Have you ever heard of quartzite, marble, slate or phyllite? Well, these are all types of metamorphic rocks. Sometimes a rock, especially when buried deep under the earth’s surfcace becomes altered by a process called metamorphism. It is basically the effect of heat and pressure, as well as chemical processes that account for the various types of metamorphic rocks. All the types of metamorphic rocks essentially fall under two categories: foliated metamorphic rocks and non foliated metamorphic rocks.
The most common foliated types of metamorphic rocks are: gneiss, slate, and schist. More specifically, abundant quartz and feldspar minerals contained in granular mineral grains make up gneiss and justify its banded appearance. Slate, on the other hand, is a metamorphic rock that is created when shale goes through a deep process of metamorphism. As a metamorphic rock it is considered of low grade and it can be split into thin pieces very easily. As for the last of these three foliating types of metamorphic rocks, schist is also a rock that can be broken into thinner pieces due to the great amount of mika contained in it and its well developed foliation.
Moving on to non-foliating types of metamorphic rocks, one comes across hornfels, amphibolite, soapstones, quartzite, and of course marble. Marble is by far the most famous among all types of metamorphic rocks, which occurs when the process of metamorphism ‘meets’ limestone. The fact that it consists basically of calcium carbonate is what accounts for its whitish/grayish color, and it is a very common choice both for you house’s floor and for sculpting. Quartzite, on the other hand, is made of sandstone that has gone through metamorphism, and as the name suggests it primarily contains quartz. The third non-foliating metamorphic rock is soapstone which contains basically talk and minerals like micas, amphiboles, pyroxenes, carbonates, and chlorite. Although it is soft in texture, it is a very dense rock that can resist heat. Soapstone has been used for centuries both for practical and for artistic reason. Even a few thousand years back, soapstone was a very common material used to make bowls, cooking slabs and other utensils that served people’s everyday needs; but it was also used by people with a need for artistic expression in order to make ornaments. All in all, soapstone has the following characteristics as a rock: besides being heat resistant (as mentioned above), it is also resistant to acids and alkalis, it is nonporous and non absorbent, and it has low electrical conductivity.
The last two types of metamorphic rocks in this category are hornfels and amphibolite. Hornfells can be created only if a rock is so close to a heat source that it becomes ‘baked’, as we say in urban language. Hornfels do not have a fixed composition, since they can be formed by various types of minerals and grains as long as the ‘baking’ requirement is met. Last but not least, amphibolite, which is composed plagioclase, quartz, and of course amphibole, goes through a special process of metamorphism called recrystallization that occurred due to extreme directed pressure. There are also other types of metamorphic rocks that are less common, each one, however, with an interesting story to tell.