Important metamorphic minerals mineral diagnostic

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Important Metamorphic Minerals Mineral Diagnostic Mineral Properties Metamorphic Grade Quartz Many colors (often dull, sometimes translucent), hard (scratches glass), conchoidal fracture (can break in smooth curves), no cleavage Any Garnet Often dark red, hard (scratches glass), can have conchoidal frac- ture (break in smooth curves), no cleavage, crystal is 12-sided Medium-high or higher Calcite White, soft (scratched with a knife but not with a fingernail), cleavage in 3 directions (rhombic), reacts (fizzes) with dilute acid Any Mica Brown/translucent, soft (scratches with fingernail), 1 good cleavage (flakes off into thin sheets) Medium-low or higher Actinolite Light to dark green, hard, good cleavage, needle-shaped crystals. A type of amphibole Medium-high or higher Feldspar White/pink/tan, hard (scratches glass), good cleavage in 2 directions High Figure 5. Common metamorphic minerals and their properties. Protolith Slate Phyllite Schist Gneiss Figure 4. Progression of foliated textures. Degree of foliation is dependent upon the metamorphic grade (see next page).
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Lab #4: Metamorphic Rocks 71 Metamorphic Grade Not all metamorphic rocks are recrystallized to the same degree. The intensity of metamorphism, called metamorphic grade , depends on how much pressure and heat have been applied. Minerals tend to grow in size with increasing grade. Also, some rocks change into other metamoraphic rocks depending on the grade. However, certain rocks do not change much with increasing metamorphic grade (e.g. marble and quartzite). The chart below (Fig. 6) is an approximate guide to the pressure and temperature ranges for different metamorphic grades. Different metamorphic grades are a result of varying pressure and temperature conditions. To help you with the the visualization of where in the different conditions occur in the lithosphere refer to Fig. 7. Figure 7. P-T diagram for metamorphic rocks showing the different metamorphic grades. 100 200 300 400 500 600 700 800 5 10 15 20 25 5 1 2 3 4 6 7 8 Temperature ( o C) Depth (kilometers) Pressure (kilobars) Sedimentary conditions Igneous conditions These conditions are not found in nature Onset of melting (migmatite formation) High grade mm medium high grade mm medium low grade mm low grade mm Contact metamorphism Metamorphic Grade Pressure Range Temperature Range Common Foliated Rock Common Non-foliated Rock Low 1-4 kbar 200-325 o C slate Medium-low 1.5-6 kbar 325-450 o C phyllite Medium-high 2.5-12 kbar 450-525 o C schist High 2.5-20 kbar 525-650 o C gneiss Igneous Once a rock melts, it is no longer metamorphic. If only part of the rock melts, the liquid part is magma and the remaining solid part is still high grade metamorphic rock (this kind of rock is called a migmatite ) Figure 6. Table of approximate conditions for metamorphic grades. Units of pressure are in kilobars (kbar), where 1 bar is roughly equal to atmospheric pressure, and one kbar is roughly equal to 1,000 times atmospheric pressure.
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