Environmental Engineering Reference
In-Depth Information
Microcrystalline (microphanerites) have grains that can be perceived but are too
small to be distinguished.
Porphyries are phanerites with large conspicuous crystals (phenocrysts).
Aphantic (aphanites) contain grains too small to be perceived with the unaided
eye.
Glassy rocks have no grain form that can be distinguished.
Pyroclastic rocks are grouped as follows:
Volcanic breccia are the larger fragments which fall around the volcanic vent and
build a cone including:
Blocks — large angular fragments
Bombs — rounded fragments the size of an apple or larger
Cinders — which are the size of nuts
Tuff is the finer material carried by air currents to be deposited at some distance
from the vent, including:
Ash — the size of peas
Dust — the finest materials
Fabric
Igneous rocks generally fall into only two groups:
Equigranular , in which all of the grains are more or less the same size
Porphyritic , in which phenocrysts are embedded in the ground mass or finer
material (the term refers to grain size, not shape)
Grain Shape
Grains are described as rounded, subrounded, or angular.
Structure Nomenclature
Continuous structure is the common form, a dense, compact mass.
Vesicular structure contains numerous pockets or voids resulting from gas
bubbles.
Miarolitic cavities are large voids formed during crystallization.
Amygdaloidal refers to dissolved materials carried by hot waters permeating the
mass and deposited to fill small cavities or line large ones, forming geodes.
Jointed structure is described in Section 6.4.
Characteristics
Photos of some of the more common igneous rocks are given as Plates 5.1 to 5.10. The char-
acteristics of igneous rocks are summarized in Table 5.7.
5.2.4
Sedimentary Rocks
Origin
Soil particles resulting from the decay of rock masses or from chemical precipitates,
deposited in sedimentary basins in increasing thickness, eventually lithify into rock strata
due to heat, pressure, cementation, and recrystallization.
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