Structure of Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 7

Metamorphic rocks may be either igneous or sedimentary rocks which have been al-
tered physically by the application of intense heat and/or pressure at some time in their
geological history. Various types of rock metamorphosis caused by different processes
and conditions can be differentiated (Murawski 1983). Examples of metamorphic rocks
are listed in Fig. 2.2.
Between the main groups, various transitions exist so that naturally occurring rocks
cannot always clearly be classified into one of the three groups.
The earth's crust is estimated to be composed of 95% of igneous rocks, 5% of sedi-
mentary rocks and an insignificant proportion of metamorphic rocks. The sedimentary
rocks however make up approximately 75% of the earth's surface and are thus of con-
siderable importance in rock engineering (Leitmeier 1950, Wagenbreth 1970).
The main rock-forming minerals of igneous rocks are feldspar, quartz, amphibole, py-
roxene and mica. The major mineral constituents of sedimentary rocks are quartz, cal-
cite, clay minerals and locally also salt minerals, gypsum and anhydrite. The metamor-
phic rocks contain further minerals such as chlorite, garnet and epidote. A total of 90%
of the earth's crust is formed by the minerals feldspar, quartz, amphibole, pyroxene and
mica. Thus, only a few minerals form the principal components of rock.

2.3 Intact Rock

In the following the aggregate of mineral particles, of which a rock is basically com-
posed, will be denoted as “intact rock”. The minerals that represent the basic rock
structure normally take on the form of crystals but may exist also as amorphous mole-
cule aggregates such as silica. The fine-grained mineral fraction in which larger grains or
crystals are embedded is referred to as “rock matrix”. The mineral particles are cement-
ed together by the rock matrix or by mechanical bonding at contact interfaces between
the grains forming the “grain skeleton”.
When describing the structure of intact rock, the term “texture” is used for the de-
scription of appearance, shape, size and size distribution of the individual grains and
aggregates of mineral particles. The grain size, as in soil mechanics, is described by the
terms fine-grained, medium-grained and coarse-grained. With regard to rock mechani-
cal properties such as deformability, strength and permeability these attributes, how-
ever, are less important.
The spatial orientation of grains in the grain skeleton is described by the “grain struc-
ture”, also referred to as “grain fabric”. This term also covers imperfections in the struc-
ture such as pores, cracks, inclusions and grain boundaries.
Sedimentary and igneous rocks frequently exhibit a random grain structure. Such
structures are characterized by a statistically uniform distribution of particles. The
homogeneous sandstone shown in Fig. 2.3 represents an example for such an intact
rock. A further example of an intact rock with random grain structure is the rock
salt, illustrated in Fig. 2.4.