Page 136 - Laboratory Manual in Physical Geology

ACTIVITY ■ Glassy refers to rocks that have no visible grains,
and break along wavy, curved glossy surfaces—just
like a broken glass bottle. An example is obsidian , a
4.3 Rock-forming Minerals dense dark-colored volcanic glass. Another example is

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THINK What are rocks made of, and where and anthracite coal (hard coal), which has a glassy texture
but is not really glass. It is made of plant fragments
About It how do they form? and parts that are so small and compacted together,
that rock just looks glassy. Coal is also opaque in hand
OBJECTIVE Analyze and identify samples of some sample and less dense than true glass.

common rock-forming minerals.
■ Fine-grained refers to rocks made mostly of grains
PROCEDURES that are barely visible and too small to identify even

1. Before you begin, read about Rock Composition when magnified with a hand lend (grains generally
(page 116 and FIGURE 4.3 ). Also, this is what you < 1 mm in diameter).
will need: ■ Coarse-grained refers to rocks made mostly of
___ pencil with eraser grains that are visible and large enough to identify
___ Activity 4.3 Worksheet (p. 123 ) with either a hand lens or your unaided eyes (grains
___ optional: a set of mineral samples (obtained as generally > 1 mm in diameter).
directed by your instructor)
___ optional: a set of mineral analysis tools ■ Vesicular refers to rocks with round or oval holes,
called vesicles , that resemble the holes in a sponge or
(obtained as directed by your instructor)
Swiss cheese. The holes are bubbles of volcanic gases
2. Then follow your instructor’s directions for that bubbled through the lava that cooled to make the
completing the worksheets. rock before the bubbles could escape. Some volcanic
rocks have just scattered vesicles, but pumice is a rock
containing so many vesicles that it floats in water.
The vesicles in pumice are tiny glassy bubbles with
ACTIVITY sharp edges where they break, so pumice is used as an
abrasive in polishes and as a cosmetic exfoliant bar to
4.4 What Is Rock Texture? soften skin (pumice stones, Lava™ soap).

THINK | How are a rock’s composition and ■ Crystalline texture refers to fine- and coarse-
About It texture used to classify it as igneous, grained rocks in which the grains are intergrown
sedimentary, or metamorphic? mineral crystals that glitter when rotated in bright
light. (The light reflects off the flat crystal faces or
cleavage surfaces like tiny mirrors.) The crystals

OBJECTIVE Determine textures of rocks and classify
them based on their composition and texture. may be heterogranular (a mixture of two or more
significantly different sizes) or equigranular (all about
PROCEDURES the same size). The crystals may also be randomly
1. Before you begin , read about rock textures arranged or else foliated —a metamorphic texure in
below, and how rock composition and texture which mineral grains have been aligned or layered,
can be used to determine if a rock is igneous, causing the rock to break or reflect light in a specific
sedimentary, or metamorphic. Also, this is what direction like the layered scales on a fish.
you will need:
■ Clastic texture means that the rock is mostly
___ pencil with eraser made of clasts (fragments; broken pieces) of
___ Activity 4.4 Worksheets (pp. 124 – 126 ) minerals or other rocks (a rock made mostly of plant
___ optional: a set of rock samples (obtained as fragments or broken or separated bones and shells
directed by your instructor) is called bioclastic ). The clasts may be angular —
2. Then follow your instructor’s directions for freshly broken with sharp corners and edges, or
completing the worksheets. rounded —having corners and edges worn down
from transportation and grain abrasion. Recall that
the terms gravel, sand, silt, and clay are also textural
terms. Gravelly rocks are made mostly of gravel
Rock Texture (grains larger than 2 mm; equal to or coarser-grained
than aquarium gravel). Sandy rocks are mostly
Another very important property of rocks is texture —a made of sand (grains 1/16 to 2 mm in diameter,
description of the grains and other parts of a rock and their coarse-grained like sand in a sandbox or a sandy
size, shape, and arrangement. Carefully review the textures beach). Silty and clayey rocks are fine-grained.
below. You will need to identify them in rock samples on The silty rocks are mostly made of grains that you
Worksheet 4.2.

Rock-Forming Processes and the Rock Cycle ■ 117