Laboratory Manual for Introductory Geology 4e

FIGURE 4.6 Flowchart for determining rock classification.
What is the rock’s texture?
1. Rock has glassy texture
Rock is IGNEOUS.
(smooth, shiny, no grains).
No minerals (glass)
Rock is IGNEOUS.
2. Rock has porous
texture (numerous
holes).
Look at material
between grains.
Nonsilicate minerals
Silicate minerals
Rock is SEDIMENTARY.
Rock is IGNEOUS.
3. Rock has interlocking
grains.
Identify
minerals.
Nonsilicates
Foliated
Halite, gypsum
Calcite, dolomite
Rock is SEDIMENTARY.
SEDIMENTARY*.
Rock may be
METAMORPHIC*.
Rock is METAMORPHIC.
Silicates
Nonfoliated
Metamorphic minerals
(garnet, staurolite, etc.)
Rock is
METAMORPHIC.
No metamorphic
minerals
Rock is
IGNEOUS.
* Look for metamorphic minerals. Are interlayered rocks in the field sedimentary or metamorphic?
4. Rock consists of visible grains separated by a matrix of finer-grained material.
(a) Matrix is glassy.
(b) Matrix is interlocking crystals.
(c) Matrix is small, rounded grains that do not interlock.
(d) Matrix is too fine grained to identify.
5. Rock is very fine grained:
Identify matrix
minerals.
Rock is IGNEOUS.
Silicate mineral matrix
Nonsilicate mineral matrix
Rock is IGNEOUS or
METAMORPHIC*.
Rock is SEDIMENTARY.
*If metamorphic minerals are present, then the rock is
metamorphic.
Rock is SEDIMENTARY.
Rock could be SEDIMENTARY or IGNEOUS. Use microscope.
i. Use microscope to observe relationships among grains if thin section is available.
ii. Use field observations to determine relationships with rocks of known origin.
iii. Estimate specific gravity to get an indication of what minerals are present.
their textural features are clearly visible. Fine-grained rocks can be more challenging
to classify, and as you examine the rocks in your study set, you might want to create
a fourth category: “too fined grained to be entirely sure.” For these rocks, geologists
turn to thin sections like that shown in Figure 4.4b. We cut a small chip off a rock, glue
it onto a microscope slide, and grind away most of the chip until it is so thin that light
can pass through it. Under a microscope, the aptly named thin section reveals grain
relationships invisible to the naked eye, allowing us to distinguish igneous and sedimentary
rocks by their interlocking crystalline or clastic textures, respectively.
104 CHAPTER 4 MINERALS, ROCKS, AND THE ROCK CYCLE