Physical Geology lecture - Mineral
textures and information therein.
The following images are taken from thin sections
of rocks underneath the microscope. Thin sections are wafers
of rock glued to a glass plate and ground down to a thickness
of a few percentage of a millimeter so that light can easily pass
through most of the minerals. A mineral's optical properties (e.g.
color, cleavage, refraction index) as viewed with petrographic
microscopes can then be used to identify the mineral species present.
This is a standard way of qualitatively analyzing a rock for its
mineral composition. What information other than simple mineral
identity (very important in of itself) might be available from
such an analysis?
This is an image of monazite
{(Ce, La, Nd, Pr) PO4} grain from a granite
in the South Carolina Appalachia Piedmont. It is euhedral,
and very nicely zoned. The zonation records a history of
crystal growth similar to the way a tree's ring records a growth
history. The growth was very possibly oscillatory, and a natural
question is what caused the growth bands. Eruptions out
of the magma chamber, earthquakes, pulses of new magma injection
into the chamber are some possibilities. More detailed chemical
analysis would likely provide insight. Zonation reflecting changes
in the chemical environment with growth are not uncommon. Also
note the brown fuzzy spots - they are the result of radiation
damage to the crystal structures as this mineral often contains
radioactive isotopes.
This is image is of a plagioclase feldspar {(Ca, Na)
(Al,Si) Si3O8} grain from a metamorphosed volcanic rock in South
Carolina. It is euhedral to subhedral (note the irregular
contact in the lower right hand portion). The rectangular outline
is typical for feldspar. The formation of partial cross shape
and the light versus dark bands in the crystal interior are a
phenomenon known as twinning. Twinning reflects the very
regular changes in the symmetry of the architecture of the growing
crystal. You use one type of twinning striation to identify plagioclase
feldspar in hand specimen. Several types of twinning occur in
plagioclase feldspar and their occurrence provides more information
as to the detailed composition of the feldspar, which in turn
reflects the conditions of growth. The matrix surrounding the
larger feldspar grain (a few millimeters in length) is recrystallized
quartz, feldspar, and some opaques.
The above images are of a garnet {(Fe3Al2(SiO4)3} grain in a phyllite from drill core in South Carolina
Appalachian Piedmont rocks. Garnet is very typically euhedral.
The image on the left was digitally enhanced to delineate edges.
A careful look defines a core from which internal fractures
radiate within the garnet. These are not cleavage planes (garnet
has no cleavage). Such fractures are thought to be due to differential
mineral expansion as pressure decreases and give insight into
the uplift history of the rock. Also note the preferred orientation
of the quartz and mica grains in the matrix material. This is
due to the deformation history of the rocks and is typical for
metamorphic rocks.
This is a sphene {CaTiSiO5}
grain in a granite from the South Carolina Appalachia Piedmont,
with adjacent light grey feldspar and quartz grains. Again note
the euhedral character with a characteristic rhombic section.
Also evident are three sets of internal fractures that are the
thin section manifestation of the cleavage - two parallel
to the crystal face and one parallel to the short diagonal of
the rhomb. Sphene is a common accessory mineral in granites
meaning it occurs as a minor fraction of the rock.
Summary: Mineral textures provide: identification of
the mineral species present, information on conditions and processes
of formation/growth, and insight into the evolution of conditions
during growth.
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