Welcome to Petrographic-Microscope
The macroscopic characters of rocks, those visible in hand-specimens without the aid of the petrographic microscope, are very varied and difficult to describe accurately and fully. Though frequently insufficient in themselves to determine the true nature of a rock, they usually serve for a preliminary classification and often give all the information which is actually needed. The fine grained species are often indeterminable in this way and the minute mineral components of all rocks can usually be ascertained just by microscopic examination. A sandstone or grit consist of more or less rounded, water worn sand-grains and if it contains dull, weathered particles of feldspar, shining scales of mica or small crystals of calcite these also rarely escape observation.
Limestone is easily marked with a knife blade, effervesce readily with weak cold acid and often contain entire or broken shells or other fossils. And other simple tools include the blowpipe, the goniometer, the magnet, the magnifying glass and the specific gravity balance. The separation of the ingredients of a crushed rock powder from one to another in order to obtain pure samples suitable for analysis is also extensively practiced. A weak magnetic field will attract magnetite, then hematite and other ores of iron. Silicates containing iron will follow in definite order and biotite, enstatite, augite, hornblende, garnet and similar ferro-magnesian minerals may be successively abstracted, at last only the colorless, non-magnetic compounds, like muscovite, calcite, quartz and feldspar, will remain. The weak acid will melt calcite from a crushed limestone, leaving just dolomite, silicates or quartz.
There is wider application of methods of separation by specific gravity. Levigation or treatment by a current of water, it is extensively emplyed in the mechanical analysis of soils and in the treatment of ores, but is not so successful with rocks, as their components do not as a rule differ very greatly in a specific gravity. The crushed and separated powders, obtained by the processes described above, may be analyzed and thus the chemical composition of the minerals in the rock determined qualitatively or quantitatively. The presence of apatite in rock-sections is established by covering a bare rock-section with solution of ammonium molybdate like a turbid yellow precipitate forms over the crystals of the mineral in question. The complete chemical analyses of rocks are also widely made use of and are of the first importance, especially when new species are under description.
A chemical analysis is in itself usually sufficient to indicate whether a rock is igneous or sedimentary and in either case to show with considerable accuracy to what subdivision of these classes it belongs. By means of the balance and the pycnometer, the specific gravity of rocks is determined in the usual way. Used by archaeologists to identify the mineral components in pottery is called petrographic microscope. A petrographic microscope is a microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks.
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