Metamorphic Rocks Lab

Last Modified: 08/20/2009

METAMORPHISM: Where an increase in heat (temperature) and/or pressure produces a significant, detectable change in the texture and/or mineralogy of the original rock.

Click on the topic below to jump to that section:

• Factors that aid in metamorphism
• Metamorphic grades (Degrees of Metamorphism)
• 2 Methods (Types) of Metamorphism
• Texture
• Mineral Composition
• Bulk Chemistry vs. Mineralogy
• Metamorphic Rock types

Factors that aid in metamorphism:

• High Temperature
• High Pressure
• Presence of fluids (water)

Metamorphic Grades (Degrees of Metamorphism):

LOW: (Mild) - Small changes in texture and/or mineralogy of parent rock.

HIGH: (Extreme) - Radical changes in texture and/or mineralogy of parent rock.

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2 Types (Methods) of Metamorphism:

REGIONAL METAMORPHISM: (Associated w/ Mountain Building)

• affects a large area
• components include High Temperature/Pressure, and shear stress

CONTACT METAMORPHISM:

• achieved chiefly by application of HEAT to rock surrounding a cooling body of magma (batholith)
• So HIGH TEMPERATURES, LOW PRESSURES

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Texture

Foliation: Parallel alignment of platy or elongate mineral grains (Micas/Amphiboles) in a rock caused by directed stress.

Types of Foliation:

• SLATY CLEAVAGE: parallel alignment of microscopic platy minerals in a fine-grained metamorphic rock; therefore rock can cleave along flat planes
• SCHISTOSITY: like slaty cleavage, but platy mineral grains are large and parallel to one another
• GNEISSIC LAYERING: discontinuous light & dark layering (or banding) due to mineral segregation

Other Aspects of Texture:

• Crystals can grow in size
• Minerals can become segregated from one another (as in a GNEISS)
• Minerals can undergo plastic deformation (change shape).

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Mineral Composition

Assuming the proper chemical components are present in the original rock, as the rock is subjected to higher grades of metamorphism, new minerals stable at those new temperatures and pressures will appear in the rock. The general order of appearance is as follows (from Low to High Grade):

Chlorite-->Biotite-->Garnet and Staurolite-->Kyanite or Sillimanite

(Water-Rich) ----------------------------------------> (No Water)

NOTE: A similar chart of mineral appearances is on Fig. 10-1 p. 167.

The types of minerals present in the original rock may change with an increasing grade of metamorphism, although the BULK CHEMISTRY of the original rock won't change (except for the loss of water).

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Bulk Chemistry vs. Mineralogy

Mineralogy refers to the constituent minerals that make up a rock (and their relative percentages). Bulk Chemistry refers to the percentage of each element that makes up the rock.

For example, a rock made completely of the mineral quartz (formula: SiO₂)...

The bulk chemistry is explained by the fact that quartz is composed of the elements oxygen and silicon with the ratio of oxygen to silicon as 2:1.

How do minerals become new minerals through metamorphism?

By "reshuffling" the elements of the minerals in the parent rocks to make new minerals! This reshuffling occurs without any actual melting of the rock (remember, melting would make the resultant rock an igneous rock!).

NOTE: If the minerals in the parent rock didn't contain the chemical components necessary to make the "new" stable minerals (p. 167, Fig. 10-1), the "new" minerals won't be formed!

Example:

Say you have a quartz cemented, quartz sandstone (a sedimentary rock) that undergoes medium grade metamorphism. According to the little chart above, new minerals such as garnet would be expected to appear in the metamorphic rock. However, garnet doesn't appear! Why? Because the bulk chemistry of the sedimentary rock doesn't contain the elements necessary to form garnet. Let's see why...

Bulk Chemistry

As we can see, to form garnet we need aluminum and iron (and perhaps calcium and magnesium) in addition to the silicon and oxygen provided by the quartz-cemented, quartz sandstone. Since we don't have the necessary components, we can't make the new mineral (garnet).

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Metamorphic Rock Types:

Foliated Rocks

• Slate: dense, fine-grained metamorphic rock; splits along cleavage
• Phyllite: between schist and slate; has sheen to surface due to fine-grained mica
• Schist: rich in mica; micas are parallel to one another
• Gneiss: layered metamorphic rock (has light/dark bands)
• Amphibolite: High grade metamorphic rock rich in Hornblende

Non-Foliated Rocks

• Quartzite: metamorphic equivalent of quartz sandstone; non-foliated
• Marble: metamorphic equivalent of limestone; may be dolomite or calcite rich; non-foliated
• Serpentinite: greenish, non-foliated metamorphic rock

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