless, it is a good idea to examine hand samples with thin sections to get a sense of the appearance of metamorphosed ultramafic rocks in the field. You can see a gallery of the contact metamorphosed serpentinite ("cmSerpentinite") hand samples by clicking this button:

Hand Sample Gallery

Most of the minerals you can observe in the thin sections of these contact aureole rocks have reasonably distinctive optical properties relative to one another.

• anthophyllite (Ath): colorless-pale green, low second order retardation, intermediate relief, prismatic crystals    Ath Info
• antigorite (Atg): colorless, first order retardation, low relief, sheet silicate texture
• diopside (Di): colorless-faint green, second order retardation, intermediate relief, stubby prismatic crystals
• enstatite (En): colorless, upper first order retardation, high relief, stubby anhedral crystals
• forsterite (Fo): colorless, upper second order retardation, high relief, anhedral crystals
• talc (Tlc): colorless, third order retardation, low relief, sheet silicate textures
• tremolite (Tr): colorless, low to mid second order retardation, intermediate relief, prismatic crystals

The opaque mineral is primarily magnetite. Chlorite (low first order or anomolous colors) is found in many of the rocks (some Al is present) and talc may occur as a retrograde mineral in some samples. Ti-rich clinohumite is present in one sample, but looks a lot like the olivine. Count it as olivine for this exercise. You can see a gallery of the thin section by clicking this button:

Thin Section Gallery

Click here when you are ready to name the mineral assemblage for each sample:

Name the Assemblages