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. 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.
> 1000 C and pressure > 1000’s Mpa.
 Low-grade metamorphism:
Occurs at about 1000 C to 5000 C.
 High-grade metamorphism:
Occurs at > 5000 C
Temperature

6. 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

7. Uniform Stress
Differential Stress
min
inter
max

8. Granite Granite-Gneiss

9. 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.

10.  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

11. 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

12.  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)

13. 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.

14. Shale Slate

15. 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.

16. 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.

17. 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.

18. 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

19. 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

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

21. 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.

22.  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.

23.  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

24. 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

25.  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.

26. 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.

27. 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.

29. 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.

30. QUARTZITE

31. 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 .

32. 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.

33. 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

34. SLATE

35. 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..

36. 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.

37.  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

38. 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.

39. MARBLE

40. 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.

41. 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.