Metamorphic rocks

metamorphic rocks and their distinguishing features-megascopic and microscopic study of gneiss, schist, quartzite, marble and slate
Properties and characteristics and uses of metamorphic rocks

1. METAMORPHIC ROCKS from desk of Y S G Govind Babu

2. Metamorphic rocks When rocks are baked by heat of molten magma or squeezed by the movements of huge tectonic plates or by the pressure of overlying thick succession of rocks They are altered or changed beyond their recognition i.e. change in Chemical composition, texture and structure Metamorphic rocks

3. Metamorphism Is the process that occur in rocks due to the effects of High temperature High pressure Chemically active fluids

4. The source of temperature is either from magma or due to the depth factor Metamorphism usually result into change in min. comp. and texture of rocks ( Ig . and Sed.) which are subjected to temp. > 100 C and pressure > 1000’s Mpa . Low-grade metamorphism: Occurs at about 100 C to 500 C. High-grade metamorphism: Occurs at > 500 C Temperature

5. UNIFORM PRESSURE Pressure - increases with depth due to increase in overburden. - acts vertically downwards and affects the volume of both liquid & solids. - increases with depth upto some extent, effective in the upper part of the crust. DIRECT or Differential PRESSURE - acts in all direction and affects only on solids resulting into deformation of shape and change in mineral composition - high temperature is also associated with (due to depth factor) - high temperature is not always associated. to depth factor) - Lithostatic pressure- due to overburden - Stress- due to tectonic forces

6. Uniform Stress Differential Stress min inter max

7. Granite Granite-Gneiss

8. STRUCTURES OF METAMORPHIC ROCKS Holmes has suggested a convenient grouping of metamorphic structures into 1 1. Cataclastic structure: soft rocks like shale or tuffs develop cleavage harder rocks scatterd and finally crushed to power with the formation. 2. Maculose structure: in this porphyroblasts of strong minerals are well developed like chloritoid feature. 3. Schistose structure: this structure is formed due to predominance in a metamorphic rock of flacky , lamellar, tabular and high cleavage minerals ex: mica, chlorite etc. 4. Granulose structure: is due to the predominance of equidimensional minerals such as quartz, feldspar etc.

9. 5. Gneissose structure: a secondary rough foliation developed as a result of pressure on the di-solified rocks, the banded or folia or lenses in gneisses etc. 6. Xenoblastic structure: the crystallographic facies are rarely developed an recrystallised metamorphic mineral. 7. Idioblastic structure: a flow mineral which posses crystallizing faces, are able to assert their proper crystallize form, even against the resistance of a solid medium

10. STRUCTURES IN METAMORPHIC ROCKS Foliation: when platy, lamellar or flaky minerals ( eg . sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine), occurring in rock orient themselves parallel to one another (i.e. perpendicular to the direction of maximum pressure or stress). Random orientation Of minerals Preferred orientation Of minerals

11. Lineation: when prismatic or rod-like minerals ( eg . Hornblende, tourmaline etc.) occurring in a rock orient themselves parallel to one another (perpendicular to direction of maxi. Pressure or stress)

12. SLATY CLEAVAGE usually formed during the early stage of Low-grade Metamorphism due to lithostatic stress. New sheet-structure minerals tends to be parallel to the bedding planes during metamorphism. however, further deep burial along the continental margin; compressional forces will cause deformation (folding). hence, the sheet minerals as well as foliation will no longer be parallel to the bedding planes, such type of foliation in fine grained rocks is called slaty cleavage.

13. Shale Slate

14. PHYLLITES usually associated with intermediate grade of metamorphism; where the mineral grains grows large in size as compare to that seen in slates This develops a pronounced foliation where the preferred oriented minerals are seen.

15. SCHISTOSE STRUCTURE usually formed during intermediate and high grade metamorphism Grain size increases and can be seen by naked eye; grains tends to enlarge with increasing grade of metamorphism; the coarse grained sheet-structure minerals show preferred orientation grain size is the main difference between the slaty structure and schistos structure.

16. GNEISSIC STRUCTURE usually associated with high-grade regional metamorphism (where differential stress prevails I.e. tectonic forces) where the sheet silicates and other minerals like quartz/feldspars/hornblende/pyroxene are segregated in distinct bands in the rocks- known as gneissic banding.

17. Classification of Metamorphic rocks based on texture/structures PHYLLITE -similar to slate, but slightly coarser phyllosilicate grains -grains can be seen in hand specimen, giving silk appearance to cleavage surfaces -often cleavage planes less perfectly planar than slates SLATE -strongly cleaved rock -cleavage planes are developed due to orientation of fine phyllosilcate grains eg . Muscovite, biotite , chlorite etc. -individual grains too fine to be visible with naked eye -overall dull appearance

18. SCHIST -parallel alignment of moderately coarse grains (fabric= schistocity ) -grains are visible by eye -mainly phyllosilicates and other minerals such as hornblende, kyanite etc. GNEISS -coarse grained rock (grain size several millimetres) and -foliated (planar fabric: either schistosity or compositional layering) -tendency for different minerals to segregate into layers parallel to foliation (gneissic layering): typically quartz and feldspar rich layers tend to separate from micaceous layers. Varieties: --Orthogneiss: rocks formed from Igneous rocks -- paragneiss: rocks formed from Sedimentary rocks -metasedimentary gneisses

19. QUARTIZITE -it comprise equidimensional minerals viz. quartz and feldspars Non foliated; show GRANULOSE STRUCTURE

20. Type of Metamorphism Cataclastic Metamorphism This type of metamorphism occurs mainly due to direct pressure eg . when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding. Cataclastic metamorphism is mere mechanical breakdown of rocks without any new mineral formation, however, sometime due to intense shearing few new minerals are formed.

21. Contact Metamorphism - This type of metamorphism occurs locally adjacent to the igneous intrusion; with high temp. and low stress There is little change in bulk composition of the rock Area surrounding the intrusion ( Batholith ) is heated by the magma; metamorphism is restricted to a zone surrounding the intrusion, this zone is know as METAMORPHIC AUREOLE . The rocks formed are non-foliated fine-grained rocks called as HORNFELS .

22. Regional Metamorphism - metamorphism occurs covering larger area, which is subjected to intense deformation under direct or differential stress. Rocks formed under such environment are usually strongly foliated, such as slates, schists , and gniesses . The differential stresses result from tectonic forces, eg . when two continental masses collide with one another resulting into mountain building activity. Compressive stresses result in folding of the rock

23. Types of Metamorphic Rocks FOLIATED The common foliated rocks in the order of increasing grain size are SLATE – PHYLLITE – SCHIST – GNEISS NON-FOLIATED Quartzites and hornfels

24. Importance of Metamorphic rocks - GNEISS Gneissic rocks are rich in SILICA i.e. predominantly Quartz and Feldspars along with garnet, pyroxene, Hornblende etc. Non-porous and impermeable nature increases the strength of the rock Foliated character to some extend improves workability Load perpendicular to foliated planes gives more stronger foundation If mineral assemblage is more or less similar to Granite (with less % mafic minerals) then: It is used as building stone As aggregate for making concrete As road metals etc.

25. SCHIST Mainly composed of prismatic or platy minerals, which contributes in development of Schistose Structure. Origin: Schists are usually formed due to Dynamothermal metamorphism ofdifferent kinds of igneous and sedimentary rocks and the nature of combination of metamorphic agents . For eg : Mica schist is formed out of shale Mica – quartz schist is formed out of feldspathic sandstone Talc schists are formed out of magnesia rich Ultrabasic igneous rocks likeperidotite . Hornblende Schists are formed from basic igneous rocks under high stress and high temperature.. Chlorite schist is formed under high stress and low temperature. Eclogite Schist consists of pyroxenes, garnet and quartz formed under low stress and moderate temperature.

26. Properties and uses of civil engineering importance: Schists are consideredweak , incompetent and undesirable rocks. The minerals of schists such as talc,chlorite , biotite , muscovite and serpentine are relatively very soft and are not strong and durable. Presence of cleavage in the minerals cause weakness of rocks. Schists are unsuitable for foundations, as building stone, as aggregate for concrete making, as road metal and as railway ballast. Schists are also unsuitable in case of tunneling. One of the main factors for the failure of St. Francis dam was that it was constructed over Schists .

27. QUARTZITE: SANDSTONE (composed of quartz/feldspars/ feldspathoid minerals) when under go metamorphism result into Quartzite. Granulose texture/structure ( Granoblastic ) makes them most competent rock amongst all other metamorphic rocks. Because metamorphism of Sst . Result disappearance of cementing material, bedding planes, fossil content etc. Quartzites are compact, hard and strong; very less porous and less permeable than the parent Sst . Predominance of Quartz makes the rock very hard and suitable for road metal; can be used as concrete aggregate etc. Acts as strong foundation for any CE structure.

28. QUARTZITE

29. Color: white or pale color. Red, brown, grey, green colours also may be seen. Grain size: fine to coarse grained Texture and Structure: Granulose structure is common. No alternating color bands. No foliation occurs. Minerals present: quartz usually make up the bulk of a quartzite. The other minerals which may also occasionally occur in quartzites are mica, garnet, feldspar, pyroxenes; chlorite, kyanite , epidote , magnetite etc.. Types: Based on mineral content different varieties of quartzites are named as Micaceous quartzite and Quartzite Schist .

30. Important feature of quartzites: Lord Venkateshwara temple is located on nagari quartzites at Tirupathi – Tirumala hills as thick beds for many kilometers. Natural bridge is seen in the same quartzites which is a unique feature. Origin: Quartzites are formed due to dynamic or thermal or Dynamothermal metamorphism of sandstones. They occur as usually as bedded formations. Properties and uses of civil engineering importance: It is a silica–rich and makes highly durable and resist to weathering. It may be used as building stone in addition to road metal, as railway ballast, as load bearing beams. In case of tunneling, the quartzites doesn’t require any lining.

31. SLATES Fine grained impermeable, cleavable and soft Incompetent; cannot withstand great loads But since they are impermeable and split easily; thin large sized slabs of uniform thickness can be extracted for roofing purpose. Economic importance: Since they are bad conductor of electricity– used in electrical industries for switch board base

32. SLATE

33. It is a fine grained metamorphic rock. By virtue of its cleavage character, it splits into very thin sheets of considerable size. Diagnostic character : Extreme fine grain size, absence of reaction with acid, slaty cleavage and shining on surfaces are diagnostic characters of slate. . Color: black or grey coloured Grain size: fine grained Texture and Structure: Foliation is clearly visible Minerals present: Slates are made up of mica ( sericite ) and quartz. Other minerals which may also occur are biotite , muscovite, talc, chlorite, feldspars, calcite, pyrite, magnetite..

34. Types: Based on colors different varieties of slates are named as Black slate, grey slate etc. Phyllite is similar to slate in appearance and represents slate itself which is further metamorphosed. When Calcium is present, slate is described as calcareous slate. Origin: Generally,it is formed due to Dynamic or regional metamorphism of shales . Properties and uses of civil engineering importance: Since, slates are soft and incompetent, they cannot withstand great loads. So they are not suitable as site rocks for foundation purposes. Due to cleavage character and softness, they split easily and hence may be used as building stone.

35. MARBLE Latin word “Marmor”– Shining stone. Calcareous metamorphic rock Though it shows granulose structure it is not as hard as Quartzite because of its Calcareous composition; but can withstand reasonable load. Due to its pleasant colour and brilliant appearance when polished it is extensively used as building stone. Calcite

36. MARBLE: It is a calcareous metamorphic rock and not hard or strong or durable. Its value is due to its pleasant color, good appearance, easy workability and the ability to take an excellent polish. Color: Milky white. However, pleasant shades of green, yellow, brown, ;blue or grey colours also seen. Acid test: Marbles react vigorously even with cold and dilute acids. Grain size : Fine to medium or even coarse grained and the rock is equigranular. Texture and Structure: Granulose structure is common. . No foliation occurs.

37. MARBLE

38. Minerals present: Calcite usually make up the bulk of Marble. The other minerals which may also occasionally occur in marbles are serpentine, olivine, garnet, graphite, mica, talc, tremolite , pyrite. mica, garnet, feldspar, pyroxenes ; chlorite, kyanite , epidote , magnetite etc.. Types: Based on their colors, different varieties of marbles are named as white marble; pink marble; green marble. Important feature of marble: The famous Taj Mahal of Agra constructed out of marble, is regarded as one of the Seven Wonders of the World. Origin: Marbles are formed due to thermal metamorphism of limestones.

39. Properties and uses of civil engineering importance: Physically, the mineral calcite is not only soft but also has three sets of well developed cleavages. This inherent weakness makes the rock split or break easily under loads. Marbles provide aesthetic beauty and a pleasing appearance to the constructions and specially chosen for face works, wall panels; flooring, statue making etc. Marbles are not used as road metal, aggregate for concrete due to soft and weak characters.