Water and Sediment Mineralogy
The various minerals found in the water and sediments of lakes usually come from the rocks
and sediments surrounding them. The minerals are altered as they travel to the lakes, through
exposure to oxygen, organic molecules, bacteria, different pH environments and many other
weathering
agents. By studying the mineral composition of lake material, and comparing the results to similar
analyses of surrounding watersheds and modern lake water, we're able to determine what the
environment was like during deposition of lake sediments. At Bear Lake, for instance, changes
in magnesium (Mg) concentration in the lake seems to determine which form of
carbonate, calcite or aragonite, is deposited. The magnesium concentration increases when the
mineral springs on the east side of the lake contribute a greater portion of water to the lake.
During periods of drought, the western springs dry out compared to the eastern springs and Mg
levels in the lake increase.
Minerology is determined in several ways:
Visual Examination:
Samples can be examined without any pretreatment by looking at them through a small hand-held
lens. Placing the sample on a microscope slide allows higher magnification examination, using a
microscope. Electroplating the sample allows use of a scanning electron microscope, which creates
a highly magnified picture for visual inspection.
X-Ray Diffraction (XRD):
XRD measures the angle and intensity of diffracted X-rays as they pass through
different minerals in a sample. By examining the angles of various x-ray intensity peaks, we
can identify particular minerals. This technique is relatively fast, requires minimal
sample preparation, and does not destroy the material (so XRD samples can be used for other
analyses).
Inductively Coupled Plasma Spectroscopy (ICP):
Samples for ICP analyses are dissolved in acids, and then vaporized to form a
plasma. This process releases photons that are detected by a spectrometer. The spectral patterns
of different minerals are known, so we identify sample minerals by matching the sprectral pattern
of their photons to known patterns.
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U.S. Department of the Interior | U.S. Geological Survey