Lab 6: Metamorphic Rocks - Classification

 

There are two main types of metamorphic rocks, distinguished by texture:

 

Foliated Texture

 

The term foliation refers to the alignment of minerals that happens as a result of directed pressure, occurring as part of regional metamorphism. The alignment happens to preexisting mineral crystals and to new minerals that grow during metamorphism. New minerals tend to grow faster in the direction where growth is "easier," and that is perpendicular to the direction of collision, or directed pressure. Squeeze a book between your hands and think of the book as a mineral crystal growing during metamorphism. It would be difficult for the book to grow out to the side toward your hands, because that is the direction of pressure, but it is easier for it go grow toward the edges of the book (perpendicular to the direction you are squeezing the book). So, foliation results from this preferential alignment and mineral growth direction. Types of foliation include:

 

Slaty Cleavage

 

Alignment of clay minerals and micas resulting from their metamorphism happens in the fashion described for squeezing the book. Slate is smooth. The smoothness of slaty cleavage results from the alignment of uniformly sized clay minerals and micas. Note that foliation happens with alignment perpendicular to directed pressure, and there is usually no relationship to original sedimentary bedding or other original fabric in the rock.

 

Phyllitic Cleavage

 

One step up the scale of metamorphism (higher grade), we find the wrinkly texture of phyllite. Whereas a piece of slate is smooth (the original chalk boards were made of slate; high quality pool tables have smooth slate under the felt), the surface of a piece of phyllite will show a wrinkly, or finely wavy surface. Directed pressure causes slightly larger mica mineral crystals to be aligned on gently warped interfaces within the body of the rock.

 

Schistose Cleavage (Schistosity)

 

With higher grade metamorphism (more temperature, more pressure) than the low grade metamorphism responsible for slate and phyllite, mica mineral crystals grow larger, and new ones form to make a coarser texture than in slate. Plate-like mineral crystals give schist (pronounced "shist") a "flakey" appearance. Schist can have non-mica minerals such as quartz, feldspar, garnet, and other minerals unique to metamorphism, but the bulk of the volume is formed by so many individual mica crystals. If present, minerals such as garnet, staurolite, and andalusite will grow forcibly in three dimensions to fill out their individual framework structures, so they stand out as full bodied forms, in contrast to the mica minerals forming the bulk of the rock.

 

Gneissic Cleavage

 

With high grade metamorphism, we see a banding alignment of minerals such as biotite, feldspar, and quartz. Slate, phyllite, and schist tend to part into thin flattish pieces, but gneiss (pronounced "nice") tends to break up into blocky pieces, reflecting the stronger structure of the banding. The bands are actually composed of one or several minerals that were separated, or "segregated," during metamorphism. Gneiss is one step before melting, which would generate magma.

 

Foliated Rocks

 

The rocks go by the same names as the foliations:

 

Slate

 

"smooth"

 

Phyllite

 

"wrinkled"

 

Schist

 

"flakey" or "mica rich"

 

Gneiss

 

"banded"

 

The predecessor (parent rock) of many foliated metamorphic rocks is shale, the most common sedimentary rock. Clay minerals form the raw materials for slate, which is modified to phyllite, which is modified to schist, and so on... But, all sorts of rocks get metamorphosed, and there needn't be such a sequential pathway.

 

Nonfoliated Texture

 

In metamorphic rocks such as marble and quartzite, there usually isn't much alignment of mineral crystals, because heat is the primary agent of metamorphism for these rocks, not directed pressure. The following nonfoliated metamorphic rocks are common:

 

Quartzite

 

Quartzite is extremely hard, as you might expect from metamorphosed sandstone, wherein the original quartz grains have intergrown, and appear to be in a solid, fused configuration in hand specimen. The parent rock is sandstone, and it forms mainly in regional metamorphism.

 

Marble

 

Marble is metamorphosed limestone. If metamorphism is only slight, original shells and other fossils may appear relatively unaltered, but with higher grade metamorphism, recrystallization of calcite in the original limestone obliterates the original texture and shapes. Marble can form in any type of metamorphism, but is especially notable in contact metamorphic zones (aureoles), which may be rich in lead and other metal ore deposits.

 

Amphibolite

 

The mineral hornblende is the most well known amphibole. You recall that it is a double chain silicate, with a "splintery" appearance resulting from the cleavage planes between the double chains. Amphibolites often have basalt as a parent rock.

 

Hornfels

 

This is a contact metamorphic rock derived from heating of original sandstone, shale, other sedimentary, and even igneous rocks. Hornfels is fine-grained, even dense, and often breaks along brittle fractures, owing to the massive, hard texture.

 

Skarn

 

Skarn forms in contact metamorphic zones, often around granitic intrusions, where chemically active fluids leach various chemical elements from adjacent rock types, providing source material for growth of a large variety of minerals, including pyroxene, garnet, magnetite, topaz, corundum, fluorite, apatite, and barite. As a result of such variation, skarn is something of a general, catch-all rock type, but most commonly forms where the mineralogic variety results from a variety of source rocks lying adjacent to one another in a highly mineralized contact zone.