58 10.3 Classification of Metamorphic Rocks

Metamorphic rocks are broadly classified as foliated or non-foliated. Non-foliated metamorphic rocks do not have aligned mineral crystals. Non-foliated rocks form when pressure is uniform, or near the surface where pressure is very low. They can also form when the parent rock consists of blocky minerals such as quartz and calcite, in which individual crystals do not align because they aren’t longer in any one dimension. This distinction breaks down in zones of intense deformation, where even minerals like quartz can be squeezed into long stringers, much like squeezing toothpaste out of a tube (Figure 10.13).

Figure 10.13 Rocks from the Western Carpathians mountain range without deformation (left) and after deformation (right). Scale bar: 1 mm. Left- An undeformed granitic rock containing the mica mineral biotite (Bt), plagioclase feldspar (Pl), potassium feldspar (Kfs), and quartz (Qtz). Right- A metamorphic rock (mylonite) resulting from extreme deformation of granitic rocks. Quartz crystals have been flattened and deformed. The other minerals have been crushed and deformed into a fine-grained matrix (Mtx). Source: Farkašovský et al. (2016) CC BY-NC-ND. Click the image to view the original figure captions and access the full text.

Types of Foliated Metamorphic Rocks

Four common types of foliated metamorphic rocks, listed in order of metamorphic grade or intensity of metamorphism are slate, phyllite, schist (pronounced “shist”), and gneiss (pronounced “nice”). Each of these has a characteristic type of foliation

Slate

Slate (Figure 10.14) forms from the low-grade metamorphism of shale. Slate has microscopic clay and mica crystals that have grown perpendicular to the maximum stress direction. Slate tends to break into flat sheets or plates, a property described as slaty cleavage.

Figure 10.14 Slate, a low-grade foliated metamorphic rock. Left- Slate fragments resulting from rock cleavage. Right- The same rock type in outcrop. Source: Karla Panchuk (2018) CC BY-SA 4.0. Photos: Left- Vincent Anciaux (2005) CC BY-SA 3.0 view source; Right- Gretarsson (2006) CC BY-SA 3.0 view source

Phyllite

Phyllite (Figure 10.15) is similar to slate, but has typically been heated to a higher temperature. As a result, the micas have grown larger.  They still are not visible as individual crystals, but the larger size leads to a satiny sheen on the surface.  The cleavage of phyllite is slightly wavy compared to that of slate.

Figure 10.15 Phyllite, a fine-grained foliated metamorphic rock. Left- A hand sample showing a satin texture. Right- The same rock type in outcrop in the city of Sopron, Hungary. Source: Karla Panchuk (2018) CC BY-SA 4.0. Photos: Left- Chadmull (2006) Public Domain view source; Right- Laszlovszky András (2008) CC BY-SA 2.5 view source

Schist

Schist (Figure 10.16) forms at higher temperatures and pressures and exhibits mica crystals that are large enough to see without magnification. Individual crystal faces may flash when the sample is turned in the light, making the rock appear to sparkle. Other minerals such as garnet might also be visible, but it is not unusual to find that schist consists predominantly of a single mineral.

Figure 10.16 Schist, a medium- to high-grade foliated metamorphic rock. Top- Hand sample showing light reflecting off of mica crystals. Bottom- Close-up view of mica crystals and garnet. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photos by R. Weller/ Cochise College. Click the image for photo sources and terms of use.

Gneiss

Gneiss (Figure 10.17) forms at the highest pressures and temperatures, and has crystals large enough to see with the unaided eye. Gneiss features minerals that have separated into bands of different colours. The bands of colours are what define foliation within gneiss. Sometimes the bands are very obvious and continuous (Figure 10.17, upper right), but sometimes they are more like lenses (upper left). Dark bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable.

Figure 10.17 Gneiss, a coarse-grained, high grade metamorphic rock, is characterized by colour bands. Top- Hand samples showing that colour bands can be continuous (left) or less so (right). Bottom- Gneiss in outcrop at Belteviga Bay, Norway. Notice the light and dark stripes on the rock. Source: Karla Panchuk (2018) CC BY-SA 4.0. Click the image for more attributions.

While slate and phyllite typically form only from mudrock protoliths, schist and especially gneiss can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Schist and gneiss can be named on the basis of important minerals that are present: a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or amphibolite (Figure 10.18).

Figure 10.18 Amphibolite in thin section (2mm field of view), derived from metamorphism of a mafic igneous rock. Green crystals are the amphibole hornblende, and colourless crystals are plagioclase feldspar. Note horizontal crystal alignment. Source: D.J. Waters, University of Oxford view source/ view context. Click the image for original figure caption and terms of use.

Types of Non-foliated Metamorphic Rocks

Metamorphic rocks that form under low-pressure conditions or under the effects confining pressure, which is equal in all directions, do not become foliated. In most cases, this is because they are not buried deeply enough, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. Metamorphism that happens because of proximity to magma is called contact metamorphism. Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels.

Marble

Marble (Figure 10.19) is metamorphosed limestone. When it forms, the calcite crystals recrystallize (re-form into larger blocky calcite crystals), and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone is pure calcite, then the marble will be white.  On the other hand, if it has impurities such as clay, silica, or magnesium, the marble could be “marbled” in appearance (Figure 10.19, bottom).

Figure 10.19 Marble is a non-foliated metamorphic rock with a limestone protolith. Left- Marble made of pure calcite is white. Upper right- microscope view of calcite crystals within marble that are blocky and not aligned. Lower right- A quarry wall showing the “marbling” that results when limestone contains components other than calcite. Source: Karla Panchuk (2018) CC BY-NC-SA. Click the image for more attributions.

Quartzite

Quartzite (Figure 10.20) is metamorphosed sandstone. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Sandstone often contains some clay minerals, feldspar or lithic fragments, so quartzite can also contain impurities.

Figure 10.20 Quartzite is a non-foliated metamorphic rock with a sandstone protolith. Left- Quartzite from the Baraboo Range, Wisconsin. Right- Photomicrograph showing quartz grains in quartzite from the Southern Appalachians. In the upper left half of the image, blocky quartz crystals show some evidence of alignment running from the upper right to the lower left. Source: Karla Panchuk (2018) CC BY-SA 4.0. Photomicrograph: Geologian (2011) CC BY-SA 3.0 view source

Even if formed under directed pressure, quartzite is generally not foliated because quartz crystals do not normally align with the directional pressure. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.

Hornfels

Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Hornfels have different elongated or platy minerals (e.g., micas, pyroxene, amphibole, and others) depending on the exact conditions and the parent rock, yet because the pressure wasn’t substantially higher in any particular direction, these crystals remain randomly oriented.

The hornfels in Figure 10.21 (left) appears to have gneiss-like bands, but these actually reflect the beds of alternating sandstone and shale that were in the protolith. They are not related to alignment of crystals due to metamorphism. On the right of Figure 10.21 is a microscopic view of another sample of hornfels, also from a sedimentary protolith. The dark band at the top is from the original bedding.  Here you can see that the brown mica crystals (biotite) are not aligned.

Figure 10.21 Hornfels, a non-foliated metamorphic rock formed from a fine-grained protolith. Left- Hornfels from the Novosibirsk region of Russia from a sedimentary protolith. Dark and light bands preserve the bedding of the original sedimentary rock. The rock has been recrystallized during contact metamorphism and does not display foliation. (scale in cm). Right- Hornfels in thin section from a sedimentary protolith. Note that the brown mica crystals are not aligned. The dark band at the top reflects the layering within the sedimentary parent rock, similar to the way those layers are preserved in the sample on the left. Source: Left- Fedor (2006) Public Domain view source; Right- D.J. Waters, University of Oxford view source/ view context. Click the image for terms of use.

What Happens When Different Rocks Undergo Metamorphism?

The nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions (temperature, pressure, fluids). The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 10.1.

Source: Karla Panchuk (2018) CC BY 4.0, modified after Steven Earle (2015) CC BY 4.0 view source. Click the table for a text version.

Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees. Sandstone and limestone don’t change much either because their metamorphic forms (quartzite and marble, respectively) have the same mineral composition, but re-formed larger crystals.

On the other hand, some rocks can change substantially.  Mudrock (e.g., shale, mudstone) can start out as slate, then progress through phyllite, schist, and gneiss, with a variety of different minerals forming along the way.  Schist and gneiss can also form from sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Migmatite: Both Metamorphic and Igneous

If a metamorphic rock is heated enough, it can begin to undergo partial melting in the same way that igneous rocks do.  The more felsic minerals (feldspar, quartz) will melt, while the more mafic minerals (biotite, hornblende) do not.  When the melt crystallizes again, the result is light-coloured igneous rock interspersed with dark-coloured metamorphic rock.  This mixed rock is called migmatite (Figure 10.22). Note that the foliation present in the metamorphic rock is no longer present in the igneous rock. Liquids cannot support a differential stress, so when the melt crystallizes, the foliation is gone.

Figure 10.22 Migmatite photographed near Geirangerfjord in Norway. Source: Siim Sepp (2006) CC BY-SA 3.0 view source

A fascinating characteristic of migmatites is ptygmatic (pronounced “tigmatic“) folding. These are folds look like they should be impossible because they are enveloped by rock which does not display the same complex deformation (Figure 10.23).  How could those wiggly folds get in there without the rest of the rock being folded in the same way?

Figure 10.23 Ptygmatic folding from Broken Hill, New South Wales, Australia. Ptygmatic folding happens when a stiff layer within a rock is surrounded by weaker layers. Folding causes the stiff layer to crinkle while the weaker layers deform around it. Source: Roberto Weinberg (http://users.monash.edu.au/~weinberg) view source. Click the image for terms of use.

The answer to the ptygmatic fold mystery is that the folded layer is much stiffer than the surrounding layers.  When squeezing forces act on the rock, the stiff layer buckles but the surrounding rock flows rather than buckling, because it isn’t strong enough to buckle.

Exercise: Naming Metamorphic Rocks

Which metamorphic rock is described in each of the following?

  1. A rock with visible minerals of mica and with small crystals of andalusite. The mica crystals are consistently parallel to one another.
  2. A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation.
  3. A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them.
  4. A rock that is dominated by aligned crystals of amphibole.

References

Farkašovský, R., Bónová, K., & Košuth, M. (2016). Microstructural, modal and geochemical changes as a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia). Geologos 22(3), 171-190. doi: 10.1515/logos-2016-0019 View full text