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Introduction to Franklin micro-minerals
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Franklin minerals are even more amazing under a
microscope. The rocks from these deposits have had a long and complex history, and there
are many periods of crystal growth best studied under a microscope.
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Vugs, or open cavities, are frequently
found in Franklin-Sterling samples which contain coatings of minute, millimeter-sized and
smaller, micro-crystals.
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Vug containing micro-crystals of
zincite, barite, fluorite and chlorophoenicite in a sample of massive
franklinite-willemite ore. Franklin, NJ. Width 5 cm.
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These crystals are often
exquisitely formed, showing the greatest degree of perfection, and complexity of crystal
forms, of any of the many crystallized specimens known from the district.
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Magnified view of above sample.
Width 8 mm.
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Though much of the beauty of such micro-crystallized
specimens can be seen using a simple light stereomicroscope, such as
figured above, the scanning electron microscope offers an even more
amazing (if monochrome) view.
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SEM view of fluorite
and chlorophoenicite from sample shown above. Width 0.5 mm
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What is SEM ?
SEM stands for scanning electron microscope,
a principal research tool for the study of surfaces at high magnification.
Samples are placed in an evacuated chamber and bombarded
by a carefully focused beam of electrons, and the secondary electrons
are collected, amplified and used to form an image, typically on a
television screen, of the sample's surface.
Details smaller than one-tenth of a micron are routinely
visible with most machines. Samples are generally first made conductive
through a microscopic coating of carbon or a special metal alloy. No
colors and no internal structures are visible using SEM, but superb
surface images can be easily prepared.
A large number of Franklin-Sterling Hill minerals occur
in secondary micro-crystals, and these are often exquisitely formed.
Such samples provide fascinating material for detailed SEM study.
Take a look at some of the SEM images listed at upper
left under 'selected species'.
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