Download Metamorphic Minerals.

METAMORPHIC
MINERALS
Prepared by Dr. F. Clark,
Department of Earth and Atmospheric
Sciences, University of Alberta
August 06
INTRODUCTION
In metamorphic rocks are some minerals you likely haven’t
seen in igneous or sedimentary rocks so far. Although
these minerals are “typical” of metamorphic rocks, you
should be aware that these particular minerals, or closely
related ones, may also occur in those other two rock
types. The particular minerals developed in a
metamorphic rock depend on the combination of the
bulk chemical composition of the parent rock plus the
chemistry of any fluids present, the temperature, and
the pressure to which the rock is subjected. The
minerals presented in this file are commonly developed
during metamorphism of an “argillite parent rock”.
WHAT’S AN ARGILLITE, AND WHY
CARE?
An argillite is a rock rich in clay minerals, and may be
thought of, a bit imprecisely, as being the same as a
generic mudstone, also referred to as a pelite. Rocks of
argillitic composition are present in many metamorphic
terranes (mudstones may comprise as much as 70% of
the sedimentary record) . Rocks of this composition are
also sensitive indicators of metamorphism in that they
grow a variety of distinctive minerals with changing
metamorphic conditions (e.g. changing conditions of
pressure and temperature). The following minerals are
arranged more or less in order of their appearance in an
argillite as its metamorphic grade or intensity increases.
Chlorite.
This dark
green sheet
silicate has
flexible to
brittle cleavage
flakes, rather
than elastic
ones as in
biotite and
muscovite.
Simplistically, the first appearance or onset of the growth of chlorite
marks the beginning of metamorphism, at approximately 200º C. It
characterizes low grade metamorphic rocks and imparts a characteristic
green colour, from which these rocks derive their name “greenschists”.
Garnet.
This familiar
mineral (the
red variety is
the birthstone
for January)
typically first
occurs at
medium grade
metamorphism.
Although not very well illustrated by these specimens, its habit is as
dodecahedral crystals, that is, twelve diamond-shaped faces [yellow
arrows] with the edges “beveled off” by narrow faces [light blue
arrow]. There is no cleavage, so one sees fracture [purple arrows].
Garnet.
If metamorphic
minerals first
grow as grade
increases, then
they could be
vulnerable or
unstable when
the grade
decreases.
The combination of pressure and temperature may revert to some
lower grade of metamorphism wherein a particular mineral is no longer
stable. This retrograde metamorphism causes, in this case, green
chlorite to grow on the surface of the red garnet [yellow stars].
Staurolite.
This dark redbrown ironbearing silicate
first grows at a
slightly higher
grade than
garnet, albeit
still medium
grade.
The typical habit for staurolite is as prismatic crystals with a strongly
compressed hexagonal cross section. In addition, as these three
specimens show, it commonly occurs as a twinned crystal. There are
only two parts to the twinned crystal, so it shows simple twinning.
Staurolite – Simple Penetration Twinning
Staurolite exhibits so-called penetration twinning, because it looks like
two staurolite prisms have been forced through each other. The
illusion results from two parts of the twinned crystal [outlined on
the right in green and yellow lines respectively] growing with
different orientation, meeting at the twin plane [purple lines].
Kyanite.
This is one of
three
polymorphs of
Al2SiO5. In the
lab, we can
determine the
stability range
for each of the
three
polymorphs.
Kyanite is characteristic of so-called regional metamorphism, with both
elevated temperatures and pressures. It is usually a watery pale blue in
colour, and occurs as bladed crystals, with one very long dimension
[yellow arrows], one intermediate, and one very short.
Kyanite.
Kyanite is one
of the best
examples of a
mineral whose
hardness varies
considerably
according to
crystallographic
direction.
Asymmetry in the crystal structure means that in this case hardness
parallel to the long dimension [yellow arrows; so-called c-axis in this
case] is only 5, whereas across the face of the bladed crystals [green
arrows] it is 7. A typical knife blade falls between these two values.
Andalusite.
This polymorph
of Al2SiO5 is
stable at low
pressures but
high
temperatures,
conditions best
met next to hot
igneous
intrusions.
Andalusite typically grows as prismatic crystals [length parallel to green
arrrows] with a nearly square cross section. Carbon inclusions are
forced into a cruciform or cross shaped pattern [yellow arrows] in a
variety called chiastolite.
MINERALOGICAL SIMPLICITY
As noted in the introduction, the mineralogy of a
metamorphic rock depends in part on its original
composition. If we start with calcium carbonate
(limestone) or calcium plus magnesium carbonate
(dolostone) and don’t add anything, then we would
expect to produce a metamorphic rock which has
nothing but calcite or dolomite. This is marble. If instead
we start with a quartz sandstone and the composition
does not change, we will produce a quartzite. In both
cases, the texture is an equigranular, interlocking
aggregate of crystals of a single mineral that is
reminiscent of the texture of igneous intrusive rocks.
Marble – Metamorphosed Limestone
Cleavage faces of coarse crystals reflect light where the cleavage
direction is parallel to the face of the specimen [yellow arrows].
Variation in impurities also imparts character to these samples,
which on these bases may be desirable for decorative purposes.
Random orientation of equidimensional crystals leads to a lack of
the fabrics which characterize many metamorphic rocks.
Quartzite.
This is an
interlocking
aggregate of
quartz crystals,
rather than
adjacent clasts
of quartz as
was the parent
quartz
sandstone.
As with marble, the rock lacks metamorphic fabrics. In general, as
metamorphic grade or intensity increases, so will crystal size. This is
because stress is maximal at grain boundaries, and larger crystals have
less surface area at grain boundaries for the same volume of material.