Mineral types, as generally grouped for the igneous rocks
Felsic minerals are generally light in color and have low density and high concentrations of some combination of silica, sodium, aluminum, and potassium.
Top left: albite.
Bottom left: quartz.
Right: orthoclase.
Mafic minerals are generally dark in color, and have high density and high concentrations of iron and magnesium. They may also have smaller quantities of silica, sodium, aluminum, and potassium.
Top left: olivine.
Top right: pyroxene.
Bottom left: biotite.
Bottom right: amphibole.
How do you identify such tiny grains?
For visible crystals, color is the best property for a first guess. Then use other properties, such as how shiny the cleavage surfaces are (if any), or luster (metallic or nonmetallic).
For example, quartz and albite tend to be pale-colored and without obvious cleavages. Quartz, however, is classy clear and usually grayish whereas albite is usually milky white. Biotite and amphibole are both black, but biotite has brilliantly shiny cleavage surfaces, whereas amphibole cleavages are rougher and dull. Also remember that microscopes or hand lenses make tiny grains much larger.
An igneous plagioclase crystal, seen under the microscope
Despite their small size in igneous rocks, plagioclase crystals have the same twin striations as do larger crystals. These can be seen on some cleavage surfaces with a hand lens or a microscope.
Examples of igneous rock types
Obsidian: volcanic glass having no crystals at all.
Scoria: a vesicular volcanic rock, in this case a vesicular basalt. The crystals are too small to identify so the basalt rock type is inferred from its dark color. The vesicles (bubbles) form as dissolved gasses come out of solution of the original homogeneous silicate liquid.
Basalt: a volcanic rock that, in this case, contains no vesicles or phenocrysts. The crystals are too small to identify and the basalt rock type is inferred from its dark color.
Porphyritic andesite: this volcanic rock has a fine-grained gray matrix enclosing long, black amphibole (hornblende) phenocrysts. The matrix grains are too small to identify.
Porphyritic granite: this plutonic rock has phenocrysts of orthoclase set in a finer grained matrix of orthoclase, albite, quartz, and biotite. These crystals are all large enough to identify, distinguishing it from a volcanic rock.
Gabbro: a plutonic rock, with large, identifiable crystals of labradorite and pyroxene.
Non-porphyritic textures in volcanic and plutonic rocks
Basalt: volcanic; fine-grained and even-grained, with no crystals substantially larger than any others.
Biotite diorite: plutonic; medium-grained with all crystals about the same size.
Muscovite granite: plutonic; a coarse-grained rock, but with all crystals being about the same size.
Granite: plutonic; a coarse-grained, orthoclase-rich rock with all grains about the same size.
Hornblende diorite: plutonic; a borderline case in which some hornblende crystals are somewhat larger that the matrix crystals. In general, most geologists would call this a non-porphyritic rock since borderline cases are usually not worth the extra terminalogy.
Porphyritic textures of volcanic and plutonic rocks
Porphyritic basalt: volcanic; large yellow-green phenocrysts of olivine are enclosed in a much finer-grained matrix. Rounded dark objects are bubbles.
Porphyritic basalt: volcanic; this rock contains three kinds of phenocrysts: yellowish olivine, black pyroxene (augite), and white plagioclase, all set in a very fine-grained matrix.
Porphyritic basalt: volcanic; this photograph is of a thin section, which is a thin slice of rock mounted on a glass slide. The rock and minerals in it are transparent, and large, white plagioclase crystals can be seen.
Hornblende andesite: volcanic; this rock has long black hornblende crystals set in a very fine-grained matrix.
Porphyritic granite: plutonic; this coarse-grained rock has phenocrysts of orthoclase set in a finer-grained (but still coarse) matrix of orthoclase, quartz, biotite, and albite.
Where do magmas come from?
Magmas are produced by the melting of rocks. Melting usually takes place deep in the Earth's crust or mantle. The rock in this photograph is an amphibolite (an amphibole-plagioclase rock) that was partially melted during high-grade metamorphism. The breakdown of amphibole released water, which lowered the melting temperature of the rock to below the ambient metamorphic temperature of about 700°C. The silicate liquid (magma), migrated from thin grain boundary films into fractures as the rock was deformed. The light-colored, coarse-grained veins that you see here are the result, and they now have the composition of a diorite.
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