Petrology

The three rock classes can be separated in the field by their texture, structure, composition, fossils and other simple features.

Quick field recognition

1. Layered with fossils and rounded grains: sedimentary.
2. Glassy or interlocking crystals, no layers, bubble holes: igneous.
3. Bands, wavy flaky layers, shiny mica, splits into slabs: metamorphic.

The rock cycle is the continuous natural process by which rocks of one type are changed into another type through geological processes over millions of years. Any rock may become any other, and no rock is permanent.

        MAGMA
       /     ^
 cooling      | melting
     v         |
  IGNEOUS ----------------+
     |  weathering        |
     v  + deposition      |
  SEDIMENTS               |
     | burial,            |
     v cementation        |
  SEDIMENTARY             |
     | heat, pressure     |
     v                    |
  METAMORPHIC ------------+ (melting -> magma)

Sequence

1. Igneous rocks form when magma or lava cools and solidifies (granite, basalt).
2. Exposed rocks undergo weathering and erosion into fragments; transport by water, wind, ice and deposition form sediments.
3. Sediments undergo compaction and cementation (lithification) into sedimentary rocks (sandstone, shale).
4. Under high heat, pressure and chemical fluids, igneous or sedimentary rocks recrystallise to metamorphic rocks (gneiss, schist, slate), without melting.
5. If temperature rises further, metamorphic (or any) rocks melt into magma, which cools again as igneous rock.

Shortcuts

Igneous may directly be metamorphosed; metamorphic and sedimentary rocks may be weathered into new sediments; and uplift brings buried rocks to the surface.

The cycle is driven by the Earth's internal heat and by solar energy through the water cycle.

The type of rock affects its strength, durability, permeability and behaviour, so it is a basic consideration in selecting sites and materials.

Igneous rocks

• Generally hard, strong, dense, massive and low in porosity; granite and basalt are good foundation rocks for dams, bridges and heavy buildings.
• Used as crushed aggregate, road metal, building and dressing stone.
• Problems: fractures, joints and weathering in humid areas; sheet jointing; columnar jointing in basalt causes leakage; weathering of granite to clayey soil.

Sedimentary rocks

• Properties vary widely with cementation and layering.
• Sandstone (silica-cemented) and limestone are good building materials and aggregate; limestone is the raw material for cement.
• Bedding planes are planes of weakness, so slope and tunnel stability depends on dip direction.
• Shale is weak, slakes in water and swells, and gives landslides.
• Limestone has solution cavities, which cause leakage in reservoirs.

Metamorphic rocks

• Massive types (quartzite, marble, gneiss) are hard and durable; marble and slate are used for flooring, roofing and decoration.
• Foliated rocks (schist, phyllite, slate) split along foliation, are weak, and cause slope failure and tunnel squeezing; mica minerals reduce strength.
• Some are anisotropic: strength depends on direction of loading.

Hence the engineer needs to study the type, structure and weathering of rocks before choosing foundation level, slope angle and support.

Rock cleavage is the property of a rock to split easily along closely spaced, parallel planes, producing thin sheets or slabs. It is a secondary structure, developed in a rock after its formation, usually in fine-grained metamorphic rocks by strong directional pressure. It should not be confused with mineral cleavage, which depends on the atomic structure of a single mineral.

Formation

Under compression, platy minerals (mica, chlorite) and flattened grains reorient perpendicular to the direction of maximum stress, so the rock splits parallel to these planes, which are unrelated to the original bedding.

Types

• Slaty cleavage: very fine, perfectly parallel planes in slate; slate splits into thin slabs.
• Fracture (shear) cleavage: closely spaced parallel fractures cutting the rock into thin slices.
• Strain-slip (crenulation) cleavage: fine folding of an earlier cleavage.
• Schistosity (coarser, flaky) in schists and gneissic banding are related foliations.

 Bedding   ====== ====== ======
 Cleavage   / / / / / / / /  (cuts across bedding)

Engineering significance

• Cleavage planes are weak and allow easy splitting, so the strength of the rock is low across them.
• It decides stability of slopes, excavations and tunnel roofs (planar failure along cleavage).
• Useful for obtaining roofing slate and flagstone, but poor for aggregate and foundations.
• Allows water to seep, causing leakage and weathering.

Sedimentary rocks are classified mainly by their mode of origin into three groups, and also by grain size and composition.

1. Clastic (mechanical, detrital) rocks

Formed from broken fragments of older rocks, transported and deposited, then cemented. Classified by grain size:

• Rudaceous (> 2 mm): conglomerate (rounded gravel), breccia (angular fragments).
• Arenaceous (1/16-2 mm): sandstone, arkose, greywacke.
• Argillaceous (< 1/16 mm): shale, mudstone, siltstone, clay.

2. Chemical rocks

Formed by precipitation from solution or evaporation.

• Limestone (chemical), dolomite, rock salt, gypsum, chert, travertine.

3. Organic (biogenic) rocks

Formed from remains of plants and animals.

• Calcareous: fossiliferous limestone, coral limestone, chalk.
• Carbonaceous: peat, lignite, coal.
• Siliceous: diatomite.

 Sedimentary
   |-- Clastic    : conglomerate, sandstone, shale
   |-- Chemical   : rock salt, gypsum, chert
   `-- Organic    : coal, coral limestone

Texture of a rock is the size, shape, arrangement and degree of crystallinity of its mineral grains, that is, the overall appearance of the rock on a small scale. In igneous rocks it depends mainly on the rate of cooling of the magma.

Textures of igneous rocks

1. Based on degree of crystallinity
   • Holocrystalline: wholly crystalline (granite).
   • Hypocrystalline (hemicrystalline): partly crystals, partly glass.
   • Holohyaline (glassy): wholly glass, from very rapid cooling (obsidian).
2. Based on grain size
   • Phaneritic: crystals visible to the naked eye, from slow cooling (granite, gabbro). Coarse, medium and fine grained.
   • Aphanitic: crystals too small to see, from rapid cooling (basalt, rhyolite).
   • Porphyritic: large crystals (phenocrysts) set in a fine-grained or glassy groundmass; two-stage cooling (porphyry).
   • Pegmatitic: very coarse crystals (pegmatite).
3. Special textures
   • Vesicular: gas bubble holes (pumice, scoria).
   • Amygdaloidal: vesicles filled with secondary minerals (zeolite, calcite).
   • Ophitic: laths of plagioclase enclosed in pyroxene.
   • Graphic: intergrowth of quartz and feldspar.
4. Based on grain shape: euhedral (perfect faces), subhedral, anhedral.

Significance

Coarse, interlocking crystalline rocks are strong; glassy and vesicular rocks are weaker and more porous.

Granite (igneous, intrusive)

• Physical properties: coarse-grained, hard, massive; colour pink, grey or white with quartz, feldspar and mica; specific gravity 2.6-2.8; vitreous lustre.
• Engineering properties: compressive strength 100-250 MPa; porosity less than 1%; water absorption low; very durable; takes good polish; may have joints and sheet jointing; feldspar weathers to clay.
• Uses: foundations for dams and bridges, building and monument stone, paving blocks, curbstones, aggregate, railway ballast, decorative slabs.

Phyllite (metamorphic, foliated)

• Physical properties: fine-grained, grey, green or black; silky sheen from fine mica; wavy foliation; hardness 3-4; specific gravity 2.7-2.9.
• Engineering properties: compressive strength low to moderate (20-60 MPa), strongly anisotropic; splits along foliation; weathers easily; weak in the slope and tunnel, often prone to landslides in the Lesser Himalaya (Nepal).
• Uses: limited; roofing and flooring slabs where thick and hard; fill or road sub-base; generally unsuitable as aggregate or heavy foundation.

Limestone (sedimentary, chemical/organic)

• Physical properties: fine- to coarse-grained, white, grey or yellowish; hardness 3; effervesces with dilute HCl; specific gravity 2.5-2.8.
• Engineering properties: compressive strength 30-150 MPa; soluble in acidic water, so forms caves and cavities (karst), leading to leakage at reservoirs and settlement of foundations.
• Uses: raw material for cement and lime, aggregate, building stone, flux in metallurgy, flooring, road metal.

Intrusive igneous bodies are classified by their relation to the surrounding country rock.

 Concordant (sill)      Discordant (dyke)
 ==================      ===|##|=====
 ##################      ===|##|=====
 ==================      ===|##|=====

• Sill: horizontal sheet between beds. Laccolith: lens with domed top. Dyke: vertical sheet cutting beds. Batholith: huge irregular mass, over 100 km², of granite. Stock: smaller version of batholith.

A metamorphic rock can be recognised in the field by these features.

1. Foliation: parallel arrangement of minerals, banding or flaky layers (slate, phyllite, schist, gneiss).
2. Slaty cleavage or schistosity: splits into thin sheets, with silky or shiny mica surface.
3. Crystalline, interlocking, "sugary" texture in non-foliated types (marble, quartzite).
4. Metamorphic minerals present: garnet, kyanite, staurolite, chlorite, talc, epidote.
5. Deformed or distorted structures such as folds, stretched pebbles, and lenticular bands.
6. No fossils (or highly distorted ones).
7. Hard and dense; the rock is often found near mountain belts, fault zones or intrusions.

Sedimentary rocks form at the Earth's surface by the accumulation and hardening of sediments, in these stages.

1. Weathering (physical, chemical) breaks existing rocks into fragments and dissolved material.
2. Erosion and transportation by water, wind, ice or gravity carry the particles away. Particles get rounded and sorted by size.
3. Deposition occurs in lakes, rivers, seas and deserts when transporting energy falls, forming horizontal layers (strata). Chemical and organic matter also precipitate or accumulate.
4. Compaction: the weight of overlying sediments squeezes out water and reduces pore space.
5. Cementation: minerals such as silica, calcite and iron oxide precipitate from water and bind the grains.
6. Lithification: the loose sediments become solid rock (diagenesis).

Result: layered rocks such as sandstone, shale and limestone, with fossils and structures like ripple marks.

Texture is the size, shape and arrangement of grains and their relationship in a rock. Sedimentary textures are of two main types.

1. Clastic (fragmental) texture

Rock is made up of broken grains cemented together. Features:

• Grain size: gravel (> 2 mm), sand (2 to 1/16 mm), silt and clay (< 1/16 mm).
• Grain shape: angular (little transport, breccia) to rounded (long transport, conglomerate); sphericity.
• Sorting: well sorted (similar size) or poorly sorted (mixed sizes).
• Packing and fabric: arrangement of grains and pore space.
• Cement and matrix: silica, calcite or iron oxide cement; fine matrix between grains.

2. Non-clastic texture

• Crystalline: interlocking crystals from chemical precipitation (rock salt, gypsum, chemical limestone).
• Oolitic: small spherical grains (ooliths) in limestone.
• Bioclastic / organic: shell fragments, fossils (coquina, coral limestone).
• Amorphous or cryptocrystalline: chert.

The grain size, sorting and cement control the strength and porosity of the rock.

Both are parts of petrology.

Petrogenesis

Petrogenesis is the branch of petrology that deals with the origin and formation of rocks.

• Igneous rocks: crystallisation of magma or lava, by cooling and differentiation.
• Sedimentary rocks: weathering, transport, deposition and lithification of sediments.
• Metamorphic rocks: transformation of pre-existing rocks in the solid state by heat, pressure and chemically active fluids.

Identification of igneous rocks in the field

• Crystalline texture: interlocking crystals (coarse in granite, fine in basalt), or glassy.
• Massive structure; no layers or foliation; may show columnar jointing.
• Vesicles or amygdales in volcanic rocks.
• No fossils.
• Minerals: quartz, feldspar, mica, pyroxene, hornblende, olivine.
• Occurrence: dykes, sills, batholiths or lava flows, with baked contacts on neighbouring rock.
• Colour from light (acidic) to dark (basic).

Civil engineering significance

• Granite: hard, strong and durable (100-250 MPa); used for foundations, bridge piers, aggregate, dressing stone and monuments. Weathered or jointed granite needs care.
• Phyllite: weak, foliated, easily weathered; makes unstable slopes and tunnels in the Lesser Himalaya, so is generally unsuitable for aggregate or heavy foundations.
• Sandstone: building stone, aggregate and flooring; strength depends on cement (silica cement strong, clay cement weak); bedding affects stability; porous, so may leak.

Metamorphism is the change in mineral composition, texture and structure of a rock in the solid state, due to changes in temperature, pressure and chemical environment.

Agents (factors) of metamorphism

1. Heat (temperature)

• Source: magma intrusion, deep burial (geothermal gradient), friction on faults.
• Role: provides energy for chemical reactions, causing recrystallisation and growth of new minerals (e.g. limestone to marble, shale to hornfels). Range roughly 200-800 °C.

2. Pressure

• Confining (lithostatic) pressure: from the weight of overlying rocks; acts equally in all directions, makes rocks denser and forms minerals with compact structure.
• Directed (differential) stress: from plate movements; flattens and aligns minerals, giving foliation, slaty cleavage and schistosity.

3. Chemically active fluids

• Water and gases (CO₂, H₂O, fluorine) carry ions to and from the rock.
• Role: speed up reactions and introduce new materials (metasomatism).

Time also matters: slow, long action gives well-formed minerals.

Marble

• Origin: metamorphism (contact or regional) of limestone or dolomite.
• Physical properties: crystalline, granular ("sugary") texture; non-foliated; white, pink, grey or green with veins; hardness 3-4; specific gravity 2.6-2.8; effervesces with dilute HCl; takes a fine polish.
• Engineering properties: compressive strength 60-140 MPa; low porosity (under 1%); soluble in acidic water, reducing durability in polluted air; massive.
• Uses: flooring, wall cladding, steps, statues, monuments, decorative and ornamental stone, lime.

Slate

• Origin: low-grade regional metamorphism of shale or clay.
• Physical properties: very fine-grained, dense, dark grey, black, green or red; slaty cleavage, splits into thin, flat slabs; dull to slightly silky lustre; hardness 3-4; specific gravity 2.7-2.8.
• Engineering properties: compressive strength 50-100 MPa across the cleavage plane, but strongly anisotropic and weak along it; low water absorption; durable; fairly impermeable.
• Uses: roofing slates, flooring, damp-proof courses, paving, blackboards; it is poor for aggregate and weak for slopes and tunnels.

Amphibolite

• Origin: regional metamorphism of basic igneous rocks (basalt, gabbro) or impure carbonate rocks.
• Physical properties: medium- to coarse-grained, dark green to black; mainly hornblende and plagioclase; weakly foliated or massive; hardness 5-6; specific gravity 2.9-3.2.
• Engineering properties: hard, dense and tough; compressive strength 100-250 MPa; low porosity; good durability and resistance to abrasion, but weathers along foliation and joints.
• Uses: good foundation rock, aggregate, road metal, railway ballast, building and crushed stone.

Marble (metamorphic, from limestone), slate (metamorphic, from shale) and granite (igneous, plutonic) are common building stones. The values below are typical textbook ranges.

Physical properties

Engineering properties

• Marble: flooring, facing, steps, monuments; unsuitable for exposed outdoor use in acid rain.
• Slate: roofing sheets, flooring, damp-proof courses, paving; weak along cleavage.
• Granite: heavy foundations, bridge piers, retaining walls, dam facing, road aggregate, ballast and kerb stones; durable and fire resistant up to about 600 degrees C.

Quartzite

Metamorphic rock formed from sandstone by recrystallisation.

• Texture: granoblastic (interlocking quartz grains), fine to medium grained; original grains fused with silica.
• Structure: massive, non-foliated; sometimes relict bedding.
• Minerals: more than 90 % quartz, with traces of mica, feldspar and iron oxides.
• Engineering properties: compressive strength 150 - 300 MPa, specific gravity 2.65, very hard (Mohs 7), very low porosity, highly resistant to weathering and chemicals. Used as road metal, railway ballast, concrete aggregate and foundation rock. Very hard to dress, and it may show jointing and slip planes.

Limestone

Sedimentary rock formed by chemical or organic deposition of calcium carbonate.

• Texture: crystalline, oolitic, fossiliferous or clastic (fine to coarse grains).
• Structure: bedded, often jointed; may contain fossils and cavities.
• Minerals: calcite (CaCO3), some dolomite, clay and silica as impurities.
• Engineering properties: compressive strength 50 - 150 MPa, specific gravity 2.4 - 2.7, hardness 3, soluble in acidic water (sinkholes, caves, karst), effervesces with dilute HCl. Used for cement, lime, building stone and aggregate. Dam sites on limestone need grouting for solution cavities.

Granite

Plutonic igneous rock.

• Texture: holocrystalline, phaneritic, medium to coarse granular (sometimes porphyritic).
• Structure: massive, often with sheeting and joints; no bedding.
• Minerals: quartz (20 - 40 %), orthoclase/microcline feldspar, plagioclase, biotite/muscovite or hornblende.
• Engineering properties: compressive strength 100 - 250 MPa, specific gravity 2.6 - 2.8, hardness 6 - 7, water absorption below 0.5 %, excellent durability and polish. Used for foundations, piers, dams, facing, aggregate. Weathering can produce deep clay-rich residual soil, which must be removed from foundations.

Identifying a metamorphic rock in the field

1. Foliation: parallel alignment of minerals, banding or schistosity (slate, phyllite, schist, gneiss).
2. Slaty cleavage: splits into thin, hard slabs.
3. Crystalline, sugary texture (marble, quartzite), with no layers or fossils.
4. Typical minerals: garnet, kyanite, chlorite, mica, staurolite, talc.
5. Distorted structures: folds, crumpled bands, stretched pebbles.
6. Often hard, dense and found in mountain belts or near intrusions.

Physical properties of sandstone

• Sedimentary, clastic rock of sand-sized grains (mostly quartz) cemented by silica, calcite or iron oxide.
• Colour: white, yellow, red, brown or grey; bedded; granular texture, gritty feel.
• Specific gravity 2.2-2.7; porosity 5-25%; compressive strength 20-170 MPa depending on cement; hardness 6-7 for quartz grains.

Physical properties of granite

• Igneous, intrusive, coarse-grained, crystalline rock of quartz, feldspar and mica or hornblende.
• Colour: pink, grey or white, speckled; massive, hard; specific gravity 2.6-2.8; porosity under 1%; compressive strength 100-250 MPa; takes high polish.

Sedimentary rocks can be recognised in the field by:

1. Bedding (stratification), in parallel layers of different colour or grain size.
2. Clastic texture: rounded or angular grains cemented together; gritty feel in sandstone; fine and smooth in shale.
3. Fossils (shells, plants, bones).
4. Surface marks: ripple marks, mud cracks, cross-bedding, raindrop prints.
5. Softness: often scratched more easily than igneous rocks; limestone effervesces with dilute HCl.
6. Occur as layers in basins, rivers, valleys, with pebbles and conglomerates.

Igneous rocks are formed by cooling of magma or lava. They are classified in several ways.

1. By mode of occurrence (place of cooling)

• Plutonic (intrusive): deep, slow cooling, coarse grains (granite, gabbro, diorite).
• Hypabyssal: shallow dykes and sills, medium grains (dolerite, porphyry).
• Volcanic (extrusive): surface, fast cooling, fine or glassy (basalt, rhyolite, andesite, obsidian).

2. By silica content (chemical)

3. By texture

Phaneritic (coarse), aphanitic (fine), porphyritic, glassy and vesicular.

4. By mineral content

Felsic (quartz, feldspar rich) and mafic (iron-magnesium rich).

Types of rocks

Rocks are natural aggregates of minerals. The three classes are:

• Igneous: formed by cooling of magma or lava (granite, basalt).
• Sedimentary: formed by deposition and lithification of sediments (sandstone, shale, limestone).
• Metamorphic: formed by change of existing rocks under heat and pressure (slate, schist, gneiss, marble).

Rock cycle

The rock cycle is the continuous transformation of rocks from one type to another.

      MAGMA
     /     ^
 cooling    | melting
    v       |
  IGNEOUS --+--> weathering, transport,
    |            deposition
    |             v
    |         SEDIMENTS
    |             | compaction,
    |             v cementation
    |         SEDIMENTARY
    |  heat &     |
    |  pressure   |
    v             v
      METAMORPHIC --(melting)--> MAGMA

Igneous rocks weather into sediments, which harden into sedimentary rocks. Heat and pressure change these into metamorphic rocks, which melt to magma, forming igneous rocks again.

Major sedimentary rocks of the Nepal Himalaya and properties

Siwalik rocks are young, poorly cemented, and weak, so they cause erosion and landslides.

Formation of sedimentary rocks

Sedimentary rocks form at the Earth's surface.

1. Weathering breaks parent rocks into fragments and dissolved ions.
2. Erosion and transport by water, wind, ice or gravity.
3. Deposition in layers in basins, rivers, lakes and seas. Chemical precipitation and accumulation of organic remains also add material.
4. Compaction: weight of upper layers squeezes out water.
5. Cementation: silica, calcite or iron oxide binds the grains.
6. Lithification completes the change to rock. Examples: sandstone, shale, limestone.

Formation of metamorphic rocks

Metamorphic rocks form deep in the crust from existing rocks (igneous, sedimentary or metamorphic) in the solid state under changed conditions.

1. Heat (from magma or burial) triggers recrystallisation.
2. Pressure: confining pressure compacts the rock; directed stress aligns minerals into foliation.
3. Chemically active fluids carry ions and speed up reactions.
4. New minerals (garnet, kyanite, mica) and new textures develop without melting.

Types: contact (thermal) metamorphism near intrusions (limestone to marble), regional metamorphism during mountain building (shale to slate, schist, gneiss) and dynamic metamorphism along faults.

Examples: shale to slate to phyllite to schist to gneiss; limestone to marble; sandstone to quartzite.

Formation of metamorphic rock

Metamorphism is the change of an existing rock (parent rock, or protolith) into a new rock in the solid state, by heat, pressure and chemically active fluids, without melting.

1. A parent rock is buried deep, or comes near hot magma, or is squeezed during mountain building.
2. Heat causes recrystallisation and growth of new, stable minerals.
3. Pressure (confining and directed) compacts the rock and aligns platy minerals, forming foliation, slaty cleavage and schistosity.
4. Fluids (water, CO₂) circulate and promote reactions.

Types of metamorphism:

• Contact (thermal): near intrusions, only heat; non-foliated rocks (marble, hornfels, quartzite).
• Regional: widespread, with heat and directed pressure during mountain building; foliated rocks (slate, phyllite, schist, gneiss).
• Dynamic (cataclastic): along faults; crushed rock.

Example: shale to slate to phyllite to schist to gneiss with increasing grade.

Texture of igneous rocks

Texture depends on the cooling rate and the gas content of the magma.

• Phaneritic (coarse): slow cooling, deep, visible crystals (granite).
• Aphanitic (fine): quick cooling (basalt).
• Porphyritic: large phenocrysts in fine matrix.
• Glassy: very rapid cooling (obsidian).
• Vesicular: gas holes (pumice).
• Pegmatitic: very coarse crystals. Also holocrystalline (all crystals), hypocrystalline (crystals and glass) and holohyaline (all glass).

Texture of sedimentary rocks

Texture is the size, shape and arrangement of the grains making up a rock. In sedimentary rocks:

• Clastic texture: made of fragments of older rocks cemented together. Grain size ranges from gravel (>2 mm) to sand and clay (<1/16 mm). Grains may be angular or rounded, and well or poorly sorted. Examples: conglomerate, sandstone, shale.
• Non-clastic texture: crystalline (rock salt, gypsum), oolitic (limestone with small spherical grains), bioclastic (shell fragments) or amorphous (chert).

Rock cleavage

Rock cleavage is the tendency of a rock, usually a fine-grained metamorphic rock such as slate, to split along parallel planes produced by directed pressure, independent of original bedding. It is a secondary structure. Examples are slaty cleavage and fracture cleavage. It makes rocks weak across the cleavage, affects slope and tunnel stability, but gives roofing slate.

Petrography is the branch of petrology that systematically describes and classifies rocks on the basis of their mineral composition, texture and structure, using hand specimens and thin sections under the microscope.

Structures of sedimentary rocks

Structures are the large-scale features, seen in outcrops, showing the conditions of deposition.

1. Stratification (bedding): arrangement in parallel layers, separated by bedding planes. Beds may be thick or thin (laminae).
2. Cross-bedding: inclined layers within a bed, from wind or current action (dunes, deltas).
3. Graded bedding: grain size changes gradually from coarse at bottom to fine at top within a bed.
4. Ripple marks: small wave-like ridges from water or wind movement.
5. Mud cracks (desiccation cracks): polygonal cracks in dried mud.
6. Fossils: preserved remains of organisms.
7. Concretions and nodules: rounded masses of cementing minerals (chert nodules).
8. Rain prints, sole marks, and stylolites.

 ====== bedding ======
 //////// cross-bedding
 ~~~~ ripple marks ~~~~

Petrology is the branch of geology that deals with the origin, occurrence, composition, structure, texture and classification of rocks.

Engineering significance of metamorphic rocks

• Massive types: quartzite, marble and gneiss are strong and durable; used as aggregate, building and decorative stones, and good foundation rocks.
• Foliated types: slate, phyllite and schist split along foliation planes; strength is low across them, so slopes, tunnels and foundations can fail. Slate is used for roofing.
• Mica and talc-rich rocks are slippery and weak.
• Weathering of schists makes thick weak soils; common in the Nepal Lesser Himalaya.

Engineering significance of sedimentary rocks

• Sandstone: building stone, aggregate; silica-cemented sandstone is strong, clay-cemented is weak.
• Limestone: raw material for cement and lime, aggregate, but solution cavities cause reservoir leakage and settlement.
• Shale, mudstone: weak, swell and slake, cause slope failures, and poor foundation.
• Conglomerate: may be strong or weak depending on cement.
• Bedding planes control stability: unfavourable dip toward the cut face causes sliding.
• Siwalik sedimentary rocks of Nepal are weak and erodible.

Schist

Medium to coarse grained metamorphic rock of medium to high grade.

• Texture: crystalloblastic, lepidoblastic (flaky minerals visible to naked eye).
• Structure: well developed foliation called schistosity.
• Minerals: mica (muscovite, biotite), chlorite, talc, quartz, garnet, hornblende.
• Engineering properties: strength is low and highly anisotropic. It splits along schistosity, so it is weak for foundations, slopes (sliding along foliation) and tunnels (overbreak, squeezing). It is not good as a building stone or aggregate.

Sandstone

Clastic sedimentary rock made of sand-size grains (0.06 - 2 mm) cemented together.

• Texture: arenaceous, clastic, grains of quartz held by cement.
• Structure: bedded, with cross bedding and ripple marks; jointed.
• Minerals: quartz mainly, some feldspar and mica; cement of silica, calcite, iron oxide or clay.
• Engineering properties: compressive strength 20 - 170 MPa depending on cement (siliceous strongest, argillaceous weakest), specific gravity 2.2 - 2.7, porosity 5 - 25 %, water absorption 1 - 10 %. Used as building stone, paving and aggregate. Weak cement type weathers and softens in water.

Phyllite

Fine grained low-grade metamorphic rock between slate and schist.

• Texture: very fine, lepidoblastic, with silky sheen.
• Structure: foliated (phyllitic cleavage), often wavy or crinkled.
• Minerals: sericite mica, chlorite, quartz.
• Engineering properties: weak and easily split along foliation, weathers rapidly to clayey soil, shows creep and slope failure. Poor foundation and tunnelling rock; widespread in the Lesser Himalaya of Nepal, where it causes landslides.

Marble

• Used as a decorative stone for flooring, cladding, steps, columns and monuments because it takes a fine polish.
• Soft and easy to cut and carve, so it is cheap to dress; crushed marble is used in terrazzo and as aggregate.
• Reacts with acid and acid rain, so it is unsuitable outside in polluted areas; a calcite-rich marble body also dissolves, giving cavities.

Granite

• Very strong, hard and durable, so it is used for foundations, bridge piers, retaining walls, dam facing and kerb stones.
• Crushed granite gives excellent road metal, railway ballast and concrete aggregate.
• Resists weathering and fire up to a limit; polished slabs are used for floors and counters. Jointed granite can leak and deeply weathered granite must be removed from foundations.

Sandstone

• Used as building and paving stone, and as aggregate; strength depends on the cement (siliceous is best).
• Well-cemented sandstone is a good foundation rock; porous types are permeable and may leak in reservoirs.
• Weakly cemented sandstone weathers, loses strength and is unsuitable for exposed work.

Granite

• Foundations, bridge piers and abutments, retaining walls and dams, because of high strength and durability.
• Crushed to aggregate for concrete, road metal and railway ballast.
• Polished slabs for facing, flooring, steps, kerbs and monuments.

Marble

• Decorative flooring, wall cladding, steps, columns, statues and monuments (takes polish).
• Crushed for terrazzo flooring and as filler; used in lime and cement industry in some areas.
• Not suitable for external use in acidic environments.

1. Marble is a decorative building stone: because it is soft, fine grained and takes a high polish, it is used for flooring, wall cladding, steps, columns and monuments.
2. Crushed marble is used as aggregate in terrazzo flooring and, being calcite-rich, as a source of lime. (Being soluble in acids, it is not suitable for exposed use in polluted areas.)

Conglomerate is a coarse clastic (rudaceous) sedimentary rock made of rounded pebbles, cobbles and gravel (>2 mm) cemented in a finer matrix of sand, silt or clay by silica, calcite or iron oxide. The rounded fragments show transport by water. It is bedded and poorly sorted. Strength depends on the cement: siliceous cement is strong, clayey cement is weak. Being porous and weakly cemented in some cases, it can leak and be unstable. It is used as aggregate and in rough masonry, and is common in the Siwalik Zone of Nepal.

Shale is a fine grained clastic (argillaceous) sedimentary rock formed by compaction of clay and silt. It is thinly bedded and splits easily into thin layers along bedding planes (fissility). Main minerals are clay minerals (illite, kaolinite) with quartz. It is soft, weak, has low strength, absorbs water, swells and slakes on wetting and drying. Engineering problems: poor foundation and slope stability, swelling in tunnels. It is used in brick and cement manufacture and metamorphoses to slate.

Slate is a fine grained, low-grade metamorphic rock formed from shale or clay by regional metamorphism. Its main feature is slaty cleavage, which allows it to split into thin, strong, smooth sheets. Colours are grey, black, green or purple. Minerals are mica (sericite), chlorite and quartz. It has low porosity and water absorption and is durable. Uses: roofing sheets, flooring, paving, damp-proof course and blackboards. In engineering it is weak along cleavage, which causes slope and tunnel problems.

Marble is a non-foliated metamorphic rock formed by recrystallisation of limestone or dolomite under heat and pressure. It is composed of interlocking calcite crystals (granoblastic texture), white or coloured with veins due to impurities. Hardness 3, specific gravity about 2.7, compressive strength about 70 - 140 MPa, effervesces with dilute HCl. It takes a fine polish and is used for flooring, cladding, steps and sculpture. It is attacked by acid rain, so it is not suitable for outdoor use in polluted areas. It is found in the Lesser Himalaya (for example Godavari, Kathmandu).

Granite is a light-coloured, coarse grained plutonic igneous rock of acidic composition. It consists of quartz, orthoclase feldspar, plagioclase and mica or hornblende, in a holocrystalline granular texture. It is massive, hard (Mohs 6 - 7), strong (compressive strength 100 - 250 MPa) and has very low porosity. It is used for foundations, bridge piers, dams, facing stone, kerbs, aggregate and ballast. Weathering to clay and joints are its main engineering problems.

Quartzite is a hard, non-foliated metamorphic rock formed by recrystallisation of quartz sandstone under heat and pressure (contact or regional metamorphism).

• Texture and structure: granoblastic texture, with quartz grains fused into an interlocking mosaic; massive; relict bedding may remain.
• Composition: over 90 % quartz, with small amounts of mica, feldspar and iron oxides.
• Properties: colour white, grey, pink or brown; very hard (Mohs 7); specific gravity 2.65; compressive strength 150 - 300 MPa; very low porosity; fractures across grains (conchoidal); resists weathering and chemicals.
• Engineering uses: road metal, concrete aggregate, railway ballast, riprap, and a good foundation rock for dams and bridges. It is hard to excavate and dress and is generally jointed, so joints control seepage and stability.
• Occurrence: found in the Lesser and Higher Himalaya of Nepal, such as in the Nawakot Complex.

Lutite (argillaceous rock) is a fine grained clastic sedimentary rock composed of clay and silt-size particles (<0.06 mm). Lutites are formed by deposition of mud in quiet water (lakes, lagoons, deep sea) and later compaction.

• Types: clay, mudstone, siltstone, claystone and shale. Shale is the fissile variety.
• Texture and structure: very fine, pelitic; laminated or thinly bedded; may be fissile.
• Minerals: clay minerals (kaolinite, illite, montmorillonite), fine quartz and mica.
• Properties: soft, low strength, plastic when wet, absorbs water, swells and slakes, low permeability.
• Engineering significance: poor foundation rock, unstable in slopes and tunnels, weak when wet; however, being impermeable it is a good seal for reservoirs and dam abutments. Used for bricks, tiles, pottery and cement.
• On metamorphism lutite changes to slate, phyllite, schist.

The structures of metamorphic rocks are of two main kinds.

Foliated structures

Parallel arrangement of platy or elongated minerals due to directed pressure.

• Slaty cleavage - very fine, rock splits into thin sheets (slate).
• Phyllitic structure - fine, silky sheen, often crinkled (phyllite).
• Schistose structure - coarse flaky minerals in parallel layers (schist).
• Gneissose structure - alternate light (quartz, feldspar) and dark (mica, hornblende) bands (gneiss).

Non-foliated structures

• Massive/granulose - interlocking equigranular grains, no layering (marble, quartzite).
• Hornfels - hard, fine grained, dense, from contact metamorphism.
• Cataclastic - crushed and broken grains from dynamic metamorphism (mylonite, fault breccia).
• Augen structure - lens-shaped large grains in a foliated matrix.

Metamorphic rock and its types

Metamorphic rock is a rock formed from pre-existing igneous, sedimentary or metamorphic rock by change in texture, structure and mineral composition in the solid state under high temperature, pressure and chemically active fluids. Metamorphism is this process.Types by cause:

1. Contact (thermal) metamorphic rocks - heat from nearby magma (hornfels, marble, quartzite).
2. Regional (dynamothermal) metamorphic rocks - heat and pressure over a wide area during mountain building (slate, phyllite, schist, gneiss).
3. Dynamic (cataclastic) metamorphic rocks - mainly pressure along faults (mylonite, fault breccia).

By texture: foliated (slate, schist, gneiss) and non-foliated (marble, quartzite).

Structure of metamorphic rocks

• Foliated: slaty cleavage (slate), phyllitic (phyllite), schistose (schist), gneissose banding (gneiss).
• Non-foliated: massive, granulose structure (marble, quartzite), hornfels.
• Cataclastic and augen structures near shear zones.

Significance of igneous rocks in civil engineering

• Strong and durable: granite, basalt and gabbro are used for foundations, piers, dams and retaining walls.
• Crushed igneous rock is a main source of concrete aggregate, road metal and railway ballast.
• Dimension stones: polished granite for flooring, facing and kerb.
• Massive, non-porous and without bedding, they are suitable sites for dams and tunnels, but joints, weathering and sheeting must be examined.
• Dykes and sills may form weak zones or barriers to water movement.

Metamorphism and metamorphic rock

Metamorphism is the change in mineralogy, texture and structure of a pre-existing rock in the solid state, caused by heat, pressure and chemically active fluids. A metamorphic rock is the rock produced by this process, for example marble from limestone, slate from shale.

Types of rock cleavage

Rock cleavage is the property of a metamorphic rock to split along closely spaced parallel planes.

1. Slaty cleavage - very fine, perfectly parallel planes; rock splits into thin sheets (slate). Caused by parallel alignment of microscopic mica and chlorite perpendicular to the maximum stress.
2. Fracture (strain-slip) cleavage - closely spaced fractures along which small movement has taken place; the rock is crossed by parallel cracks.
3. Flow cleavage - arrangement of flattened grains and minerals produced by plastic flow in the rock (schist, phyllite).
4. Schistosity - coarse foliation of platy minerals in schist (sometimes treated as a separate type).

Engineering significance

• Strength: rock is weak along cleavage planes and strong across them, so strength is anisotropic.
• Slopes: cleavage dipping out of a cut slope gives sliding planes (landslides, rock slides).
• Tunnels and excavation: overbreak, roof falls, and squeezing along cleavage planes; tunnels should be aligned across the cleavage strike.
• Seepage: cleavage planes allow water to pass, so leakage under dams and reservoirs may take place.
• Useful aspect: slate cleavage gives roofing slabs and flooring stone that are easily quarried.

Rock cleavage

Rock cleavage is the tendency of a rock to split along closely spaced, parallel, planar surfaces that are not related to the original bedding. It is a secondary structure formed in metamorphic rocks (especially slate, phyllite and schist) by pressure.

Types of cleavage

1. Slaty cleavage - very fine, parallel planes; the rock splits into thin, flat sheets. Developed by parallel orientation of mica and chlorite flakes at right angles to the maximum compressive stress (slate).
2. Fracture cleavage - numerous closely spaced parallel fractures with small displacement; usually in brittle rocks and in the crests of folds. Also called shear or strain-slip cleavage.
3. Flow cleavage - formed by plastic flow and the parallel arrangement of flattened grains and recrystallised minerals; seen in phyllite and schist, where it is also called schistosity.

Civil engineering significance

• Cleavage planes are zones of weakness, so the rock strength is low parallel to them.
• They make cut slopes unstable when cleavage dips towards the free face.
• They give leakage paths for water in dams, reservoirs and tunnels.
• In tunnels they lead to overbreak and roof falls.
• On the positive side, slates are cut easily into roofing and paving slabs.