Page 221 - Laboratory Manual in Physical Geology

Chlorite

Pyrite
1 porphyroblast
1 (brassy cube)
FIGURE 7.14 Anthracite coal. Anthracite is a fine-grained,
nonfoliated metamorphic rock, also known as hard coal (because it FIGURE 7.15 Porphyroblastic texture. This texture is
characterized by large, visible crystals of one mineral occur
cannot easily be broken apart like its parent rock, bituminous or soft
coal). Anthracite has a smooth, homogeneous, glassy texture and breaks in a fine-grained groundmass of one or more other minerals.
This medium-grained chlorite schist contains porphyroblasts
along glossy, curved (conchoidal) fractures. It is formed by low- to
intermediate-grade metamorphism of bituminous coal, lignite, or peat. of pyrite (brassy metallic cubes) in a groundmass of chlorite.
The rock can be called porphyroblastic chlorite schist or pyrite
chlorite schist or pyrite greenschist.

■ Porphyroblastic texture —an arrangement of large ACTIVITY
crystals, called porphyroblasts , set in a finer-grained
groundmass ( FIGURE 7.15 ). It is analogous to porphy- 7.3 Hand Sample

ritic texture in igneous rocks.
Analysis, Classification,
■ Hydrothermal veins —fractures “healed” (filled) by and Origin
minerals that precipitated from hydrothermal fluids
(see FIGURE 7.5 ).
THINK How are rock composition and texture
■ Folds —bends in rock layers that were initially flat, About It used to classify, name, and interpret
like a folded stack of paper (see FIGURE 7.4 ). metamorphic rocks?
■ Lineations —lines on rocks at the edges of foliations,

shear planes, slaty cleavage, folds, or aligned crystals. OBJECTIVE Determine the names, parent rocks
(protoliths), and uses of common metamorphic rocks,
based on their textures and mineralogical compositions.
Classification of Metamorphic Rocks
PROCEDURES
Metamorphic rocks are mainly classified according
1. Before you begin , read about Description and
to their texture and mineralogical composition.
Interpretation of Metamorphic Rock Samples.
This information is valuable for naming the rock
Also, this is what you will need :
and determining how it formed from a parent rock
(protolith). It is also useful for inferring how the ___ Activity 7.3 Worksheets (pp. 202–204 ) and pencil
metamorphic rock could be used as a commodity for ___ optional: a set of metamorphic rock samples
domestic or industrial purposes. You can analyze and (obtained as directed by your instructor)
classify metamorphic rocks with the aid of FIGURE 7.16 , 2. Then follow your instructor’s directions for

which also provides information about parent rocks completing the worksheets.
and how the metamorphic rocks are commonly used.

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