Laboratory Manual for Introductory Geology 4e

FIGURE 7.8 Varieties of quartzite.
(a) Pink quartzite.
(b) The original clast outlines are visible in
this raw specimen of quartzite.
(c) Crystalline quartzite (orthoquartzite).
or beige. Impurities in the protolith (clay, quartz,
feldspar) may react to form unique metamorphic
minerals.
Quartzite: A fine- to medium-grained, nonfoliated, very
tough rock formed by metamorphism of silica-cemented
quartz sandstone. Typically white or gray, but
green and red varieties are also common (FIG. 7.8).
Outlines of individual clasts may be visible (as in Fig.
7.8b), depending on original grain size and degree
of recrystallization, but at high grades of metamorphism,
quartz cement and quartz clasts recrystallize
and can no longer be distinguished.
Metasandstone: A nonfoliated rock that retains the bedding
and other characteristics of its sandstone protolith
(FIG. 7.9). Its minerals may include quartz,
plagioclase, and potassium feldspars (from protolith
clasts) as well as biotite and muscovite (produced
from clay-rich protolith matrix). Primary sedimentary
features, such as graded bedding and load casts,
a type of bedding structure formed by sinking sediments,
may be preserved.
Hornfels: A term that is sometimes applied to nonfoliated
metamorphic rocks that contain platy minerals.
By this usage, the metasandstone in Figure 7.9 would
be called hornfels, as would the metamorphosed
mudstone in the same figure.
Anthracite: A black, shiny rock formed by metamorphism
of bituminous coal. Anthracite, which is also
called “hard coal,” commonly exhibits conchoidal
fracture and may resemble obsidian (FIG. 7.10), but
is softer and, because it is nearly pure carbon, less
dense.
FIGURE 7.9 Metasandstone.
Outcrop showing metasandstone (light color) interbedded with
metamorphosed mudstone layers (dark color).
FIGURE 7.10 Anthracite.
7.4 STUDYING METAMORPHIC ROCKS
185