METAMORPHIC MINERALS

1. METAMORPHIC MINERALS Prepared by Dr. F. Clark, Department of Earth and Atmospheric Sciences, University of Alberta August 06

2. INTRODUCTION In metamorphic rocks are some minerals you likely haven’t seen in igneous or sedimentary rocks so far. Although these minerals are “typical” of metamorphic rocks, you should be aware that these particular minerals, or closely related ones, may also occur in those other two rock types. The particular minerals developed in a metamorphic rock depend on the combination of the bulk chemical composition of the parent rock plus the chemistry of any fluids present, the temperature, and the pressure to which the rock is subjected. The minerals presented in this file are commonly developed during metamorphism of an “argillite parent rock”.

3. WHAT’S AN ARGILLITE, AND WHY CARE? An argillite is a rock rich in clay minerals, and may be thought of, a bit imprecisely, as being the same as a generic mudstone, also referred to as a pelite. Rocks of argillitic composition are present in many metamorphic terranes (mudstones may comprise as much as 70% of the sedimentary record) . Rocks of this composition are also sensitive indicators of metamorphism in that they grow a variety of distinctive minerals with changing metamorphic conditions (e.g. changing conditions of pressure and temperature). The following minerals are arranged more or less in order of their appearance in an argillite as its metamorphic grade or intensity increases.

4. Chlorite.This dark green sheet silicate has flexible to brittle cleavage flakes, rather than elastic ones as in biotite and muscovite. Simplistically, the first appearance or onset of the growth of chlorite marks the beginning of metamorphism, at approximately 200º C. It characterizes low grade metamorphic rocks and imparts a characteristic green colour, from which these rocks derive their name “greenschists”.

5. Garnet.This familiar mineral (the red variety is the birthstone for January) typically first occurs at medium grade metamorphism. Although not very well illustrated by these specimens, its habit is as dodecahedral crystals, that is, twelve diamond-shaped faces [yellow arrows] with the edges “beveled off” by narrow faces [light blue arrow]. There is no cleavage, so one sees fracture [purple arrows].

6. Garnet.If metamorphic minerals first grow as grade increases, then they could be vulnerable or unstable when the grade decreases. The combination of pressure and temperature may revert to some lower grade of metamorphism wherein a particular mineral is no longer stable. This retrograde metamorphism causes, in this case, green chlorite to grow on the surface of the red garnet [yellow stars].

7. Staurolite.This dark red-brown iron- bearing silicate first grows at a slightly higher grade than garnet, albeit still medium grade. The typical habit for staurolite is as prismatic crystals with a strongly compressed hexagonal cross section. In addition, as these three specimens show, it commonly occurs as a twinned crystal. There are only two parts to the twinned crystal, so it shows simple twinning.

8. Staurolite – Simple Penetration Twinning Staurolite exhibits so-called penetration twinning, because it looks like two staurolite prisms have been forced through each other. The illusion results from two parts of the twinned crystal [outlined on the right in green and yellow lines respectively] growing with different orientation, meeting at the twin plane [purple lines].

9. Kyanite.This is one of three polymorphs of Al2SiO5. In the lab, we can determine the stability range for each of the three polymorphs. Kyanite is characteristic of so-called regional metamorphism, with both elevated temperatures and pressures. It is usually a watery pale blue in colour, and occurs as bladed crystals, with one very long dimension [yellow arrows], one intermediate, and one very short.

10. Kyanite.Kyanite is one of the best examples of a mineral whose hardness varies considerably according to crystallographic direction. Asymmetry in the crystal structure means that in this case hardness parallel to the long dimension [yellow arrows; so-called c-axis in this case] is only 5, whereas across the face of the bladed crystals [green arrows] it is 7. A typical knife blade falls between these two values.

11. Andalusite.This polymorph of Al2SiO5 is stable at low pressures but high temperatures, conditions best met next to hot igneous intrusions. Andalusite typically grows as prismatic crystals [length parallel to green arrrows] with a nearly square cross section. Carbon inclusions are forced into a cruciform or cross shaped pattern [yellow arrows] in a variety called chiastolite.

12. MINERALOGICAL SIMPLICITY As noted in the introduction, the mineralogy of a metamorphic rock depends in part on its original composition. If we start with calcium carbonate (limestone) or calcium plus magnesium carbonate (dolostone) and don’t add anything, then we would expect to produce a metamorphic rock which has nothing but calcite or dolomite. This is marble. If instead we start with a quartz sandstone and the composition does not change, we will produce a quartzite. In both cases, the texture is an equigranular, interlocking aggregate of crystals of a single mineral that is reminiscent of the texture of igneous intrusive rocks.

13. Marble – Metamorphosed Limestone Cleavage faces of coarse crystals reflect light where the cleavage direction is parallel to the face of the specimen [yellow arrows]. Variation in impurities also imparts character to these samples, which on these bases may be desirable for decorative purposes. Random orientation of equidimensional crystals leads to a lack of the fabrics which characterize many metamorphic rocks.

14. Quartzite.This is an interlocking aggregate of quartz crystals, rather than adjacent clasts of quartz as was the parent quartz sandstone. As with marble, the rock lacks metamorphic fabrics. In general, as metamorphic grade or intensity increases, so will crystal size. This is because stress is maximal at grain boundaries, and larger crystals have less surface area at grain boundaries for the same volume of material.