Guide to

Mineral Collecting in Minnesota

G. R. Rapp, Jr. and D. T. Wallace

Department of Geology and Geophysics

University of Minnesota

Illustrations by Ann Cross

WISCONSIN

OF MINNESOTA

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l A K E S AN 0 RIVERS

IOWA

Printed 1966, revised 1974, second revised edition 1979

I SSN 0544-3083

The University of Minnesota is cOlllllitted to the pol icy that all persons shallhave equal access to Its progra s, facilities, and emplo)'!'lent without regardto race, creed, color, sex, national origin, or handIcap.

M ineral collecting appeals to more than six millionAmericans. Rocks and minerals provide many clues

to what we know about nature. Our knowledge of the ageof the earth, the nature of prehistoric life, and therecord of the great ice ages comes from what we candetermine from the study of rock strata.

This booklet is about rocks and minerals in Minne­sota. It is intended for the general public, particu­larly for those individuals that are just awakening toor are renewing an earlier interest in rocks. We hopeto point the beginner in a direction that will providean interesting and rewarding hobby. To do this we offersome essential background on rocks and minerals and adetailed guide to many of Minnesota's more attractiverock and mineral specimens.

Although the terms minerals and rocks are often usedinterchangeably, it is not entirely correct to do so.Minerals are distinct chemical species. Each mineral hasits own chemical formula and crystal structure. Indeed,minerals are often referred to as crystals, especiallyif they exhibit crystal faces. Rocks, on the other hand,are aggregates of several minerals. Just as minerals arecomposed of certain specified elements, each rock typeis composed of a certain group of minerals. For example,the rock granite is composed chiefly of the mineralsquartz and feldspar.

About 2,000 distinct ~ineral species are known al­though less than 100 are considered common or abundant.Collectors are interested in gathering these minerals fora variety of reasons. Some collectors are students ofsystematic mineralogy and assemble suites of minerals ina manner similar to stamp collecting. Some even aspireto obtaining nearly all of the 2,000 mineral species.Others are interested only in minerals exhibiting goodcrystal form. One type of collector specializes incollecting micromounts, tiny perfect crystals. Many are

interested primarily in semi-precious gem rocks and min­erals that may be cut, polished, or faceted to bring outtheir natural beauty. A small group collects rocks andminerals that have been fashioned by primitive man intoimplements such as axes, arrowheads, spearheads andpeacepipes (see pages 6 and 15).

IDENTIFYING MINERALS

Collectors should learn to identify as many mineralsas possible. There are several scientific tests thataid in mineral identification. Because each mineral hasa distinct chemical composition and crystal structure,each has distinct physical properties such as hardness,shape or form, streak, luster, specific gravity, cleavageand, to a certain extent, color.

Ha rdness

Hardness is the quality of resistance to scratching.Gem stones must possess reasonable hardness, otherwisethey are too easily marred or disfigured. For comparisona hardness scale known as Mohs scale is used. Ten min­erals, each representative of a certain hardness, formthis scale:

1. Talc 6. Orthoclase2. Gypsum 7. Quartz3. Calcite 8. Topaz4. Fluorite 9. Corundum5. Apatite 10. Diamond

Diamond is the hardest of all natural substances.If you are testing an unknown mineral and the mineralscratches orthoclase but will not scratch quartz, thenits hardness lies between 6 and 7. As an added test,quartz should scratch this mineral. Two minerals ofapproximately the same hardness will scratch eachother.

2

Several other materials can be used in lieu of amineral hardness set:

Hardness

Hardness 3

Hardness 4

Hardness 5

Hardness 6

Hardness 7

Hardness 8

& 2 -- can be scratched with fingernail

will scratch fingernail; can bescratched by edge of a pennywill scratch a penny; can be easilyscratched by a knifebarely scratched by knife; will notscratch most glasswill scratch glass

will scratch a knife

will scratch quartz

Specific Gravity

Specific gravity is the weight of a mineral as com­pared to the weight of an equal volume of water. It canbe used as an aid in distinguishing minerals. Pure goldhas a specific gravity of 19.3, whereas pyrite has aspecific gravity of about 5; thus, gold is nearly 4 timesas heavy as an equal volume of pyrite.

Streak

Streak is the color of the powdered mineral obtainedwhen the mineral is rubbed against a flat piece of un­glazed, white porcelain. Among the iron minerals, mag­netite leaves a black streak, hematite a red streak, andgoethite a yellowish-brown streak. Pyrite, which isbrass-yellow in color, leaves a black streak.

Shape and Form

Mineral shape and form have two entirely differentmeanings. CrystaZ form means the type of crystal facesoccurring on a mineral. Because each mineral has a

3

S ria ed P te Cube

stlnc crys a 5 ruc ret only cer a n inds of crystalaces can form 0 example pyre e common y crys all ezes

en t e ape of a cube as sho n above This externalshape contra e by he arrange en of atoms i hecrysta Shape and form are also used 0 describe thegenera exte nal appeara ce of a mlnera it respeco he shape of 0 her hings For examp e, one of the

common shapes of hema ete and goe hi e is called kedneyore or reni arm 5 r c re orne common forms found inrni erals incl de: feb 0 5, radea lng, botryoida J

5 alacti c and dendri iCe

S a oe hi

Botryoidal Goethite

Cleavage

Cleavage is the way a mineral splits. If it splitswith a smooth, flat surface, the mineral has good cleavage.Cleavage is described by the number of different direc­tions or planes along which the mineral splits. Asillustrated on page 6, mica has a single cleavage plane;feldspar splits in two directions, calcite in three,fluorite in four. Many minerals including quartz donot have any cleavage direction. Quartz breaks as glassdoes, with a shell-like, conchoidal fracture.

Color

Color, luster, transparency, reflectivity, index ofrefraction, and fluorescense result from the interactionof electrons in the mineral with light waves. Theseoptical characteristics determine the natural beauty ofa mineral. Great differences in color can be found in

5

uscovite ica Feldspar

Fl uori e

Calcite

Quartz

c sing e mineral species and he property of co or mustIe used i h caution as an aid 0 identifica ~on. Inmeta c minerals color is ar more n·form han In

onmetall·c inerals. Therefore, co or ·s a safeguide for he identifica ion of metall·c minerals.

6

Luster

Luster is a property governed by the manner in whichlight is reflected from the surface of a mineral. Lusteris called vitreous when the reflection is similar tothat shown by broken glass. Adamantine luster is thatparticular bright reflection shown by diamond. Metallicluster needs no description.

The mineral identification chart on pages 22 and 23lists many of the common minerals of Minnesota that areof interest to the mineral collector, and describes thephysical properties that are useful aids in their identi­fication.

WHERE TO COLLECT MINERALS

Rocks and minerals of interest crop out at manyplaces. They are found along the North Shore of LakeSuperior, on woodland trails, and in the banks of open­pit mines, quarries, and gravel pits. Nearly every daynew finds are uncovered by excavation for buildings andhighways in our State.

As has been mentioned, each rock type is an aggregateof minerals. Therefore, to find a particular mineralone must hunt in an area where the rocks that containthis mineral are exposed. It would thus be useful toconsider the general geology of Minnesota before embark­ing on collecting trips. To aid you in your study andplanning, a map and explanation of the bedrock geologyof Minnesota are printed on pages 8 and 9. We suggestyou color the separate rock units on the map and explana­tion; by doing so, the map will be more useful andmeaningful.

There are three major rock types -- igneous, sed­imentary and metamorphic.

7

Bedrock Geologic Map of Minnesota

8

EXPLANATION

•Quartzose and arkosic sandstone, shale, limestoneand dolomite (unmetamorphosed)

Quartzite (unmetamorphosed)

~.""~

Graywacke, si Itstone, argi I I ite, slate, quartziteand iron-formation [shown black] (weakly tomoderately metamorphosed)

Basaltic lava flows and related rocks (weaklymetamorphosed)

oMetasedimentary (graywacke, volcanogenicsediments and iron-formation) and interlayeredmetavolcanic (basalt, andesite, pi I low lavas)rocks (weakly metamorphosed except near graniteand other related igneous rocks)

~~

Gabbro and related igneous rocks

Granite and related igneous rocks

Metasedimentary and meta-igneous gneiss, schistand migmatite (extensively metamorphosed)

Fault

Contact

9

Igneous Rocks

Igneous rocks are formed by the solidification andcrystallization of molten material called magma. Onetype, volcanic rocks, forms from magma that reaches theearth1s surface before solidifying. Most of the zeoliteminerals that we will describe are in the ancient lavaflows along the North Shore of Lake Superior. Granite,anorthosite, and porphyry are types of igneous rocksreferred to as intrusive rocks because they crystallizedeep within the earth1s crust. Uplift and erosion havecombined to bring the intrusive rocks to the earth1ssurface in northeastern and central Minnesota.

Sedimentary Rocks

Sedimentary rocks are formed by the cementing ofmineral grains that were carried and deposited by water,wind, and ice or were chemically precipitated in streams,lakes, or oceans. The weight of later sediments aids incompacting the mineral grains into a sedimentary rock.Sedimentary rocks are found most commonly in southeasternMinnesota.

Metamorphic Rocks

Metamorphic rocks, which contain many minerals notpresent in the other two rock types, are formed by thetransformation of pre-existing rocks by heat and pressure.Staurolite, illustrated on page 36, and garnet are examplesof minerals found primarily in metamorphic rocks. Meta­morphic rocks may be found in all but the southeasternpart of the State.

Glacial Drift

Knowledge of the glacial history of Minnesota isparticularly useful to agate collectors. The spread ofthe great ice sheets over the State during the PleistoceneEpoch constructed the present topography or the face of

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IOWA93°

EXPLANATION

a - outwash (sands and grovels)

I - glacial lake deposits

p - peat

e - loess (eol ion si Its)

D - Des Moines lobe gray drift

W- Wadena lobe gray drift

G - Old Gray drift

R - Rainy lobe brown drift

S - Superior lobe red drift

92°

Surficial (Glacial) Geologic Map of Minnesota

The map will be more useful if it is colored,with the lightest to darkest colors in the order theyappear in the explanation, so that the Superior lobered drift stands out. Shaded areas on the map are themajor modern lakes.

1I

Minnesota. During the last or Wisconsinan stage, whichbegan about 40,000 years ago, mile-high ice sheetsentered and retreated from the State during severalseparate phases and along four distinct routes. Theearly Wisconsinan-stage ice sheets traveled via the RedLakes, Rainy Lake, and the Lake Superior-Minneapolislowland routes. Later, ice advanced along the Red Riverof the North and the Minnesota River lowlands into Iowawith sublobes advancing into the Minneapolis-St. Pauland Mesabi Range areas. For about 4,000 years, from13,000 to 8,000 years ago, the waters of the famousLake Agassiz covered large parts of northwestern Minn­esota. During the melting of the last glacial ice,other smaller lakes covered areas of northeasternMinnesota.

Glacial ice that moved into Minnesota from the north­west picked up and later deposited fragments of shale andlimestone as well as other rocks. The resulting glacialdeposits, called drift, are gray or buff, generallyclayey, and have substantial amounts of calcium carbonate.In contrast, the glaciers that overrode northeasternMinnesota picked up rocks that now form a red sandy driftthat contains little clay and has a wide variety ofinteresting igneous and siliceous rocks, including agates.Therefore, the best hunting ground for agates is in thered drift deposited by the ice sheets from the LakeSuperior area. These deposits are delineated on thesurficial map on the preceding page.

A Nonrenewable Resource

Collectors are reminded that access to many gravelpits, quarries and mines may be limited. The owners areconcerned about both personal injury to collectors anddamage to their property. Permission should always besought before entering private property. Collecting isprohibited in State and National Parks, which weredeveloped to protect the natural treasures they contain.

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In a later section on mineral localities in Minne­sota we have indicated that some minerals are abundantat certain sites. It is evident that the minerals becomeless abundant each year as collectors visit the sites.If collectors take more than they can use from theselocalities t the supply of many Minnesota minerals willsoon be exhausted. Minerals are a nonrenewabLe resource.The young collectors that we all hope to inspire mayfind nothing left when they are ready to go into thefield unless steps are taken to conserve some of theminerals at the better collecting localities.

If you find something you believe to be rare or insome other way of exceptional interest t show it to amore advanced collector. He may suggest that you submitit to the University for positive identification byX-ray or petrographic analysis. In the past t out­standing amateurs have been involved in many new mineraldiscoveries. The hours spent by amateurs in searchingand collecting are much greater than those spent by thefew professional mineralogists in the State.

COLLECTING AND PREPARING SPECIMENS

A few items of equipment are necessary for collect­ing minerals in the field. Because most specimens are

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found as part of a rock mass, a geologic hammer or pickis an essential tool. Many collectors carry a heavysledge hammer and a light-weight trimming hammer. Ageologic pick has a square head with either a chisel orpick edge opposite. A cold chisel, also a useful tool,is used with a hammer to pry apart large chunks of rockor to help separate good crystals from the rock. Safetyglasses or some form of eye protection should be used.Extreme care must be taken to prevent injury to handsas well as damage to crystals.

For identifying small crystals or for studyingcleavage a la-power hand lens is indispensable. Whencollecting in iron-ore country, a magnet is standardequipment. Many collectors find a streak plate(unglazed porcelain) useful in the field. Othersprefer to do most testing after returning home. Agood canvas collecting bag will give long service as anaid in getting the sometimes hard-won specimens home.

Collectors and professional mineralogists and geo­logists have learned the wise practice of carrying afield notebook and labels into the field. The date ofthe trip, the locality, the number and kinds of rocksand minerals collected, and other relevant informationshould be written in the field notebook. Labels can besimply pieces of paper or cardboard with the itemizeddata concerning the specimen. A small supply of suchlabels is included on the last two pages of this booklet.The label, when properly filled out, is wrapped witheach specimen or group collected. Experienced collectorsalways carry a supply of newsprint for wrapping specimens.Proper labeling and note taking are important. Aspecimen without locality information has lost much ofits scientific value. Remember: the smallest note isbetter than the best memory.

Systematic mineral collectors invariably have agood place to store minerals and often have a displaycabinet for their prize specimens. Material collectedin the field should be prepared for the collection as

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soon as possible. This includes trimming away excessmaterial, cleaning, and proper labeling and catalogingfor storage or display. A small spot of white enamelmay be placed on an inconspicuous part of the specimen.A catalog number in india ink may then be written onthe spot and the specimen remains forever tied to itslabel or file-card number.

For many mineral hobbyists the real interest is inwhat can be done with the material that is collected.These people take pleasure in cutting and polishing gemmaterial. Because of the hardness of most gem materials,either diamond or silicon carbide wheels are requiredto cut and shape them. Many Minnesota rocks and min­erals can be used to make beautiful cabochons to bemounted and worn in rings, pendants, brooches, and otherforms of jewelry. Some hobbyists make flat cabinet

Agate

Feldspar

,

Pipestone

15

specimens by cutting and polishing a flat surface to lendbrill iance to, and bring out the details of, the colorsand textures of the materials. Another lapidary approachis to mass-polish gem minerals in a tumble barrel. Thisprocess is 1imited to small -- up to one inch -- specimensbut it is a good way to finish the small agates one finds.It takes about 500 hours to polish a batch in this manner.A small three-pound tumbler will handle about 250 indivi­dual pieces. Cutting and polishing need not be limitedto jewelry items. Book-ends, paperweights, mosaics andsimilar objects can be made from many Minnesota rocksand minerals such as those illustrated on page 15.

MINERALS OF MINNESOTA

Quartz Minerals

Quartz presents a fantastic variety of appearances,variegated in color and texture, and marked by a brilliant

Ouartz Crystals

luster. Mankind has been attracted to this useful mineralsince the Old Stone Age, when primitive man used flint,

16

,

jasper, and other quartz minerals in fabricating toolsand weapons. When man learned the art of cutting andpolishing agate, carnelian, and similar materials, hefashioned them into beads and other ornaments.

The diverse modes of formation and consequent dissimi-lar appearances of quartz minerals, coupled with theirwide occurrence, has given rise to a bewildering numberof names ranging from agate and amethyst to zonite. Quartzminerals occur in all three major rock types, igneous,sedimentary and metamorphic, but find their greatest devel­opment in low-temperature chemical sediments depositedfrom percolating waters. These varieties are microcrystal­line or cryptocrystall ine and include chalcedony, carnelian,agate, onyx, jasper, flint, and chert.

All quartz family minerals are composed of silicondioxide (SiOZ) and owe their variety of colors to traceamounts of impurities. The types of quartz common toMinnesota include the following:

Crystals

Large terminated crystals of quartz are rare in Minne­sota. Drusy masses may be found in a number of rocks alongfractures or in vugs (as illustrated on page 16). Thecommon, milky white, shoe-button variety found in theglacial drift is of disappointing gem quality.

Amethyst

This form of lavendar to purple quartz is found locallyin geodes and in cavities in agates. It has been collectedfrom veins, cracks, and crevices in rocks in the area ofthe Gunflint Trail.

Cold-Water Agate

This variety develops in cavities in weathered lime­stone, and is recovered as water-worn fragments along thedry washes of the Zumbro River at Mazeppa, Minnesota. Itis commonly associated with drusy quartz.

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Lake Superior Agate

This variety was carried here by the glaciers thatflowed from central Canada. This highly-sought agate isnot found in all drifts, but is restricted to the red orLake Superior drift (refer to map on page 11). Whenscouting in an unfamiliar area, look first for the moreabundant sand-sized agates. If they are present, thelarge ones will also be present.

The drawings below show banded or fortificationagates of the Lake Superior type. The color is basicallyred with white bands. Banding results from rhythmicdeposition of sil ica gel from percolating waters whichenter cavities in various rocks. Banding may be extremelyvaried, regular and rhythmic, or absent. Because of thetransparency and luster of agates, it is easier to locatethem when the search area is in direct sunlight.

Lake Superior A9ates

18

In the illustrations on page 18 you will note thatone half of each pair shows an unpolished exterior surface.This is usually the only part exposed in the field, andit is important to be able to recognize this dull, slight­ly waxy and pitted surface, which retains the imprint ofthe cavity in which it formed.

Paradise Beach Agate

This tiny, well-formed agate is a made in Minnesotaproduct. You may chip this variety from its enclosingamygdaloid in the basalt along the North Shore of LakeSuperior approximately thirteen miles east of Grand Marais.It is generally red-orange with strong white banding, andspecimens are rarely larger than one's thumb.

Paradise Beach Aqate

Thunder Egg Agate

Concretionary nodules whose center portion is composedof red and white banded agate may be found weathering fromthe hillside 300 feet north of U. S. Highway 61 near the

~ Highway Patrol Target Range at Five Mile Rock, east ofGrand Marais.

Jasper

In a general way, jasper includes all the red andreddish-brown varieties of microcrystalline quartz. Itis quite abundant in Minnesota and, when sufficiently

19

Thunder Egg Agate

( ro uoh )(cut & polished)

attractive. is in demand for lapidary work. The patterns.textures. and colors show wide variation. and jasper hasbeen given several subvariety names. In Minnesota oneof these is the Mary Ellen Jasper. named for its occur­rence in the Mary Ellen Mine on the Mesabi Range. one milewest of Biwabik. on State Highway 35. This subvariety isa beautiful red to pink material containing red to whiteswirls of Precambrian fossil algae. If it isn't tooporous it will take a high pol ish. Jasper grades intoflint and chert. These varieties make excellent arrow-

Mary Ellen Jasper

20

heads, but are not suitable for jewelry. Any of theglacial drifts may produce jasper. One jasper commonlyrecovered from glacial deposits is a flecked or freckleddeep red to purple-colored variety.

Bin hamite

Binghamite is composed of quartz and fibrous sili­cates with inclusions of goethite or hematite. Materialof gem quality, which is usually red or yellow, exhibitsa chatoyant luster similar to African Tiger's-eye. Smallspecimens of red Binghamite can be found on the dump ofthe Portsmouth Mine at Ironton and at other CuyunaRange localities.

Silks tone

A similar but more opaque and more coarsely fibrousrock than Binghamite, known as silkstone, is commonlyfound in the same veins. Generally silkstone is brown,yellow, grayish green, grayish blue, or any combinationof these. I t has a marked wavy chatoyancy and is common­ly associated with asbestos. Silkstone is found sparinglyat the Arco Mine and other localities on the Cuyuna Range.

Silkstone

(cut & polished)

( rough)

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TABLE FOR DE TERMINATION OF COMMON MINNESOTA MINERALS

MINERAL COMPOSITIONHARD- CLEAVAGE SPECIFIC

STREAK COLOR LUSTER OTHER PROPERTIESNESS OR FRACTURE GRAVITY

Quartz SiOZ 7.0 conchoidal white colorless vitreous Z.6 impurities produce a variety ofcolored quartz

Thomsonite NaCa Al Si ° ·6H ° 5.5 planar white l white to greel' vitreous Z.3 habit prismatic to radialZ 5 5 ZO Z

Hovlandite (Xonotlite) CaSi03

· ZHZO 6.5 planar white

fwhite to pink vitreous Z.7 habit fibr ous

Pectolite NaCaZSi

30

8(OH) 5.0 Z-directions white white to gray subvitreous Z.8 luster may be silky, habit fibrous

Prehnite CaZAlZSi3°10(OH)Z 6.0- weak planar white light green vitreous Z.9 tabular groups have cock's comb

6.5 appearance

Hematite Fe ° 5.5 uneven fracture red red, gray, dull to bright 5.2 appearance varies widelyZ 3 black metallic

Goethite HFeOZ

5.0 planar yellowish- yellowis h-to dull to 4.3 habit may be reniform, radial,

brown blackish-brown adamantine bladed, etc.

Magnetite Fe3° 6.0 uneven black black dull to bright 5.2 lnagnetic; habit octahedral

4 metallic

Ilmenite FeTi035.5 conchoidal black black submetallic to 4.7 distinguished from magnetite only by

fracture metallic lack of magnetism

Pyrite FeS 6.5 uneven greenish- brass metallic 5.0 habit varied, striated cubes toZ black yellow pyritohedra

Pyrrhotite FeS 4.0 uneven gray bronze metallic 4.6 tarnishes yellowish- brown; weakly

black yellow magnetic

Ar 5 enopyr ite FeAsS 5.5 uneven grayish- silver white to metallic 6.0 wedge-shaped crystals; emits garlic-

black steel gray like odor when struck

Siderite FeC034.0 rhombohedral white reddish-brown vitreous 4.0 rhombohedral habit

Pyrolusite MnOZ

Z.O - Z-directions black iron gray to metallic 5.0 commonly 50ft enough to soil fingers

6.0 black

Manganite MnO(pH) 4.0 planar brown to steel gray to submetallic 4.3 prismatic crystals, commonly

black iron black striated

Groutite HMn°Z3.5 planar brownish- jet black submetallic 4.1 curved, wedge-shaped crystals

black

Rhodochrosite MnC034.0 rhombohedral white pink to gray vitreous 3. 7 distinct crystals uncommonj cleaved

fragments common

Calcite CaC03

3.0 rhombohedral white \ commonly white vitreous Z.7 wide variety of forms and habits

or colorless common

t

Native Copper Cu 2.5 - uneven metallic J copper to metallic 8.9 distorted crystals, plates, and wir e-

3.0 brown like form

Staurolite Fe silicate 7.0 uneven gray brown vitreous 3.7 cruciform twins common

Garnet silicate 7.0 subconchoidal white wide variety vitreous 3.6-4.3 dodecahedral habit; color depends on

fracture compositionj commonly reddish-brown

22 23

Basanite

This dense, velvety-black variety of fine-grainedjaspery quartz has been collected and used by man sinceancient times. It was the touch stone or Lydian Stonethat the early Greeks used to "assay" gold and otherprecious metals. Its hard black surface provided anexcellent background for comparing the streaks of metalsdrawn across it. Different alloys give different streakcolors on its surface, and gold-plated objects quicklyshow a change in streak as the base metal begins to showthrough. Basanite is found in Minnesota on the beachat Grand Marais as polished, water-worn pebbles as muchas one inch in diameter. The pebbles resemble similarwater-worn pebbles of black basalt; basanite can bedistinguished, however, by its greater hardness.

Zeolites and Associated Minerals

The zeolites are a group of hydrous aluminum silicatesof sodium, calcium, and potassium. The water of hydrationis pecul iar in that it is lost gradually and continuouslyon heating without destroying the crystal structure. More­over, the water thus lost is readily regained upon exposureto water. The zeolites also possess the property of alkaliion-exchange -- the basis for their use as water softeners.Zeolites are commonly found as good crystals, but arerather soft and have a hardness varying from about 3.5to 5.5.

Tho mson it e

24

Thomsonite

The most colorful occurrence of this gem mineral isin Minnesota rocks. Pink and white thomsonite, commonlyforming radial patterns giving the appearance of little'Ibloodshot ll eyes, occurs in vesicle fillings -- as

amygdules -- within the basaltic lavaflows that outcrop along the NorthShore, about 5 miles southwest ofGrand Marais, near Good Harbor Bay.These specimens may be collected fromthe beach sands or directly from thehost basalt. Unfortunately, most goodradial thomsonite localities are now

closed to the public or are on private land. Other fibrous,sheaf-I ike thomsonite may be found in the beach sands atTofte, Minnesota. These specimens are of a white non-gemvariety. A more or less structureless variety of thomsoniteexhibiting no apparent radial or fibrous pattern, may berecovered -- along with agate -- on the beach at the Cook­Lake County line. Also at this local ity, white to pinkamygdules, commonly flecked like bird1s eggs, may becollected. Thomsonite is one of the hardest of the zeol ites,having a hardness greater than 5. Thomsonite nodules areoften cut into attractive gem stones.

Li nton i te

This green to grayish-green variety of thomsonite isassociated with the other varieties of thomsonite at mostof the localities described above. Because of its trans­lucency, size, and shape, it has the appearance of a jellybean. Thread-I ike inclusions often spoil the cuttingqualities of this mineral.

Othe r Zeo 1i tes

Systematic mineral collectors will also find the zeo­lites stilbite and heulandite in cavities in the basalticrocks along the North Shore. In add it ion, the zeo liteslaumontite and analcite have been found with native copperin the Snake River district near Pine City.

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Hovlandite

Hovlandite is a variety of xonotlite, not a truezeolite. However, it is often grouped with the zeolitesbecause of its matted fibrous appearance and its occur­rence as a secondary mineral in basic igneous rocks.Hovlandite is found as white needles and rarely as fansof pink fibers distributed throughout a green diabasecontaining black pyroxene crystals. Veins and water-wornpebbles of this material can be found near the Grand Maraissanitation plant. In some specimens the entire mass willcut and polish well.

Hovlandite

Pectolite

This silicate mineral also is not a zeolite, but isfound in association with zeolites as cavity fillings orin veins within the gabbroic and basaltic rocks of theNorth Shore. Its white, silky, needle-like crystals areeasily separated and are sufficiently sharp to puncturethe skin of the unwary. The mineral is subtranslucentand grayish-white, and has a silky to subvitreous luster.

Epidote

Tiny, bright, pistachio-green crystals of epidote canbe found in cavities in the basalts of the North Shore,and occasionally within rocks of the glacial drift.

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Prehnite

Another secondary silicate mineral found in vugs inthe basic igneous rocks of Good Harbor Bay and other areasalong the North Shore is prehnite. This pale-green min­eral is hard (6 to 6.5) and commonly has a cock's combshape. It is also found in water-worn pebbles. Some ofthe prehnite is flecked with native copper.

The Iron Minerals

Much of the romance in the mineral history of Minne­sota is bound up in the lore of the Mesabi Range: the storyof the Seven Iron Men; the record tonnages of reddish-brown material sent eastward on ships; the rise of magnetictaconite as the king of Minnesota iron ores. Iron is thefourth most abundant element in the earth1s crust (afteroxygen, silicon, and aluminum). Considering the totalmass of the planet earth, iron is probably the mostabundant element. No wonder that large deposits of thisuseful element are numerous. It is Minnesota's fortunethat geological events in the far-distant past -- approxi­mately 2.0 billion years ago -- were favorable for thedeposition of the great thicknesses of sedimentary iron­formations or taconites.

agnetic Taconite

27

Taconi e can be either magne ic or non-magnetic.ag etec taconete, the type presen ly used by the ·ro

endustry consists of dark gray to black layers of magnetete alternateng with somewhat thicker and lightercolored quartz·te layers as illustrated above. Aftercrushing to a ine powder, the magnet·te is separatedfrom the si ica by electromagnets. Then small quantitiesof ben onite clay and coal are added to the magnetitepowder. The mixture is then rolled into marble-sizedpel ets which are roasted to gain sufficient strengthfor shipment to the steel mills. The non-magneticvariety consists of hematite, iron si icates or ironcarbonate layers alternating with the quartzite layers.These rocks are not presently being exploited in Minnesota.

The three major iron oxide minerals are magnetite,hematite, and goethite. They may be distinguished mostreadily by their streaks. Magnetite has a black streak,hematite has a red streak, and goethite has a brown toyellowish-brown streak.

Magnetite

This heavy black variety of iron oxide is attractedby a magnet. It is the common are mineral of the taconite

Magnetite Crystals

28

i dustry. So e e e -0 ie ed crys a s iact as a a ura or Zodestone ads 0 9 Y attract

ron objec s. q ali y 10 es one is no found eninnesota. he black grains of e 'black sand' a ong t eorth Shore of La e Supe io are predominan ly magneti eagne ie-ric sands occur sparing y In the glacial dreft

deposi ed by La e Superior ice sheets.

Hematite

This abundant mineral occurs n a variety of shapesand forms, ranging from he ery hard meta lec-black reni­form types to he sheny uster of specu arite to he bright-

He rna tit e R0 S e s

(cut polis ed)o ~

Bo yoidal He at; e

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red, earthy varieties. Whatever the color, the streakalways is red. The name hematite is derived from theGreek word for blood. All three Minnesota iron ranges,the Mesabi, the Cuyuna, and the Vermilion, contain agreat diversity of hematite varieties. Gem hematiteis rare except as water-worn pebbles from the Cretaceousoverburden of mines near Grand Rapids on the MesabiRange. Martite, a variety of hematite derived from theoxidation of magnetite, commonly retains the externalform of the original magnetite crystals.

Goethite

This mineral, a hydrated form of iron oxide, is themajor constituent in limonite. Limonite is dominantlyan earthy iron oxyhydroxide with additional capillarywater. In massive form this material constitutes thecommon iron ocher. It occurs in both the Mesabi andCuyuna Ranges, and in the Fillmore County District insouthern Minnesota. Well-crystallized goethite developsmany interesting commonly fibrous forms, including themammillary and 'stalactitic types (see page 4). Itsstreak is brown to yellowish-brown.

Py rite

The fool's gold of prospector1s lore, pyrite, is abrassy-yellow metallic iron sulfide. It is much harderthan gold; in fact it will scratch a knife blade. Pyritecrystallizes in the cubic system and often is found asnearly perfect cubes. Pyrite cubes are available inthe dump of the Portsmouth Mine at Ironton and also inone deposit near Glen Lake in Aitkin County. Pyriteis unstable in the presence of moist air and slowlyoxidizes to goethite. It tarnishes to a brass yellowcolor similar to pyrite.

Pyrrhotite

This mineral is weakly magnetic iron sulfide that iseasily mistaken for pyrite. Pyrrhotite can be distin­guished from pyrite by its lower hardness, its weak

30

Pyrite Cubes

magnetism, and its brown tarnish. It occurs in smallamounts in the Duluth Complex and in parts of the magnetictaconite near the eastern end of the Mesabi Range.

Ilmenite

This mineral is an oxide of iron and titanium. Itis black and metallic, and can be distinguished from mag­netite only by its lack of magnetism. Ilmenite occurswith magnetite and is a common constituent of the darkgray igneous rocks of the Duluth Complex of northeasternMinnesota. It occurs as nearly pure masses along thenorthern margin of the complex in Lake and Cook counties.

Arsenopyrite

Arsenopyrite is a silvery-white mineral composed ofiron, arsenic, and sulfur. It can be found in veins atthe west end of Loon Lake in Cook County and in rectangularforms in the Roberts Mine of the Cuyuna Range. The latterspecimens make good polished flat-cabinet exhibits.Arsenopyrite may tarnish to a brass yellow color similarto pyrite.

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Siderite Rhombs

Siderite

The iron carbonate, siderite, occurs in the BiwabikIron Formation of the Mesabi Range as massive aggregatesof tiny crystals It is softer (4) than a knife bladeand generally is tan or brownish in color. As shownabove, siderite has perfect rhombohedral cleavage.

The Manganese Minerals

Many chemical elements impart a characteristic colorto minerals. Iron silicates are commonly green; ironoxides are mainly red, black, or brown. Manganese char­acteristically imparts a pink color to the silicates orcarbonates, and black or gray colors to the oxides.

anganese is of major importance in the manufac ureof steel. It improves the ro ling and forging propertiesas well as the hardness of steel, acting a the same timeas a deoxidizeng and desu urizing agent. Geological ythe genesis of manganese oxide ores is similar to theiron ores and innesota's Cuyuna Range is well knownas a so rce of manganiferous eron ores.

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Pyrolusite

This mineral is most easily recognized as steel-grayto black, metal I ic, needle-like crystal masses in mineson the Cuyuna Range. When in unaltered museum-qualitycrystals, pyrolusite is very hard (up to 6.5) and iscalled polianite. However, it is more commonly found ina softer (I to 2.5) form that will soil the fingers black.In addition to the needle-I ike crystals, pyrolusite mayoccur as pseudomorphs after manganite, in reniform coat­ings, in concretionary forms, and as the well-knowndendritic growths on fracture surfaces of rocks. Lacy,dendritic stains on limestone pebbles may be recoveredfrom the glacial drift, particularly near Lauderdale, asuburb of Minneapol is-St. Paul. Pyrolusite is often calledblack oxide of manganese and is essentially pure manganesedioxide. The streak is black or bluish-black.

Manganite

This hydrated oxide of manganese is found as prismaticcrystal groups in pockets or vugs in more massive man­ganese ore. The black crystals may be striated and have abrownish-black streak. With the loss of its constituentwater, it alters to pyrolusite. It occurs in the RobertsMine dump one mile north of the village of Cuyuna, Minne­sota.

Manganite

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Ps·lomelane

Th·s hard hydrous manganese ox·de is a true m· eralspecies a though i invariably occurs as fine-grained,massive material in botryoidal, reniform, or stalactiticforms rather than as ind·vidual crystals. n theseshapes it strongly resembles similar forms of blackgoethite and hematite However pSllomelane will have ablack rather than the yellowish-brown or red streaks ofthe iron minerals. is both idely distr·buted andplentiful wherever manganese ores are found.

Groutite

Groutite

Groutite a hydro s manganese oxide, was first·dentified as a new mineral from the Sagamore Pit on heCuyuna Range, and was named by Professor John W. Grunerafter the late Professor F. F. rout of t e Universityof ·nnesota. It is very difficu t to d·stinguish frommangani e in co or, luster, streak, ardness, or speci ·cgravity. It is found as wedge-shaped crysta s that linevugs in manganese ore. The crysta's have a h-gh submetallic

us er and give a dark brown streak. Sril iant reflectionsmay be ob erved from erfect c eavage su faces_

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Ramsdellite

The collector of rare minerals may be interested inthis exceedingly rare manganese dioxide. It has been re­ported from the Monroe-Tener Mine near Chisholm, on theMesabi Range, where it occurs as clusters of radiating,shiny-black crystals about 6 mm. long in a dull graymatrix. It must be noted that these crystal aggregatesare quite delicate and fall apart easily.

Rhodochrosite

The manganese carbonate, rhodochrosite, is not acommon mineral, but specimens with a beautiful pink colorare sought as semi-gem material. Its relatively lowhardness of 4 prevents it from being classed as a gemmineral. In common with other rhombohedral carbonatessuch as calcite, it has perfect cleavages in threedirections. Impure gray rhodochrosite is moderatelyabundant in rocks of the Cuyuna District, but the rose­red variety is scarce. Rose-colored rhodochrosite maybe found in veins in the overburden in some Cuyuna Minesand in the dump at the Hopkins Mine at Crosby-Ironton.

Miscellaneous Minerals and Rocks

Native Copper

Copper is the only metal found abundantly in thenative state. It is a major ore in lavas of the KeweenawanPeninsula of Michigan. Rocks of similar age and typeoccur in Minnesota, and have been prospected for thismineral. Minor amounts of copper have been recoveredfrom drill cores and mines in the vicinity of Pine Cityand in the Knife River area northeast of Duluth. Nativecopper is light-rose in color on freshly-broken surfacesbut changes quickly to copper-red and eventually tobrown or even green when exposed to weathering. It isboth ductile and malleable. It may be rarely found inthe glacial drifts of the Cuyuna District, the upper St.Croix Valley, the Kettle River area, and southern Minnesota.

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Calcite

More than a hundred different crystal habits andvarieties have been described for this mineral. Crystalsare common, but rhombohedral cleavage fragments are ob­served most frequently (see page 6). Calcite may becolorless, white, blue, green or nearly any color of thespectrum. It has a white or slightly grayish streak andsome is fluorescent. Clear cleavage fragments are doublyrefracting. Calcite of optical quality has been obtainedfrom a cavernous zone of basalt in Iceland, hence thecommon name Iceland Spar for optical grade material.Calcite is an abundant and widely distributed mineral inthe earth's crust. It is the major constituent of lime­stone, marble, and chalk.

Calcite crystals are abundant as fillings in crackswithin the basalts along the North Shore and in veinletsand vugs in the limestone beds of southeastern Minnesota.It can be distinguished from quartz by being softer,having a hardness of 3, and by effervescing with weakhydrochloric acid or even vinegar.

Staurolite Crystals

36

Staurolite

Staurolite, an iron silicate, is best known for thevariety of twinned crystals formed in abundance in meta­morphic rocks. Cruciform twins, including the fairycr08S~ and X-shaped twins are common. The name stauroliteis derived from the Greek, stauros meaning cross andlithos meaning stone. The mineral is brown and has agray streak. It is abundant below the power dam westof Royalton. At this local ity cruciform twins are rareand commonly malformed, but they can be improved bylapidary work. Single crystals may be collected fromeither the schists at the base of the dam or from theriver sands below. Additional outcrops of staurolite­bearing schists occur farther up the Mississippi Rivertoward Little Falls.

Pipestone

Catlinite or pipestone is the name given to the toughindurated clays interbedded in the Precambrian SiouxQuartzite, which crops out in both Rock and Pipestonecounties. This mahogany-red material is the 'pipestone"of the American Indian tribes. The catlinite bed atPipestone is about 16 inches thick and is composedchiefly of soft hydrous layer silicates, including sericitemica. In places it is replaced by a yellowish mineral,pyrophyllite. Catlinite may be shaped easily with aknife blade when freshly quarried (see page 15).

Garnet

Many people think of garnet as a deep-red gem mater­ial. Actually garnet ranges widely in color from nearlycolorless through red, green, yellow, brown, and black,depending upon its chemical composition. Garnets aresilicates of calcium, magnesium, iron, aluminum, manganese,and chromium. The dodecahedron (12-sided crystal) is thecommon crystal form of most garnets, and good crystalsare abundant in many "metamorphic rocks. However, crystalsof a size adequate for lapidary work or ordinary displaysare rare in Minnesota. Tiny crystals can be found in

37

the schists near Royalton, Minnesota, in boulders in theglacial drift, and in outcrops of biotite gneiss alongthe Minnesota River Valley.

Anorthosite

This coarse-grained rock is composed mainly of plagio­clase feldspar. It forms many of the individual smallpeaks in Lake County and Carlton Peak in Cook County.Many outcrops occur along Highway 61 near Silver Bay. Thecolor varies from nearly white through a pleasant trans­lucent green to a bluish black. Anorthosite is not gemmaterial, but has been used for riprap for breakwalls.The Minnesota Mining and Manufacturing Co. initially wasorganized to develop the northeastern Minnesota deposits.

Marble

A metamorphic rock composed of the minerals calciteor dolomite, marble comes in a variety of colors. Marbleis used for carvings and ornamental stone work not subjectto extensive wear, and often is attractive when polished.In Minnesota, marble is found at the surface only in thearea southeast of the village of Denham.

Granite

This coarsely crystalline igneous rock is composeddominantly of feldspar, quartz, and minor amounts of micaand/or hornblende. Many different color and texturalvarieties are known, but pink to red colors predominatein Minnesota. Many granite quarries are located in theSt. Cloud region as well as in the upper Minnesota RiverValley and near Lake Mille Lacs. Note the areas ofigneous rocks on the Geologic Map on page 8. Granitepebbles and boulders occur virtually everywhere in theglacial and stream deposits of the State. Granite is hardand durable, and will take the high polish as shown onmany monument stones. Attractive book ends and paper­weights can be made from the many varieties found evenin a single gravel pit.

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Algal Structures

Beds of red and white (and in places green) massivefossil ized algal structures occur in the taconites of theMesabi Range. The beds generally are composed of uniform­ly fine-grained gray chert containing red laminations ofjasper. These laminations form gnarled patterns or whorlsand upwardly convex arches (see page 20). Because ofits siliceous nature, this rock is quite durable and takesa high polish. Formed some two billion years ago, itrepresents evidence of some of the earliest life on theearth. Mines near Biwabik on the Mesabi Range containexposures of good material, and many dumps contain largeblocks.

WHERE TO FROM HERE

As a beginner's interest in rocks and minerals devel­ops, he seeks to broaden and deepen his knowledge and tocommunicate with others of similar interests. There area number of ways to 1ccomplish these objectives. Mineralstudy can progress by visiting the many museums exhibit­ing mineralogical and geological specimens. Good modernmuseum exhibits are designed to do more than merelypublicly display fine specimens. The geological, chemical,and physical relationships among minerals and the rest ofthe natural world are presented in a manner best describedas an educational exhibit. The Department of Geology andGeophysics at the University of Minnesota, Minneapoliscampus, the Department of Geology at the University ofMinnesota, Duluth, and other public and college museumshave such displays.

The University of Minnesota through its General Exten­sion Division offers both Evening and Correspondencecourses in geology and mineralogy. On occasion publicschools have offered adult education classes in gem cuttingand jewelry work.

39

Comparing notes and sharing ideas and specimens withother collectors is facilitated by joining a rock andmineral club. There are several advantages to belongingto a club. Association with more advanced hobbyistsoffers a source of guidance in developing the hobby andlearning the whereabouts of first-class collectinglocalities. Club programs are designed to improve theknowledge and skills of the club members, and club fieldtrips provide possibilities to visit localities that arenot open to individuals.

Many mineral and equipment dealers are sincerelyanxious to assist the amateur. However, one should notalways expect commercial collectors of choice specimensto disclose the location of sites that may have takenthem years to discover.

Rock and mineral collecting allover the world isreported in many magazines and books, some of which arelisted in the Appendix.

Additional information on the geology of Minnesotacan be secured from books, reports and maps of the Minne­sota Geological Survey, University of Minnesota, 1633Eustis Street, St. Paul, and the United States GeologicalSurvey, Washington, D. C.

ACKNOWLEDGMENTS

The authors wish to acknowledge the assistance ofArthur Anderson, Joseph Heininger, and Raymond Lull ingin the preparation of this booklet.

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APPENDIX

References for Rock and Mineral Collectors

Journals

"Earth Sciencell, Earth Science Publishing Co., Colorado

Springs, Colorado

IIGems and Minerals", Box 687, Mentone, CA 92359

"Lapidary Journal", Box 80937, San Diego, CA 92138

"Mineralogical Record", Box 783, Bowie, MD 20715

IIRock and Gem", Behn-M i 11 er Pub 1is hers, Inc., 16001Ventura Blvd., Encino, CA 91316

"Rocks and Minerals ll, Heldref Publications, 4000 Albe­

marle Street N.W., Washington, D.C. 20016

Books

For independent study there are a large number ofgood books on minerals and collecting including:

Dinwiddie, Donal, and Russell P. MacFall, 1978, PopularMechanics Complete Book of Rocks, Minerals, Gems,Fossils: Popular Mechanics Books, New York.

Fay, Gordon S., 1972, The Rockhoundls Manual: Harper &Row, New York.

Pough, Frederick H., 1976 (4th ed.), A Field Guide toRocks and Minerals (Peterson Field Guide Series):Houghton Mifflin Co., Boston.

Rapp, George R., Jr., 1970, Color of Minerals: HoughtonMifflin Co., Boston.

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Roberts, W.l., George R. Rapp, Jr., and Julius Weber,1974, Encyclopedia of Minerals: Van Nostrand Rein­hold, New York.

Schwartz, G.M., and G.A. Thiel, 1963, Minnesota1s Rocksand Waters: University of Minnesota Press, Minneapolis.

Sinkankas, John, 1964, Mineralogy for Amateurs: VanNostrand Co., Princeton, N.J.

Zim, Herbert S., and Elizabeth K. Cooper, 1971, Minerals-­Their tdentification, Uses, and How to Collect Them:Harcourt Brace and World, New York.

Zim, Herbert S., and Paul R. Shaffer, 1957, Rocks and Min­erals--A Guide to Familiar Minerals, Gems, Ores andRocks (a Golden Nature Guide): Golden Press, New York.

Maps

Topographic maps and geologic maps and reports canbe obtained from the Minnesota Geological Survey, 1633Eustis Street, St. Paul, MN 55108. Generalized small­scale maps include:

Map M-24, Bedrock Geologic Map of Minnesota, at1:3,168,000 scale (about 8-1/2 by 11 inches), in color.Price $.50 plus 2¢ tax.

Map S-4, Quaternary (surficial) Geologic Map ofMinnesota, also at 1:3,168,000 scale, in color. This mapis in press; it will be available in the fall of 1979.

A Directory of the Midwest Federation of Mineralogicaland Geological Societies may be obtained from Clinton I.Heckert, MWF Director of Supplies, 725 Stewart Avenue,Elgin, Illinois 60120. It is printed annually and givesinformation on clubs and events. 1978 price: $1.50 pre­paid.

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Date No. Date No.

Name Na meFor rna t; on FormationLoca 1i t.y LocalityCollector Collector

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Name NameFor rna t ion Forma t ionLocality LocalityCollector Collector

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Name NameFormation For rna t ionLocality LocalityCollector Collector

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Na me Na meFormation FormationLocality LocalityCollector Collector

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Name Na meFormation Forma t; 0 nLocality LocalityCollector Collector

I,

I,

Date No. Date No.

Name Na meFor rna t i on FormationLocal it.y LocalityCollector Collector

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Name NameFor rna t i on Forma t ionLocality LocalityCollector Collector

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Name NameFor rna t ion For rna t ionLocality LocalityCollector Collector

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Na me Na meF0 rrna t ion F0 rrna t ionLocality LocalityCollector Collector

Date No. Date No.

Name Na meFormation Forma t ionLocality LocalityCollector Collector