this impressive m winneshiek county. such cave decorations are … · 2014-08-18 · lake superior...

MINERALS

Pointed crystals of white calcite and translucent yellowcubes of fluorite edge this gray limestone collected nearPostville in Allamakee County. Calcite (calcium carbonate)is the commonmineral in lime-stone, while fluoriteis rare. Such crystalgrowths are foundalong open vugsand fracture traceswithin the rock.

FRONT COVER"Dog-tooth spar" is the name given

to sharply pointed crystals of white calciteas seen on this massive piece of gray limestone

from Mahaska County. Also prominent arebrass-colored masses of pyrite crystals,

known as "fools gold."

Photo by Tim Kemmis

Geodes have adrab roundedexterior with ahard outer layerand partiallyhollow interiorlined with in-wardly project-

ing mineral crystals. This large geode, containing pinkand gray calcite and glassy quartz crystals, was col-lected near Keokuk from the Warsaw Shale, a rockformation that outcrops along stream beds in Iowa'ssoutheastern counties.

This wind polished and lichen covered rockof Sioux Quartzite is from Lyon County innorthwest Iowa. Quartzite is composed ofcompacted quartz grains cementedtogether with silica, giving the rock aglassy appearance and a hard surface. Itsresistance to weathering makes it usefulas highway and railroad aggregate.

OFIOWA

Minerals are the building blocks of

the Earth's rocks. They have a

specific chemical composition and a

characteristic crystal form. The Iowa miner-

als shown here display an intriguing range

of color and shape.

Many people are first introduced to the

field of geology through the fun of search-

ing for and collecting minerals. Beautiful

varieties can be found in Iowa's sedimentary

rocks that outcrop in road cuts, quarries,

strip mines, and along natural stream banks

or valley sides. Eye-catching crystals com-

pose many coarse-grained igneous and

metamorphic cobbles and boulders that lie

in pastures and farm fields where they were

left by melting glaciers. Gravel pits along

Iowa's valleys and gravel bars within river

channels are also good places to find a wide

assortment of mineral specimens.

In addition to their crystalline beauty,

information about a mineral's geologic age

and origins can be obtained from its chemi-

cal isotopes and from its association with

other minerals. Mineral resources play a

significant role in our daily lives, and Iowa's

mineral industries are a valuable contributor

to the state's economy.

Geological Survey Bureau109 Trowbridge Hall

Iowa City, Iowa 52242-1319www.igsb.uiowa.edu

Educational Materials: EM-27 (2001)

Iowa Department of Natural RIowa Department of Natural RIowa Department of Natural RIowa Department of Natural RIowa Department of Natural Resourcesesourcesesourcesesourcesesources

This impressive stalactite is from a cave inWinneshiek County. Such cave decorations are

composed of the mineralcalcite, and are deposited indistinctive shapes by the slowdripping of lime-richgroundwater.

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Galena has adistinctmetallic-grayluster and acube-shapedcrystal form. Itis quite heavyand is the prin-cipal ore of

lead. This mass of crystals is from DubuqueCounty, where lead ores were mined forover 300 years from veins in the dolomitebedrock.

Coal is a combustible rock, rich in carbon andformed by compaction of fossil plant remainssimilar to peat. Thin veins in this piece are filledwith pyrite. Coal was mined from seams in thePennsylvanian-age rocks of south-central Iowa,with peak production during the early 1900s.

Limonite is a distinctively yellowish brown ore ofiron. It takes many forms, including the cellularstructure seen in this sample from the historic Iron

Hill area nearWaukon in Alla-makee County.

The tall slender crystal of gypsum, a varietyknown as selenite, is from Appanoose County. It

has a soft, easilyscratched surface.Below it is aspecimen of band-ed gypsum from theFort Dodge area.Gypsum is minedin Webster and DesMoines countiesfor wallboardproduction.

This pyramidcrystal oftranslucentcalcite is fromMahaskaCounty.Calcite is theprincipalmineral inlimestone,chalk, andmarble. Itoccurs in avariety ofcolors and bubbles vigorously when a drop of dilutehydrochloric acid is applied.

Barite is anunusually heavymineral. ThisFayette Countysample is composedof curved masses ofradiating crystals.Barite is used in themanufacture ofpaints and drillingmuds.

Heavy nuggets ofnative copper, agood conductor ofheat and electricity,are found on rareoccasions in Iowa'sglacial deposits. This67-pounder, tar-nished with greenishoxides, probablyoriginated in theLake Superior regionof Michigan's UpperPeninsula.

Known tomineralcollectorsas "riceagate," these polished stones of black chert (flint) consistof a dense variety of silica found in the sedimentary rocksof Montgomery County. The "rice" pattern comes fromnumerous white shells of fossil fusulinids, a tiny marineprotozoan.

These agates (varieties ofdense but translucent quartz,chalcedony, and opal) arefrom Mississippi River graveldeposits in Clayton County andhave been tumbled to a highpolish. They include the prizedLake Superior agates, knownfor their fine alternatingbands of rich colors.

Metallic clusters of pyrite crystals (“fool's gold”) form bumpson this piece of limestone collected in Black Hawk County. Thepattern of mineralclusters is a resultof mineral growthin the honey-combed openingsof a fossil colonialcoral within thelimestone.

Feldspar isa widespreadmineral,especiallycommon inigneous rockssuch asgranite. Thisfragment ofcrystallinefeldspar wasfound ingravels along the Cedar River in Linn County. Itprobably weathered out of a granite boulder carriedinto Iowa by a glacier.

Petrified wood can becollected from glacial-agegravels along Iowa's rivers.This water-worn piece fromthe Cedar River in LinnCounty shows silica, inthe form of chalcedonyor opal, has replaced theoriginal tissue. Tan andbrown bands reveal theoriginal wood grain.

Non-credited photos by Paul VanDorpePhoto setups by Patricia Lohmann