Geoscience Reference
In-Depth Information
The classification of metamorphic rocks depends on
the nature of the original rock and the degree to which
it was metamorphosed. At moderate temperatures, low-
grade metamorphic rocks are formed. These are very
fine-grained rocks such as slate. At higher temperatures,
the rates of chemical change increase and new minerals
are able to grow to larger sizes. This forms medium-grade
metamorphic rocks such as schist, metaquartzite, and
marble. At the highest temperatures, the original fabric is
obliterated to form coarse-grained rock that is foliated
and consists of minerals that are stable at high
temperatures. Gneiss is the main high-grade
metamorphic rock type. A classification of metamorphic
rocks for engineering purposes is provided in
Table 10
.
All rock types may be transformed into metamorphic
rocks long after the original time of their formation.
Metamorphism results from alteration by heat and
pressure either from local metamorphism at contact
aureoles and fault zones, or from regional metamorphism
in mountain belts. The mineral fabric of the pre-existing
rocks may be reformed to give larger crystals of the
original minerals, or form new minerals that are stable in
the newly imposed environment. The overall chemistry
(elemental composition) of the rock remains the same,
unless new chemicals were introduced.
Table 10
Geological classification of metamorphic rock for engineering purposes (a modified form of this
table appears in BS 5930:1999)
Foliated rocks
Nonfoliated rocks
Grain size (mm)
Grain size
>20
description
Gneiss
2-20
Well developed but widely spaced
Marble
foliation sometimes with schistose
bands
Metaquartzite
Coarse
Migmatite
Granulite
Irregularly foliated: mixed schists
and gneisses
Hornfels
Amphibolite
Schist
Medium
Well developed undulose foliation;
Serpentinite
0.06-2
generally much mica
Phyllite
0.002-0.06
Slightly undulose foliation;
sometimes 'spotted'
Fine
Slate
<0.002
Well developed plane cleavage
(foliation)
Mylonite
Amorphous or
Glassy
Found in fault zones, mainly in
cryptocrystalline
igneous and metamorphic areas
