Laboratory Manual for Introductory Geology 4e

FIGURE 4.3 Typical grain shapes.
(a) Garnet crystals in metamorphic rock.
(b) Halite crystals in sedimentary rock.
(c) Rounded grains.
(d) Angular grains.
or cubes, like the garnets in FIGURE 4.3a or the halite crystals in FIGURE 4.3b, are
equant. Those that resemble rods, sheets, or ovals, or have an irregular form, like
those in FIGURE 4.3c and d, are inequant. Next, we look at to see if the grains are
rounded, angular, or some combination of these shapes. Finally, we ask, are all grains
the same shape, or do they vary depending on mineral type or on position within the
rock? Each of these characteristics tells us something about the formation of the rock.
4.3.1c Relationships among Grains During the formation of most igneous and
metamorphic rocks, and during some sedimentary rock–forming processes, crystallizing
minerals interfere with one another and interlock like a three-dimensional
jigsaw puzzle (FIG. 4.4a, b), creating what we refer to as a crystalline texture. A very
different texture results from sedimentary processes that deposit grains and then
cement them together (FIG. 4.4c, d). Such sedimentary rocks are said to have a
clastic texture, and individual grains in these rocks are called clasts.
4.3.1d Grain Orientation and Alignment The orientation of grains relative to one
another often provides a key clue to the classification of a rock. For example, in most
igneous rocks, the inequant grains are randomly oriented in that they point in a
variety of directions. In many metamorphic rocks, however, inequant minerals, such
as platy micas or elongate amphiboles, are oriented parallel to one another. Thus,
when looking at a rock, you should ask, are the inequant grains aligned parallel to
one another, or are they randomly oriented throughout the rock? If the grains are
94 CHAPTER 4 MINERALS, ROCKS, AND THE ROCK CYCLE