The James River Park System - Richmond, VA

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North Geology Tour

Stop 6b: Made of Minerals

DIRECTIONS: Look at some pieces of granite. Notice the different colors of the rock. (37.52959° N, 077.45529° W)

Petersburg Granite is made of 3 MINERALS. Look very closely and try to find:

  1. BIOTITE - shiny black mica specks that look like glitter
  2. QUARTZ - clear or yellow crystals that look like foggy glass
  3. FELDSPAR - color varying (pink or yellow-orange-tan-white), large bits often have squared edges

MINERALS occur in nature as inorganic solids. Each type of mineral has a specific chemical makeup and crystalline structure.

Different combinations of minerals form different types of rock. Petersburg Granite is composed primarily of the minerals you just identified: BIOTITE, QUARTZ, and FELDSPAR. As the granite is weathered by the three agents of erosion (WIND, WATER and ICE), feldspar and mica become clay, and quartz becomes sand.

Minerals may be more visible in granite that is rough and weathered as opposed to granite that has been smoothed by the river.

close up of granite

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