ECOS Guide to the Ecology of the Northern Rockies

		ECOS Guide to the Ecology of the Northern Rockies

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Montana Rock Descriptions and Distribution

Brief descriptions of common Western Montana rocks.

Rock Types:

Extrusive
Intrusive
Metamorphic
Sedimentary

Extrusive Rocks
Lithology	Rock Type	Mineralogy	Appearance	Tectonic Environment	Uses	Western MT Occurrence
andesite	extrusive	plagioclase (<50% anorthite), biotite; also magnetite, ilmenite, quartz, hornblende, pyroxene, glass, olivine, orthoclase, anorthoclase	blackish-brown or greenish; porphyritic texture with plagioclase and biotite phenocrysts	lava flows and domes, usually in association with basalt	porphyry copper ore	none
basalt	extrusive	plagioclase (>50% anorthite), pyroxene; also magnetite, hematite, ilmenite, apatite, quartz, olivine, glass, amphibole, biotite	black to dark gray; holocrystalline to hypocrystalline to vitreous texture; fine crystals with rare palgioclase, pyroxene, and olivine phenocrysts	most common extrusive rock, formed from mantle sources	road paving and railroad ballast	sparse
latite	extrusive	Sanidine, plagioclase, quartz, biotite, hornblende; also anorthoclase, olivine, feldspathoid, magnetite, ilmenite.	Gray with pink, green or brown hues	lava flows and dikes in tectonically stable environments	none	some Cenozoic formations Map
rhyolite	extrusive	quartz, alkaline feldspar; also glass, biotite, albite, magnatite, ilmenite, tridymite, cristobalite, amphibole, pyroxene	very light (except in glassy varieties such as obsidian); porphyritic texture	forms from rapid cooling of viscous granitic magma, occurring in domes, chimneys, dikes, and lava flows	acoustic and light thermal insulation, chemical washing processes, and rock wool	sparse
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Intrusive Rocks
Lithology	Rock Type	Mineralogy	Appearance	Tectonic Environment	Uses	Western MT Occurrence
aplite	intrusive	quartz, alkaline feldspar, mica; also tourmaline	with or gray; mosaic texture; fine crystalline	fracture filling in felsic intrusive and veined rocks	none	sparse
diabase	intrusive	plagioclase, clinopyroxene; also orthopyroxene, olivine, hornblende, magnetite, ilmenite, apatite, calcite, pyrrhotite, chalcopyrite, glass, serpentine, chlorite	dark, green to black; porphyritic texture	sills and dike swarms	copper occurrences and building stone	swarm NW of Missoula Map
diorite	intrusive	plagioclase (<50% anorthite), hornblende; also magnetite, ilmenite, titanite, allanite, quartz, orthopyroxene, clinopyroxene, orthoclase, biotite	dark to blackish gray; granular to porphyritic texture	limited to marginal zones of granitic plutons and transitional facies of gabbroic plutons	building stones	discontinuously ditributed along eastern front of Rocky Mtns Map
gabbro	intrusive	plagioclase (>50% anorthite), olivine, clinopyroxene; also chromite, magnetite, ilmenite, apatite, sulfide, titanite, rutile, corundum, amphibole, garnet, biotite	dark, green-gray	forms in deep parts of large intrusions	building stones, sometimes chromium, nickel, cobalt, iron, and platinum ore	sparse
granite	intrusive	quartz, potassic feldspar, plagioclase, biotite; also magnetite, ilmenite, apatite, pyrite, zircon, allanite, tourmaline, muscovite, hornblende, pyroxene, garnet	with, gray, pink; granular texture, sometimes with large feldspar grains	large homogeneous batholiths and plutons	building stone	Beartooth Plateau Map
granodiorite	intrusive	quartz, plagioclase, potassic feldspar, biotite, hornblende; also magnetite, ilmenite, apatite, titanite, allanite, zircon, pyroxene, muscovite	light to dark gray; granular texture with large hornblende or K-feldspar	small plutons and marginal regions of large granitic batholiths	building material	Bitterroot and southern Sapphire Mountains Map
granophyric rock	intrusive	potassic feldspar, quartz; also plagioclase, biotite, amphibole, sodic pyroxene, magnetite, ilmenite, muscovite, apatite, zircon, molybdenite, topaz, fluorite	pink; porphyritic texture	veins and peripheral parts of granitic plutons	building stone, ornamental gravel	southern Bitterroot Mountains Map
monzonite	intrusive	potassic feldspar, plagioclase, clinopyroxene, hornblende; also magnetite, ilmenite, quartz, titanite, biotite, titaniferous orthopyroxene, nepheline, olivine	dark gray, greenish, reddish; granular texture	small intrusive laccolith or sill bodies, associated with rift zones	building stone	associated with the Boulder Batholith near Butte Map
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Metamorphic Rocks
Lithology	Rock Type	Mineralogy	Appearance	Tectonic Environment	Uses	Western MT Occurrence
gneiss	metamorphic	feldspar, mica; also epidote, apatite, tourmaline, allanite, magnetite, ilmenite, zircon, monazite, titanite, pyrite, pyrrhotite, quartz, chlorite, kyanite, sillimanite, andalusite, cordierite, garnet, hornblende, augite	usually light with compositional banding	medium to high metamorphic grade; most gneiss comes from sedimentary rocks (paragneiss); some gneiss comes from granitic rocks (orthogneiss)	building stone	typically associated with basement rocks Map
marble	metamorphic	calcite; also graphite, pyrite, ilmenite, dolomite, quartz, mica, chlorite, plagioclase, epidote, diopside, pyroxene, tremolite, wollastonite, vesuvianite, forsterite, talc, brucite, serpentine, periclase	white, sometimes streaked with green, gray, brown, and red; granoblastic texture	limestone recrystallized by regional or contact metamorphism	building material	scarce, associated with basement
quartzite	metamorphic	quartz; also mica, feldspar, heavy detrital minerals, garnet, graphite, calcite, sulfides	very white if pure quartz, grading to black with other mineral presence; granoblastic texture	metamorphism of quartz-rich sedimentary rocks	construction material, glass, ceramics	somewhat common in SW MT Map
schist	metamorphic	quartz, mica; also chlorite, apatite, tourmaline, zircon, pyrite, ilmenite, magnetite, graphite, andalusite, cordierite, kyanite, sillimanite, garnet, staurolite, epidote, calcite, plagioclase, glaucophane	variable, depending on mineralogy; schistose texture; thin compositional banding	medium to high metamorphic grade; regional metamorphism of shales and arkoses	gravel	associated with basement
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Sedimentary Rocks
Lithology	Rock Type	Components	Appearance	Depositional Environment	Uses	Western MT Occurrence
argillite	sedimentary	calcite and clay minerals, with quartz, mica, and carbonates	light to dark gray, brown, or green; clastic texture with very fine grain size; stratification barely visible	marine or lacustrine clastics, lengthy movement before deposition	cement	associated with Belt rocks Map
arkose	sedimentary	mainly feldspar, with quartz, biotite, muscovite, and other rock fragments; silicate, calcite, or limonite cement	gray, pink, or reddish; clastic texture with coarse grain size; poorly sorted; angular grains; poor stratification; no fossils	fluvial, lacustrine, or shallow marine deposition; granitic source; short distance of travel	building material	sparse Map
coal	sedimentary	carbon, hydrocarbons, pyrite, quartz	dark brown to black, shiny, hard, compact	deposited in lagoons and coastal lakes	fuel	scarce Map
conglomerate	sedimentary	pebbles of any type of rock; sandy or muddy matrix; calcite cement	variable color; roundish pebbles (diameter >4mm)	deposition in shallow turbulent water	building stone	sparse
dolomite	sedimentary	dolomite and calcite, with clay, quartz, pyrite, marcasite, and bitumen	light, gray, yellow, pink; does not effervesce with dilute HCl	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	reservoir rocks for petroleum	sparse Map
limestone	sedimentary	clacite, with secondary dolomite and aragonite; also chalcedony, quartz, clay minerals	variable; white, yellow, brown, pink, red, dark brown, black; variable texture, from compact porcelain to coarse grains; fossils can be common	accumulation of calcareous skeletons of marine organisms	construction material	common in paleozoic formations in the fold and thrust belt Map
sandstone	sedimentary	rock fragments <2mm in diameter; commonly quartz, feldspar, mica, and calcite; silica, calcite, dolomite, or clay cement	variable color; white, gray, yellow, green, red, brown; clastic texture, well sorted, rounded grains; well stratified; fossils and sedimentary structures are common	accumulations of clastic material transported by wind, river, and sea.	building stone	common east of Rocky Mountain Front Map
shale	sedimentary	clay or mud	black, gray, green, red; thin layers	deposited in slow moving or standing water, such as lake, lagoon, floodplain, and offshore environments	building material	common through western Montana Map
siltstone	sedimentary	grains <62 micrometers	similar to shale, but slightly coarser	deposited in slow moving water	building material	silt is common in river valleys Map
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The ECOS program is sponsored by the University of Montana's Division of Biological Sciences, and the College of Forestry and Conservation. Carol Brewer Program Director, Division of Biological Sciences. Paul Alaback Program Co-Director, College of Forestry and Conservation.

ECOS is supported by the GK-12 Program of the National Science Foundation.
Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.