Rock Types and their Mineralogy

 Minerals / General / Rock Types and Mineralogy
1. Igneous Rocks

I.Plutonic

Granite - composed of quartz, alkali feldspar and plagioclase as essential minerals and small amounts of mafic minerals(biotite &/hornblende), alkali feldspar predominates over plagioclase.

Granodiorite - composed of quartz, alkali feldspar and plagioclase as essential minerals and small amounts of mafic minerals(biotite &/hornblende), plagioclase predominates over alklai feldspar.

Tonalite - composed of quartz, and plagioclase as essential minerals and small amounts of potash feldspars and mafic minerals(biotite &/hornblende).

Trondhjemite - quartz, and plagioclase(albitic) as essential minerals and is free from mafic minerals.

Monzonite - composed of equal portion of plagioclase and K-feldspars.

Syenite - composed of alkali feldspar and plagioclases as essential minerals and small amounts of mafic minerals(bioitte &/hornblende).

Diorite - composed of plagioclase(An<50) and hornblende as essential minerals with accessory quartz and potash feldspar.

Gabbro - composed of plagioclase(An>50) and clino pyroxene as essential minerals (+ or -) orthopyroxene and olivine.

Norite - composed of plagioclase and ortho pyroxene as essential minerals.

Troctolite - composed of olivine and plagioclase of equal proportions.

Pyroxenite - composed either of clino pyroxene or ortho pyroxene(>90% of mafics).

Websterite - composed of both clino pyroxene and ortho pyroxene.

Peridotite -

Harzburgite - composed of olivine and ortho pyroxene.

Wehrlite - composed of olivine and clino pyroxene.

Lherzolite - composed of olivine and both clino pyroxene/ortho pyroxene.

Dunite - monomineralic rock comprising >90% olivine.

Anorthosite - monomineralic rock containing >90% plagioclase(An>70).

Hornblendite - monomineralic comprising >90% hornblende.

II.Volcanic

Rhyolite - comprising quartz,alkali feldspar and plagioclase as essential minerals,alkali feldspar predominates over plagioclase.

Dacite - comprising quartz,alkali feldspar (usually sanidine) and plagioclase as essential minerals plagioclase predominates over alklai feldspar.

Trachyte - comprising alkali feldspar(usually sanidine/anorthoclase) and plagioclase as essential minerals,characterised by flow/trachitoid texture.

Andesite - an intermediate rock comprising pyroxene/hornblende and plagioclase as essential minerals with quartz and potash feldspar as accessories (SiO2>52% and An <50)

Obsidian - is a glassy rock of rhyolite - dacite composition.

Basalt - comprising plagioclase and clino pyroxene as essential minerals,often contains vesicles and amygdules and pillows,variolites etc.

Komatiite - is an ultramafic volcanic rock that contains at least 18% MgO.It is charecterised by Spinifex texture,with skeletal olivine quench crystals.

Broad volcanic and plutonic equivalents

Rhyolite = granite,alkali feldspar granite and silica rich granodiorite.

Dacite = Granodiorite and tonalite.

Trachyte = Syenite.

Andesite = Quartz monzodiorite,quartz diorite and diorite.

Basalt = Gabbro.

Dolerite - comprising plagioclases and clino pyroxene (+ or -) olivine, Opx with ophitic and sub-ophitic texture.

Carbonatite is an intrusive or extrusive igneous rock that contains >05% of carbonate minerals like calcite,dolemite,siderite and ankirite.

Lamprophyre (both calc alkaline and alkaline) is a broad term to a group of hypabyssal igneous rocks (rarely extrusive) containing amphibole and or biotite + olivine, phroxene, melilite as phenocrystic phases and feldspars/feldspathoids as ground mass phase. They typically exhibit panidiomorphic and ocellar texture.

Kimberlite and lamproite (both hypabyssal and volcanic) are hybrid rocks containing crustal and mantle xenoliths and xenocrysts including diamond as well as crystallisation products or ultramafic-ultrapotassic magma. Both the rock types are charecterised by olivine, diopside and phologophite as major phenocrysts and ground mass phases and minor apatite, perovskite, Cr-ilmenite and Cr-spinel.

All these minerals or one of leucite, alkali-amphibole, enstatite and serpentine are characteristically present in lamproite and absent in kimberlite. Monticellite is present in kimberlite and is absent in lamproite. Kimberlites and lamproites are generally charecterised by porphyritic texture, Pelletal textures etc.


2. Sedimentary Rocks
  • Conglomerate is a clastic sedimentary rock comprising rounded pebble sized clasts bounded by arenaceous/argellceous matrix.
  • Oligomictic conglomerate - only one type of clast.
  • Polymictic conglomerate contains more than one type of clast.
  • Intraformational conglomerate contains pebbles of early formed volcanic and sediment clasts which were originated within the basin of deposition.
  • Sandstone is a clastic sedimentary rock comprising predominantly quartz and minor feldspar.
  • Arkose is a clastic sedimentary rock comprising quartz and appreciable amount of feldspar.
  • Greywacke is a clastic sedimentary rock comprising quartz and appreciable amount of feldspar and rock fragments set in a matrix.
  • Shale/Argillite is a very fine grained clastic sedimentary rock comprising quartz, feldspar and clay minerals.
  • Limestone is a non-clastic sedimentary rock (precipitates) comprising calcim carbonate.
  • Dolomite is a non-clastic sedimentary rock comprising calcium and magnesium carbonates.

3. Metamorphic Rocks
  • Slate is very low grade and very fine-grained metamorphic rock that splits into thin, smooth-surfaced layers. The individual minerals can not be recognized by naked eye or hand lens. The parent rock may be only partially altered so that some of the mineralogy and sedimentary bedding are commonly preserved.
  • Phyllite is a low grade and fine-grained metamorphic rock primarily composed of quartz, sericite mica, and chlorite. Small flaky minerals can be recognized by hand lens. Chlorite sheen is characteristic.
  • The rock represents a gradation in the degree of metamorphism between slate and schist.

  • Schist is a medium and medium-to-coarse-grained, metamorphic rock consisting of flaky minerals like micas and chlorite with variable proportions of quartz and feldspar.
  • Gneiss A coarse-grained medium to high grade metamorphic rock in which bands of light coloured, granular minerals (commonly quartz and feldspars) alternate with bands of dark flaky or elongate minerals (biotite/hornblende).
  • Mylonite is a strongly foliated and lineated rock which has under gone grain size reduction by dominant crystal-plastic processes. Resistant grains with in matrix are called porphyroclasts.
  • Phyllonite is a mylonite rich in mica and chlorite.
  • Cataclasite is fine grained crushed rock formed due to brittle deformation.
  • Pseudo tachylite is a dark glassy rock, derived from local melting of the rock by frictional heating during brittle deformation. It is commonly seen in fault zones and occurs as veins injecting the surrounding rock.
  • Migmatite is a composite hybrid rock consiting of two or more petrographically different parts, one of which is country rock (metamorphic) and the other is of igneous component. It generally consists of
  • a) paleosome i.e. the original or slightly modified country rock.

    b) neosome i.e. the newly formed rock portion.

    1. Charnockite - Orthopyroxene granite
    2. Enderbite - Orthopyroxene tonalite
    3. Opdalite - Orthopyroxene granodiorite
    4. Mangerite - Orthopyroxene monzonite
  • Khondalite - a high grade paragneiss of granulite terrain composed of quartz, feldspar, garnet, sillimanite (+ or -) graphite.
  • Marble - a recrystallised limestone.
  • Calc-granulite - a recrystallised impure granulite composed of carbonates and calc silicates.
  • Leptynite - a leucocratic garnetiferous quartzo-felspathic gneiss in granulite terrain.
  • Leptite - a leucocratic quartzo-felspathic gneiss without garnet.

4. Pyroclastic Rocks
  • Ash - any unlithified deposit made of ash (<2mm grain size).
  • Tuff - any lithified ash (varieties-vitric tuff, crystal tuff and lithic tuff).
  • Agglomerate - any deposit made of volcanic bombs/blocks (>64mm size).
  • Tephra - any unlithified pyroclastic deposit.
  • Volcaniclastics : A pyroclastic rock without having genetic connotation (sedimentary plus volcanic clasts).

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