Here is a list of seven important metamorphic rocks: 1. Slate 2. Schist 3. Gneiss 4. Quartzite 5. Marble 6. Phyllite 7. Hornfels.
Slate is an extremely fine-grained metamorphic rock characterized by a slaty cleavage by virtue of which it can be readily split into thin sheets having parallel smooth surfaces. The slaty cleavage is due to parallel arrangement of platy and flaky minerals of the slate under the dominant stresses operating during the process of metamorphism.
Mineralogically, slate is made up of very fine flakes of mica, chlorite and microscopic grains of quartz, felspar, oxides of iron and many other minerals, all of which cannot be easily identified even under microscope because of their fine grain size.
Slate is a product of low-grade- regional metamorphism of argillaceous rocks like clays and shales.
When slate is subjected to further action of dynamothermal metamorphism, recrystallisation leads to the development in number and size of some minerals, especially micas. Such metamorphic rocks with conspicuous micaceous constituents and general slaty appearance are termed PHYLLITES. The phyllites on further metamorphism change into rocks called SCHISTS that show megascopically crystalline texture and foliated structure.
Slate is used locally (where available) for construction purpose as a roofing and paving material only.
Schists are megascopically crystalline foliated metamorphic rocks characterised by a typical schistose structure. The constituent flaky and platy minerals are mostly arranged in parallel or subparallel layers or bands.
Texture and Structure:
Schists show wide variation in their texture and structure. Most varieties are coarsely crystalline in texture and exhibit a typical schistose structure. Microcrystalline schists are also not uncommon. Quite a few types show lineation and porphyroblastic fabric.
Platy and rod-like acicular minerals form the bulk of most of the schists. Micas (both muscovite and biotite), chlorite, hornblende, tremolite, actinolite and kyanite are quite common constituents of most of the schists. Quartz and felspars are comparatively rare but not altogether absent. Porphyroblasts of granular minerals like staurolite, garnet and andalucite make their appearance in many schists.
Specific names are given to different types of schists on the basis of predominance of any one or more minerals. Thus some commonly found schists are- muscovite schists, biotite – schists, sericite-schist, and tourmaline-schist etc.
Sometimes schists are grouped into two categories on the basis of degree of metamorphism as indicated by the presence of index minerals:
(a) Low-Grade Schists:
Formed under conditions of regional metamorphism at low temperature. These are rich in minerals like albite, muscovite and chlorite that are unstable at high temperature. Mica-schist, chlorite-schist and talc-schist are a few examples from this group.
(b) High-Grade Schists:
These are formed under conditions of regional metamorphism and are rich in minerals that are stable at high temperatures such as andalusite, cordierite, garnet, staurolite and sillimanite etc. Garnet-schists, cordierite-schists and staurolite-schists are common examples.
Slates and schists are generally the product of dynamothermal metamorphism of argillaceous sedimentary rocks like clays and shales. These indicate the final and stable stage in the metamorphism of shales through the intervening stages of slates and phyllites.
Some schists like chlorite-schists and talc-schists may also be formed from basic and ultra-basic igneous rocks respectively.
A gneiss (pronounced as neis) is a megascopically crystalline foliated metamorphic rock characterised by segregation of constituent minerals into layers or bands of contrasting colour, texture and composition. A typical gneiss will show bands of micaceous minerals alternating with bands of equi-dimensional minerals like felspars, quartz and garnet etc.
Gneisses are generally rich in the minerals of parent rocks that are simply recrystallised during the process of metamorphism. Felspar and quartz are more common in gneisses than in schists. Dark minerals of pyroxene and amphibole groups are also common, as are the typical metamorphic minerals like staurolite, sillimanite, garnet, kyanite and epidote etc.
Texture and Structure:
Gneisses show a variety of textures and structures, the most common being coarsely crystalline texture and the gneissose structure. The so- called augen-gneisses show a typical cataclastic structure in which the hard minerals are flattened and elongated.
Specific names of gneisses are generally based on their mode of origin and structure rather than on their mineralogical composition (as in schists).
Important types are:
(i) Orthogneiss formed as a result of metamorphism of granites and other igneous rocks.
(ii) Paragneiss these are formed from the metamorphism of sedimentary rocks like sandstones;
(iii) Banded gneiss -typical gneiss in which the tabular and flaky minerals are segregated in very conspicuous bands of alternating dark and light colours.
(iv) Augen gneiss- It is a gneissic rock formed as a result of dynamic metamorphism of granites and sedimentary rocks showing a typically cataclastic structure. In this rock stronger minerals are elongated in the form of lense-like form under operating stresses.
(v) Injection gneiss- It is a type of gneiss which is also termed as composite gneiss, migmatite or permeation-gneiss and exhibits a typically banded structure in which bands of igneous and metamorphic minerals occur in an alternating manner. The rock is generally of mixed origin.
According to one view, igneous material penetrated in an originally quartzo-feslpathic rock along its planes of foliation where the mafic minerals of igneous nature got crystallized. This process of penetration of igneous rocks along planes of weakness of another rock is termed as lit-per-lit injection. Hence the name of the rock.
Gneisses of all varieties are generally the result of advanced stages of metamorphism of a variety of parent rocks such as sandstones, conglomerates, granites and rhyolites etc. There is difference of opinion on the origin of the granitic gneisses; their mineralogical composition is close to granites but in structure they appear more metamorphic.
Compact, dense and massive varieties of gneisses find applications as roadstones and in some cases as building stones.
Quartzites are granular metamorphic rocks composed chiefly of intersutured grains of quartz. The name Orthoquartzite is used for a sedimentary rock of similar composition but having a different (sedimentary) origin, in which quartz grains are cemented together by siliceous cement.
In metamorphic quartzite, called Paraquartzite, the grains are fused (Intersutured) together in such a way that when subjected to heavy loads they fracture through the grains unlike the sedimentary quartzite where fracturing takes place along the grains disbanding the cement.
Besides quartz, the rock generally contains subordinate amounts of micas, felspars, garnets and some amphiboles which result from the recrystallisation of some impurities of the original sandstone during the process of metamorphism.
Metamorphic quartzites (paraquartzites) result from the recrystallisation of rather pure sandstones under the influence of contact and dynamic metamorphism.
The rock is generally very hard, strong, dense and uniformly grained. It finds extensive use in building and road construction.
Marble is essentially a granular metamorphic rock composed chiefly of recrystallised limestone (made of mineral calcite). It is characterized by a granulose texture but the grain size shows considerable variation in different varieties. It varies from finely sachhroidal to highly coarse grained. Marbles often show a banded structure also; coarse varieties may exhibit a variety of structures.
Marble is composed chiefly of mineral Calcite (CaCO3) in its recrystallised form. Small amounts of many other granular minerals like olivine, serpentine, garnet and some amphiboles are also present in many varieties, which are derived from the impurities present in the original limestone during the process of metamorphic recrystallisation.
Various types of marble are distinguished on the basis of their colour, composition and structure. White marble, pink marble and black marble are known on the basis of their colours, which is basically due to fine dispersion of some impurity. Dolomitic marble is a variety distinguished on the basis of composition; it may show slightly schistose structure.
Marble is formed from contact metamorphism of carbonate group of sedimentary rocks- pure white marble results from pure limestone; coloured marbles from those limestones that have some impurities and dolomitic marbles from magnesian limestones.
Marble is commonly used in the construction of palatial and monumental buildings in the form of blocks, slabs, arches and in the crushed form as chips for flooring. Because of its restricted occurrence and transport costs, it is mostly used as ornamental stone in costly construction.
It is a medium – to fine-grained foliated metamorphic rock of complex silicate composition. All the individual minerals making a phyllite may not be easily recognised with unaided eye; the presence of muscovite (white mica) is quite conspicuous in many phyllites. The rock shows a foliated structure and represents an intermediate stage in the metamorphic transformation of slates to schists.
Phyllites consist chiefly of minerals like chlorite, muscovite and quartz grains all of which are in fine state but may be seen with the help of a magnifying lense. In fact this is the main point of difference between slates and phyllites.
Phyllites are formed as a result of dynamothermal metamorphism of clay-bearing sedimentary rocks like shales and represent an intermediate stage of metamorphic change of these rocks to schists through slate.
The term hornfels is generally applied to a group of metamorphic rocks developed commonly in the contact zones of igneous intrusions under the influence of intense heat action. These are characterized by fine-to medium-grained textures and a maculose structure. They are formed from fine-grained argillaceous rocks like shales.
A very convenient way to achieve a grasp of different rocks of a group is to compare their properties. There must be some distinguishing features between different rocks. The students are advised to prepare tables of comparative studies of different rocks which may be otherwise similar in broader appearance.
We give below such a table for three closely related metamorphic rocks – slate, phyllite and schist: