Page 164 - Laboratory Manual in Physical Geology

A CTIVIT Y 5.3 Estimate Rock Composition

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. Estimate mafic color index (MCI) for each rock below, using a Visual Estimation of Percent Chart (cut from GeoTools 1 or
2 at the back of the manual ). For the rocks with visible crystals, determine the percent of each mineral by point counting
along the line provided and by using the Visual Estimation of Percent Chart. Based on all methods, determine the rock’s
compositional group. All of these rocks are shown at their actual size (x1).

Mafic Color Index (MCI): Mafic Color Index (MCI):

Compositional Group: Compositional Group:

Mafic Color Index (MCI): Mafic Color Index (MCI):

Percent Based Percent Based Percent Based Percent Based
Mineral on Visual on Point Mineral on Visual on Point
Estimation Counting Estimation Counting
Plagioclase feldspar Potassium feldspar
Hornblende Plagioclase feldspar
Quartz
Compositional Group:
Biotite

Compositional Group:

B. REFLECT & DISCUSS
1. Which method do you think is the best one to use for determining the rock’s compositional group? Why?

2. Is there any benefit to using two or all three methods to determine the rock’s compositional group? Why?

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