# 7.2: Activity 7A - The Mafic Color Index (MCI)

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For this section you will need igneous rock samples selected by your instructor. Locate these samples before you begin.

Take a close look at the rocks; how are they different? Do they reflect light? Are they dull looking? Have small shiny surfaces? Are they extremely shiny and smooth? Have holes?

As a first attempt at classifying igneous rocks separate the samples into three groups based on their overall color (dark, light or intermediate). Light colors typically include beige, white and pink. Whereas dark colors typically include black, dark gray, and green. Intermediate colors are typically gray or black and white. (Hint, think back to the mineral unit. Minerals like pyroxene and amphibole were black or dark green, while quartz and K-spar were much lighter in color).

1. Select the correct sample provided by your instructor. Indicate the color index (CI)?

1. 0-15% CI (felsic)

1. 16-45% CI (intermediate)

1. 46-85% CI (mafic)

1. >85% CI (ultramafic)

1. Using the same sample from above, examine the pink colored minerals present. Which mineral is this?

1. Quartz

1. Biotite

1. Ca-rich plagioclase

1. Potassium feldspar

1. Using the same sample from above, examine the small, dark colored minerals present. Which mineral is this?

1. Quartz

2. Biotite

3. Ca-rich plagioclase

4. Potassium feldspar

1. Select another sample provided by your instructor. Indicate the color index (CI)?

1. 0-15% CI (felsic)

1. 16-45% CI (intermediate)

1. 46-85% CI (mafic)

1. >85% CI (ultramafic)

1. Using the same sample from above, examine the white colored minerals present. Which mineral is this?

1. Quartz

1. Biotite

1. Ca-rich plagioclase

1. Potassium feldspar

1. Select the last sample provided by your instructor. Indicate the color index (CI)?

1. 0-15% CI (felsic)

1. 16-45% CI (intermediate)

1. 46-85% CI (mafic)

1. >85% CI (ultramafic)

1. Using the same sample from above, examine the green-colored mineral present. Which mineral is this?

1. Quartz

1. Biotite

1. Pyroxene

1. Olivine

1. According to Bowen’s Reaction Series, this sample should start melting at what temperature?

1. ~2400 °F

1. ~2000°F

1. ~1500 °F

1. ~1000 °F

This page titled 7.2: Activity 7A - The Mafic Color Index (MCI) is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Chloe Branciforte & Emily Haddad (ASCCC Open Educational Resources Initiative) .