Physical Geology lecture - Minerals under the microscope and what can be learned.

The following images are taken from thin sections of rocks underneath a special 'petrographic' microscope. Thin sections are wafers of rock glued to a glass plate and then ground down to a thickness of a small fraction of a millimeter so that light can easily pass through most of the minerals. In addition, filters that polarize the light are used. 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 and textures, and is learned in GEOL 2750 - our mineralogy course. What information other than simple mineral identity (very important in of itself) might be available from such an analysis?


This is a thin section of a meta-volcanic rock from South Carolina The large rectangular crystal in this photomicrograph is plagioclase feldspar. The crystal is twinned, with two intergrown portions with crystal lattice orientations at right angles to each other. The surrounding matrix is a finer-grained matrix of quartz, feldspars and opaques. This was a volcanic rock that crystallized quickly, hence the fine-grained matrix. The larger crystal is a phenocryst, and likely represents a crystal that grew more slowly, but for much longer, in the magma chamber that fed the volcanic eruption that originally formed this rock. The finer grained material shows mineral shape alignment that “flows” around the phenocryst. This fabric could have been acquired either as the lava was both flowing and crystallizing, or it could be due to later deformation associated with the Appalachian mountains. The scale bar is .1 mm in length.


This is a photo mosaic of a North Carolina Blue Ridge dunite. Dunites are made almost entirely of olivine and are fairly rare rocks. In this photomicrograph the olivine has a distinctive polygonal texture. However, the polygons do have a longer direction that defines a fabric in the rock, in this case from lower right towards upper left. The texture is a result of solid-state deformation and recrystallization, and is an indication this is a metamorphic and not igneous rock. In addition, altered fractures are sub-perpendicular to this fabric. One place dunites originate from is the mantle. If this was the case the question arises as to how part of the mantle was incorporated into the Blue Ridge province.


These are hornblende porphyroblasts from Georgia. The long needle like crystals here are an amphibole group mineral known as hornblende. The finer-grained matrix is polymineralic, but grains with diagnostic striping (twinning) are plagioclase feldspar. This is a metamorphosed volcanic rock. One question is as to whether the large hornblende crystals were formed as phenocrysts as the original lava crystallized, or whether they are porphyroblasts - crystals that grew larger than the surrounding matrix during metamorphism. A close look shows a matrix fabric, defined by alignment of the grains, including dark and tabular biotite and opagues. These oriented dark grains also occur inside the hornblende, suggesting that the hornblende overprinted the fabric, a relationship most consistent with these hornblende crystals being metamorphic porphyroblasts and not phenocrysts.


 

This image shows calcite (CaCO3) as the lighter yellow-brown crystals with good crystal terminations (the somewhat pointed and angular boundaries) best developed in the lower right quadrant of this image. The calcite served as the growth substrate for the radial and hemispherical form of very fine-grained fibrous quartz (known as chalcedony) which is in bright bluish, purple and yellow colors here. The different colors represent how the quartz (SiO2) lattice structure is oriented differently throughout, but has a systematic radial pattern. The spherical bands represent zones rich in inclusions in the chalcedony that mark growth fronts during slightly fluctuating conditions. There is a lot of information in these textures. This specimen comes from a chalcedony vein in the Badlands National Park, and the horizontal scale is approximately 3 mm.


In this microscope image two minerals, gypsum (CaSO4-2H2O) and anhydrite (CaSO4) exist. Gypsum is basically the 'wet' form where water is incorporated into the lattice structure, and anhydrite is the dry form, without water. A variety of factors determine which one forms and under what conditions one can be changed to the other. Here the light gray portion represents highly sheared gypsum, and the brightly colored grains are anhydrite that has formed from (overprints) the gypsum. Note the consistent well developed fractures (cleavage) evident in the anhydrite.


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 images to the left 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 gray 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. It can also be an ore mineral when concentrated enough.

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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