Page 220 - Laboratory Manual in Physical Geology

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FIGURE 7.12 Hornfels. Hornfels is a fine-grained, nonfoliated
metamorphic rock having a dull luster and a microcrystalline
FIGURE 7.11 Marble. Marble is a fine- to coarse-grained,
nonfoliated metamorphic rock with a crystalline texture formed texture (that may appear smooth or sugary). It is usually very hard
and dark in color, but it sometimes has a spotted appearance
by tightly interlocking grains of calcite or dolomite. Marble forms
by intermediate- to high-grade metamorphism of limestone or caused by patchy chemical reactions with the metamorphosing
magma or hydrothermal fluid. Hornfels forms by contact
dolostone. Marble is a dense aggregate of nearly equal-sized
crystals (see photograph), in contrast to the porous texture and/or metamorphism of any rock type.
odd-sized grains of its parent rock.

Photomicrograph ( 26.6)
Original sample width is 1.23 mm

Quartz sand grains

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FIGURE 7.13 Quartzite. Quartzite is a medium- to coarse-grained, nonfoliated metamorphic rock consisting chiefly

of fused quartz grains that give the rock its sandy texture . Compare the fused quartz grains of this quartzite sample (see

photomicrograph) with the porous sedimentary fabric of quartz sandstone in FIGURE 7.2 . Sand grains can often be rubbed
from the edges of a sandstone sample, but never from quartzite (because the grains are fused together).
Metamorphic Rocks, Processes, and Resources ■ 195