Petrographic Descriptions of Important Metamorphic Rocks

Metamorphic rocks form when existing rocks are subjected to changes in temperature, pressure, and chemically active fluids, resulting in new mineral assemblages and textures. Here is a detailed petrographic description of some common and important metamorphic rocks:

1. Slate

• Texture: Very fine-grained, foliated
• Mineralogy: Mainly quartz, muscovite, and chlorite; sometimes biotite or feldspar
• Appearance: Typically dull, dark gray to black color; exhibits excellent slaty cleavage
• Formation: Low-grade metamorphism of shale or mudstone under low temperature and pressure
• Key Features: Fine foliation allows it to split into thin sheets; fine mica crystals are microscopic; surfaces are smooth

2. Phyllite

• Texture: Fine-grained with a silky or satiny sheen (phyllitic luster)
• Mineralogy: Muscovite and chlorite are dominant, quartz is present, minor biotite
• Appearance: Shiny surfaces with wavy foliation; colors range from gray to greenish or silver
• Formation: Metamorphism slightly higher grade than slate, intermediate-low grade
• Key Features: Foliation is more pronounced than slate, mica crystals are slightly larger and visible under microscope, gives a silky sheen

3. Schist

• Texture: Medium to coarse-grained, well-developed foliation called schistosity
• Mineralogy: Abundant mica (biotite or muscovite), quartz, feldspar, garnet, staurolite, chlorite, and other metamorphic minerals
• Appearance: Sparkly due to visible mica flakes; foliation planes are prominent and parallel
• Formation: Medium-grade regional metamorphism, often from mudstone or shale protoliths
• Key Features: Large mica crystals aligned giving a shiny, flaky appearance; sometimes porphyroblasts of garnet or staurolite are present

4. Gneiss

• Texture: Coarse-grained, banded or foliated texture called gneissic banding
• Mineralogy: Alternating bands of light-colored minerals (quartz, feldspar) and dark minerals (biotite, hornblende)
• Appearance: Distinct light and dark mineral bands; colors vary widely depending on composition
• Formation: High-grade regional metamorphism, usually from sedimentary or igneous protoliths
• Key Features: Strong compositional layering (gneissic banding), granular texture with minerals segregated into bands

5. Amphibolite

• Texture: Medium to coarse-grained, foliated or massive
• Mineralogy: Dominated by amphibole (mainly hornblende) and plagioclase feldspar; minor quartz, biotite
• Appearance: Dark-colored rock, often black or dark green; foliated varieties show alignment of amphibole crystals
• Formation: Medium to high-grade metamorphism of mafic igneous rocks like basalt or gabbro
• Key Features: Hornblende crystals are elongate and aligned; typically lacks quartz-rich bands found in gneiss

6. Granulite

• Texture: Coarse-grained, granoblastic (equigranular) texture, generally non-foliated
• Mineralogy: Feldspar (plagioclase and K-feldspar), quartz, pyroxene, garnet; little to no mica
• Appearance: Light to medium color, often brownish or gray; hard and dense
• Formation: High-grade metamorphism at high temperature and moderate to high pressure, deep crustal conditions
• Key Features: Absence of hydrous minerals (like mica); presence of pyroxene distinguishes granulites from other metamorphic rocks

7. Eclogite

• Texture: Coarse-grained, granular texture
• Mineralogy: Dominated by red garnet (pyrope) and green omphacite (clinopyroxene)
• Appearance: Striking red and green colors; very dense and hard
• Formation: Very high-pressure metamorphism, typically of basaltic protoliths at great depths (subduction zones)
• Key Features: Distinctive red-green mineral assemblage; absence of quartz distinguishes it from granulite; indicator of deep crustal or mantle conditions

8. Marble

• Texture: Medium to coarse-grained, non-foliated, granoblastic texture
• Mineralogy: Mainly calcite or dolomite crystals
• Appearance: Typically white, gray, or pinkish; can have veining or impurities
• Formation: Metamorphism of limestone or dolostone under low to high-grade conditions
• Key Features: Reacts vigorously with dilute hydrochloric acid; crystalline texture with interlocking calcite/dolomite grains

9. Quartzite

• Texture: Medium to coarse-grained, granoblastic texture, usually non-foliated
• Mineralogy: Dominated by quartz
• Appearance: Usually white, gray, or reddish; very hard and resistant
• Formation: Metamorphism of quartz-rich sandstone under medium to high-grade conditions
• Key Features: Very hard rock; quartz grains recrystallized and fused together; no foliation; often shows sugary texture

Summary Table

Rock	Texture	Dominant Minerals	Grade	Key Feature
Slate	Very fine, foliated	Quartz, muscovite, chlorite	Low	Slaty cleavage, smooth surface
Phyllite	Fine, silky sheen	Muscovite, chlorite, quartz	Low-medium	Silky sheen, wavy foliation
Schist	Medium-coarse, foliated	Mica, quartz, garnet, staurolite	Medium	Shiny mica flakes, schistosity
Gneiss	Coarse, banded	Quartz, feldspar, biotite, hornblende	High	Distinct compositional banding
Amphibolite	Medium-coarse, foliated	Hornblende, plagioclase	Medium-high	Dark color, amphibole alignment
Granulite	Coarse, granular	Feldspar, pyroxene, garnet	High	No mica, pyroxene present
Eclogite	Coarse, granular	Red garnet, green omphacite	Very high	Red-green colors, very dense
Marble	Medium-coarse, non-foliated	Calcite or dolomite	Low-high	Reacts with acid, crystalline
Quartzite	Medium-coarse, non-foliated	Quartz	Medium-high	Hard, sugary texture