Laboratory Manual for Introductory Geology 4e

can reveal aspects of its history, such as what agents of metamorphism were involved
and how intense those agents were.
7.4.1 How to Determine if a Rock Is Metamorphic
Texture and mineralogy are the keys to recognizing that a rock is metamorphic. In
some cases, texture or mineralogy alone will point to metamorphism, whereas in
others, it is necessary to consider both. As with igneous rocks, we first note a rock’s
most visible property—its texture—and then identify the minerals it contains.
STEP 1 Observe the texture.
Foliated or lineated textures: If a rock is foliated or lineated (see Fig. 7.1c), it
must be metamorphic, because stress causes these preferred mineral orientations,
and metamorphic rock is the only class of rock in whose formation
stress plays a role.
Gneissic banding: The alternating bands of light and dark minerals in gneiss
(FIG. 7.4) are characteristics of high-grade metamorphism. They are different
texturally from the depositional layering of sedimentary and fragmental igneous
rocks in that (1) the grains are not cemented clasts or compacted pyroclastic
debris, but rather interlock with one another, and in that (2) one set of bands is
typically foliated or lineated (as in Fig. 7.4a, in which the dark bands are strongly
foliated biotite flakes). They also differ mineralogically in that the grains in gneiss
are interlocking grains of metamorphic minerals.
Nonfoliated textures: Some metamorphic rocks contain interlocking grains
with no preferred orientation. For some, this is because stress was not one of
the agents of their metamorphism, so neither protolith grains nor new minerals
could be aligned. For others, stress was indeed a metamorphic agent, but
their grains were neither platy nor rod-like, and were therefore unable to show
preferred alignment (FIG. 7.5). You’ve seen that interlocking grains also occur
in igneous rocks and in crystalline sedimentary rocks, so an interlocking
texture alone cannot prove or disprove a metamorphic origin. For that, it is
necessary to identify the minerals the rock contains.
STEP 2 Identify the minerals present. Some minerals form almost exclusively during
metamorphism, so their presence immediately identifies a rock as metamorphic;
unfortunately, most of these minerals are not usually found in student
mineral sets. You have heard of some—andalusite, sillimanite, and kyanite—
even if you’ve never seen them before. Other metamorphic minerals, such as
muscovite and biotite, can not only form during metamorphism, but can also
FIGURE 7.4 Two examples of gneissic banding in which light and dark minerals have been separated.
(a) Pink potassic feldspar layers (unfoliated) separated by foliated
biotite-rich layers.
(b) Similar to (a), but light layers are quartz and gray-white feldspar.
7.4 STUDYING METAMORPHIC ROCKS
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