Page 51 - Petrophysics

GLOSSARY (CHAPTER 1) 25

Fissility: the property of breaking along thinly spaced sheets, or planes,
parallel to the depositional bedding orientation.
Foliation: directional property of metamorphic rocks caused by layered
deposition of minerals.
Lithification: the process of changing accumulated unconsolidated
sediments into a rock. The grains are compacted by the overburden
sediments and cemented by deposition (from interstitial water) of silica,
calcite, clays, iron oxide, and other minerals, between the grains.
Hardness 0: arbitrary scale of approximately equal steps between
and
numerical hardness numbers, except for 9 to 10, which is a very
large step (the hardness value is followed by a mineral that represents
that value): 1 -Talc, 2-Gypsum, 3-Calcite, 4--Fluorite, 5--Apatite,
6 -Orthoclase, 7 -Quartz, 8 -Topaz, 9- Corundum, 1 0- Diamond.
The minerals 1-3 can be scored by a fingernail, 4-5 by a copper penny,
5-6 by a knife or piece of glass, 6-8 by a piece of quartz, but 9-10 cannot
be scored by any of the above.
Igneous rocks solidify from a melt, or magma. They are classified
according to texture and mineralogy; however, they are not uniform in
either composition or texture. A homogenous magma produces a variety
of chemically different rocks by the process of fractional crystallization,
or differentiation. Igneous rocks that are rich in light-colored mineral
are generally referred to as felsic because they contain a relatively large
amount of feldspar. Composition and texture (grain size) are used for
classification. The common groups of rocks fall into various steps in
the differentiation of a basaltic magma according to the Bowen series.
Igneous rocks occur in two ways: intrusive (below the surface) and
extrusive (on the surface). The source is magma from the upper part
of the mantle.
Luster: reflection of light by a clean surface.
Metamorphic rocks form as a result of a new set of physical and
chemical conditions being imposed on pre-existing rocks. Metamorphic
rocks differ significantly in mineralogy and texture. Most are regional
and related to orogenic events. The naming of metamorphic rock is
based principally on textural features, but some names are based on
composition. Most have distinct anisotropic features: foliation, lineation,
and rock cleavage.
Obsidian: a dark-colored, or black, essentially non-vesicular volcanic
glass. It usually has the composition of rhyolite.
Pegmatic: having crystals greater than 1 cm.
Porphyritic: named for the texture of the matrix. Porphyritic basalt
is fine-grained dark rock, with inclusions of large crystals. Porphyritic
granite is coarse-grained granite with much larger crystals imbedded in it.