Rocks of Western Montana

  • Andesite

    Rock type: extrusive

    Mineralogy: plagioclase (<50% anorthite), biotite; also magnetite, ilmenite, quartz, hornblende, pyroxene, glass, olivine, orthoclase, anorthoclase

    Techtonic environment: lava flows and domes, usually in association with basalt

    Uses: porphyry copper ore

    Western Montana occurrence: none

  • Aplite

    Rock type: intrusive

    Mineralogy: quartz, alkaline feldspar, mica, also tourmaline

    Techtonic environment: fracture filling in felsic intrusive and veined rocks

    Uses: none

    Western Montana occurrence: sparse

  • Argillite

    Rock type: sedimentary

    Mineralogy: calcite and clay minerals, with quartz, mica, and carbonates

    Techtonic environment: marine or lacustrine clastics, lengthy movement before deposition

    Uses: cement

    Western Montana occurrence: associated with belt rocks

  • Arkose

    Rock type: sedimentary

    Mineralogy: mainly feldspar, with quartz, biotite, muscovite, and other rock fragments, silicate, calcite, or limonite cement

    Techtonic environment: fluvial, lacustrine, or shallow marine deposition, granitic source, short distance of travel

    Uses: building material

    Western Montana occurrence: sparse

  • Basalt

    Rock type: extrusive

    Mineralogy: plagioclase (>50% anorthite), pyroxene; also magnetite, hematite, ilmenite, apatite, quartz, olivine, glass, amphibole, biotite

    Techtonic environment: most common extrusive rock, formed from mantle sources

    Uses: road paving and railroad ballast

    Western Montana occurrence: sparse

  • Coal

    Rock type: sedimentary

    Mineralogy: carbon, hydrocarbons, pyrite, quartz

    Techtonic environment: deposited in lagoons and coastal lakes

    Uses: fuel

    Western Montana occurrence: scarce

  • Conglomerate

    Rock type: sedimentary

    Mineralogy: pebbles of any type of rock, sandy or muddy matrix, calcite cement

    Techtonic environment: deposition in shallow turbulent water

    Uses: building stone

    Western Montana occurrence: sparse

  • Diabase

    Rock type: intrusive

    Mineralogy: plagioclase, clinopyroxene; also orthopyroxene, olivine, hornblende, magnetite, ilmenite, apatite, calcite, pyrrhotite, chalcopyrite, glass, serpentine, chlorite

    Techtonic environment: sills and dike swarms

    Uses: copper occurrences and building stone

    Western Montana occurrence: swarm NW of Missoula

  • Diorite

    Rock type: intrusive

    Mineralogy: plagioclase (<50% anorthite), hornblende, also magnetite, ilmenite, titanite, allanite, quartz, orthopyroxene, clinopyroxene, orthoclase, biotite

    Techtonic environment: limited to marginal zones of granitic plutons and transitional facies of gabbroic plutons

    Uses: building stones

    Western Montana occurrence: discontinuously distributed along eastern front of Rocky Mountains

  • Dolomite

    Rock type: sedimentary

    Mineralogy: dolomite and calcite, with clay, quartz, pyrite, marcasite, and bitumen

    Techtonic environment: primary dolomites are rare, most are due to metasomatic substitution during diagenesis of aragonite, present in fossil shells in sea water and brackish lagoons or lakes

    Uses: reservoir rocks for petroleum

    Western Montana occurrence: sparse

  • Gabbro

    Rock type: intrusive

    Mineralogy: plagioclase (>50% anorthite), olivine, clinopyroxene, also chromite, magnetite, ilmenite, apatite, sulfide, titanite, rutile, corundum, amphibole, garnet, biotite

    Techtonic environment: forms in deep parts of large intrusions

    Uses: sparse

    Western Montana occurrence: building stones, sometimes chromium, nickel, cobalt, iron, and platinum ore

  • Gneiss

    Rock type: metamorphic

    Mineralogy: feldspar, mica, also epidote, apatite, tourmaline, allanite, magnetite, ilmenite, zircon, monazite, titanite, pyrite, pyrrhotite, quartz, chlorite, kyanite, sillimanite, andalusite, cordierite, garnet, hornblende, augite

    Techtonic environment: medium to high metamorphic grade, most gneiss comes from sedimentary rocks (paragneiss), some gneiss comes from granitic rocks (orthogneiss)

    Uses: building stone

    Western Montana occurrence: typically associated with basement rocks

  • Granite

    Rock type: intrusive

    Mineralogy: quartz, potassic feldspar, plagioclase, biotite, also magnetite, ilmenite, apatite, pyrite, zircon, allanite, tourmaline, muscovite, hornblende, pyroxene, garnet

    Techtonic environment: large homogeneous batholiths and plutons

    Uses: building stone

    Western Montana occurrence: Beartooth Plateau

  • Granodiorite

    Rock type: intrusive

    Mineralogy: quartz, plagioclase, potassic feldspar, biotite, hornblende, also magnetite, ilmenite, apatite, titanite, allanite, zircon, pyroxene, muscovite

    Techtonic environment: small plutons and marginal regions of large granitic batholiths

    Uses: Bitterroot and southern Sapphire Mountains

    Western Montana occurrence: building material

  • Granophyric rock

    Rock type: intrusive

    Mineralogy: potassic feldspar, quartz, also plagioclase, biotite, amphibole, sodic pyroxene, magnetite, ilmenite, muscovite, apatite, zircon, molybdenite, topaz, fluorite

    Techtonic environment: veins and peripheral parts of granitic plutons

    Uses: building stone, ornamental gravel

    Western Montana occurrence: southern Bitterroot Mountains

  • Latite

    Rock type: extrusive

    Mineralogy: sanidine, plagioclase, quartz, biotite, hornblende; also anorthoclase, olivine, feldspathoid, magnetite, ilmenite

    Techtonic environment: lava flows and dikes in tectonically stable environments

    Uses: none

    Western Montana occurrence: some Cenozoic formations

  • Limestone

    Rock type: sedimentary

    Mineralogy: calcite, with secondary dolomite and aragonite, also chalcedony, quartz, clay minerals

    Techtonic environment: accumulation of calcareous skeletons of marine organisms

    Uses: construction material

    Western Montana occurrence: common in paleozoic formations in the fold and thrust belt

  • Marble

    Rock type: metamorphic

    Mineralogy: calcite, also graphite, pyrite, ilmenite, dolomite, quartz, mica, chlorite, plagioclase, epidote, diopside, pyroxene, tremolite, wollastonite, vesuvianite, forsterite, talc, brucite, serpentine, periclase

    Techtonic environment: limestone recrystallized by regional or contact metamorphism

    Uses: building material

    Western Montana occurrence: scarce, associated with basement

  • Monzonite

    Rock type: intrusive

    Mineralogy: potassic feldspar, plagioclase, clinopyroxene, hornblende, also magnetite, ilmenite, quartz, titanite, biotite, titaniferous orthopyroxene, nepheline, olivine

    Techtonic environment: small intrusive laccolith or sill bodies, associated with rift zones

    Uses: building stone

    Western Montana occurrence: associated with the Boulder Batholith near Butte

  • Quartzite

    Rock type: metamorphic

    Mineralogy: quartz, also mica, feldspar, heavy detrital minerals, garnet, graphite, calcite, sulfides

    Techtonic environment: metamorphism of quartz-rich sedimentary rocks

    Uses: construction material, glass, ceramics

    Western Montana occurrence: somewhat common in SW MT

  • Rhyolite

    Rock type: extrusive

    Mineralogy: quartz, alkaline feldspar, also glass, biotite, albite, magnatite, ilmenite, tridymite, cristobalite, amphibole, pyroxene

    Techtonic environment: forms from rapid cooling of viscous granitic magma, occurring in domes, chimneys, dikes, and lava flows

    Uses: acoustic and light thermal insulation, chemical washing processes, and rock wool

    Western Montana occurrence: sparse

  • Sandstone

    Rock type: sedimentary

    Mineralogy: rock fragments <2mm in diameter, commonly quartz, feldspar, mica, and calcite, silica, calcite, dolomite, or clay cement

    Techtonic environment: accumulations of clastic material transported by wind, river, and sea

    Uses: building stone

    Western Montana occurrence: common east of Rocky Mountain Front

  • Schist

    Rock type: metamorphic

    Mineralogy: quartz, mica, also chlorite, apatite, tourmaline, zircon, pyrite, ilmenite, magnetite, graphite, andalusite, cordierite, kyanite, sillimanite, garnet, staurolite, epidote, calcite, plagioclase, glaucophane

    Techtonic environment: medium to high metamorphic grade, regional metamorphism of shales and arkoses

    Uses: gravel

    Western Montana occurrence: associated with basement

  • Shale

    Rock type: sedimentary

    Mineralogy: clay or mud

    Techtonic environment: deposited in slow moving or standing water, such as lake, lagoon, floodplain, and offshore environments

    Uses: building material

    Western Montana occurrence: common through western Montana

  • Siltstone

    Rock type: sedimentary

    Mineralogy: grains <62 micrometers

    Techtonic environment: deposited in slow moving water

    Uses: building material

    Western Montana occurrence: silt is common in river valleys