Management of Commercially Generated Radioactive Waste - U.S. ...

8.17
Salt deposit structures can be flat-lying, folded, or jointed. Jointing is generally
parallel to bedding. Included within beds are large crystal masses, large rock masses of
solidified impurities with lateral continuity, and lateral lithologic changes (Office of
Waste Isolation 1978a). Joints can be anhydrite-filled, near vertical, unopen, moderately
spaced, and generally extensive.
B.6.2 Granite Properties
Granite is an intrusive igneous rock with an equigranular, medium-to-coarse crystalline
texture. It is generally light colored, composed principally of feldspar, quartz and, typi-
cally, hornblende and biotite. Granites are generally homogeneous in composition, with
variations primarily in accessory minerals and secondary rock features. Granites are found
as plutons, which are bodies of igneous rock that have formed beneath the earth's surface by
consolidation from magma. Typical granite plutons include batholiths and smaller-scale
stocks; they are very deeply rooted and enlarge with depth (Verhoogen et al. 1970, Holmes
1978).
Igneous rocks may have similar physical characteristics but range in chemical and min-
eralogical composition from granite to closely related rocks such as granodiorite. In many
respects other closely related igneous rocks are similar to or identical to granite, but,
because they vary significantly in major element, trace element and mineralogic composition,
they are not considered to have the same disposal media properties as granite. The loca-
tions of potential repository granites within the contiguous 48 states are illustrated in
Figure 8.6.2. The areas identified represent large granite masses at or near the surface.
Granites are formed beneath the earth's surface. Their texture is a dense matrix of
equigranular coarse grains. The porosity is low, with little or no natural moisture content.
Intergranular permeability is extremely low. Also, strength is considered to be very
high. Most component minerals are hard, resulting in high durability. Granites are gener-
ally very rigid, with little ability to deform under earth stress, but may exhibit fractures
that could conduct water if they are open and water is available. Granites are basically
resistant to temperature effects up to several hundred degrees Celsius. However, thermal
expansion of particular minerals may be sufficient to cause fracture of the rock and pos-
sibly surface heave.
Granite is mostly composed of silica, alumina, and alkali elements, and forms minerals
of quartz, feldspar, hornblende, and mica. Typical chemical composition of a granite is
included in Ekren et al. (1974, Table 5.1.3). Mineral components of granite are almost
inactive chemically under ambient temperature and pressure conditions. However, more data
are needed about waste-granite reactions under repository conditions.
Granites have no bedding because of their intrusive igneous mode of formation, but may
be layer-like. Joints tend to be blocky or sheet-like on a large scale, and their orienta-
tions may be vertical and intersect at right angles and/or horizontal and subparallel to the
topographic surface. Joints, which range from sealed to partially opened and extensive
often have little mineralization. Granite masses may contain dikes, veins and occasionally
fragments of other rock material.