i.?PEi{ Dli.Mineral and Rock ldentification

MI N ERAL IDENTIFICATIONTable C.3 lists many of the more common minerals. Representa-

rive chemical formulas are provided for reference. Some appear

complex because of opportunities for solid solution; some have

been simplihed by limiting the range of compositions repre-

sented, although additional elemental substitutions are possible.

A few general identification guidelines and comments:

Hardness is at approximate measure of how readily a min-

eral scratches, oris scratched by, other minerals. Values ofhard-

ness range from I (softest) to l0 (hardest) and are measured

against the ten reference minerals of the Mohs hardness scale

(table C.l). For example, a mineral that scratches quaÍz and is

scratched by topaz would have a hardness of 7%.

ruThe Mohs Hardness Scale

H¡rdnesses ofsome common materiah are i¡dicaEd for reference'

¿rsrel describes the surface sheen of a mineral sample, and

the tems (such as metallic, pearly, earthy) are self-explanatory.

Cleavage is a mineral's tendency to break preferentíally along

certain planes of weakness in the crystal structure.

Minerals showing metallic lustq are usually sulfides (or

native metals, but these are much rarer). Native metals have

been omitted from table C.2. Those few that are likely to be en-

countered, such as native copper or silver, may be identified by

their resemblance to household examples of the same metals.

Of the nonmetals, the silicates are generally systematically

harder than the nonsilicates. Hardnesses of silicates are typi-cally over 5, with exceptions principally among the sheet sili-cates; many of the nonsilicates, such as sulfates and carbonates,

are much softelDistinctive luster, cleavage, or other identifying Foperties

are listed under the column "Other Characteristics." In a few

cases- this column notes resrictions on the occurrence of certain

minerals as a possible clue to identification; for example, "found

only in metamorphic rocks," or "often found in pegmatites."

A Note on Mineral FormulasEach chemical element is denoted by a otre- or two-letter sym-

bol. Many of these symbols make sense in terms of the Engüsh

name for the element-O for oxygen, He for helium, Si for silicon, and so on. Other symbols reflect the fact that, in earlier

centuries, scientists were generally versed in Latin or G¡eek:

The symbols Fe for iron and Pb for lead, for example, are de-

ited from ferrum and plumbum, respectively, the Latin names

for these elements.The chemical symbols for the elements can express the

compositions of substances very precisely. Subscripts after a

symbol indicate üe number of atoms/ions of one element pres-

ent in proportion to the other elements in the formula. For ex-

ample, the formula FqAl2Si3Or2 represents a compound inwhich, for every twelve oxygen atoms, there ale three iron

atoms, two aluminum atoms, and three silicon atoms. (This hap-

pens to be a vadety of the mineral garnet.) The chemical

talc

gyp§um

calcite

fluorite

apatite

orthoclase

qratlz 'l

fop^z 8

corundum 9

diamond l0

<- (hngernail)

e (copper penny)

<- (glass, pocketknife blade)

I2

4

5

Mineral

518

fo¡mula is much briefer than describing the composition inwords. It is also more exact than the mineral name ,,garnet,,'for

there arc several compositions of garnets with the same basickind of formula and crystal structure: Other examples include acalcium-aluminum garnet with the fomula CarAlrSirO,, and acalcium-chromium garnet, Ca,CrrSi,O,. Moreover, chemicalformulas are understood by all scientists, while mineral namesare known primarily to geologists.

Formulas can become very complex, especially when differ-ent elements can substitute for each other in the same site in thecrystal stucture. Iron and magnesium often do this in silicates.Biotite, a cornmon, dark-colored mica, may be rich in iron andhave a formula of KFe,AlSirO,o(OH)r, or it may be rich in magnesium and have a fomula of KMg,AlSiaOl¡(OH)2, or, morecommonly, it may contain some iron and some magnesium, whichtogether total three atoms per formula. The generalized formula isthen K(Fe,Mg),AlSiaO,¡(OH)2, as it appears in table C.2.

ROCK IDENTIFICATIONOne approach to rock identification is to decide whether thesample is igneous, sedimentary or metamorphic and then lookat the detailed descriptions in the corresponding chapter Howdoes one identify the basic rock type? Here are some generalguidelines:

1 . Glassy rocks or rocks containing bubbles are volcanic.

2. Coarse-grained rocks with tightly interlocking crystals arelikely to be plutonic, especially if they lack foliation.

3. Coarse-grained sedimentary rocks differ from plutonic¡ocks in that the grains in the sedimentary rocks tend to

Mineral and Rock ldentification APPENDIX C

be more rounded and to interlock less closely. A brecciadoes have angular fragments, but the fragments in a b¡ec-cia are typically rock fragments, not individual mineralcrystals.

4. Rocks that are not very cohesive, that c¡umble apart easilyinto individual grains, are generally clastic sedimentaryrocks. One other possibility would be a poorly consolidatedvolcanic ash, but this should be recognizable by the natureof the grains, many of which will be glassy shards. (Note,however, that extensive weathering can make even a gran-ite crumble.)

5. More cohesive, fine-grained sedimentary rocks may be dis-tinguished from fine-grained volcanics because sedimen-tary rocks are generally softer and more likely to show a

tendency to break along bedding planes. Phenocrysts, ofcoulse. indicate a (porphyritic) volcanic rock.

6. Foliated metamorphic rocks are distinguished by their foli-ation (schistosity, compositional banding). Also, rocks con-taining abundant mica, gamet, or amphibole are commonlymetamorphic rocks.

7. Nonfoliated metamorphic rocks, like quartzite and marble, re-semble thei¡ sedimentary parents but are harder, denser, andmore compact. They may also have a shiny or glittery appear-a¡ce on broken surfaces, due to rccrystallization duing meta-moryhism.

Once a preliminary determination of category (igneous,/

sedimentary/metamorphic) has been made, table C.3 can b€

used in conjunction with the appropriate text chapter to identif]the rock type. (Keep in mind, however, that üe key and re\r fo-cus on relatively common rock types.)

5',1 9

IEilgA Brief Mineral ldentification Key

Formula

amphibole (e.9., hombleude)

apatite

azuaite

barite

beryl

biotite (a mica)

bomite

c¿lcite

chalcopyrite

cblorite

cinmba¡

co¡undum

covellite

dolomite

epidote

fluorite

galena

garnet

graphite

gyp§um

halite

hematite

kaolinite

kyanite

limonite

magnetite

malachite

molybdenite

muscoüte (a mic¿)

olivine

phlogopite

plagioclase feldspar

potassium feldspa¡

pynte

py¡oxene (e.g., augite)

quartz

sery€ntine

sillimanite

sphalerite

staurolite

sulñ¡¡

sylvite

talc

toumaline

(Na,Ca)z(Mg,Fe,Al)sSi8O22(OH)2

Cq(PO¿)¡(RCl,OH)

Cu3(C03»(OH),

BaSOn

Be3Al2Si6O rs

K(Mg,Fe»AlSi3O ro(OH),Cu5FeS¿

CaCOr

CUFeS?

(Mg,Fe)j(Si,Al)aOro(OH»

Hgs

Arzo¡CuS

CaMg(CO3),

CarRAlrSi3Orr(OH)

CaF,

PbS

(Ca,Mg,Fe»(Fe,Al)rSi30 D

cCaSOa ' 2HrO

NaCl

Fqo¡Alrsiro5(oH)¿Alrsio5FeaO3(OH)6

FerOo

CurCO3(OH)2

MoS2

KAt3Si3Oro(OH),

(Fe,Mg»Sioa

KMg,AlSi O'6(OH),(NaCaXAl,Si)rsirOs

KAtSi3Os

FeS2

(Na,Ca,Mg,FeÁl»Siro6

sio2M&SirOlOH)aAl2sio5

ZnS

F%AleSiaoro(OH),

sKCI

Mg.SioO,¡(OH),(NaCa),(Li,Mg,AlXAl,Fe,Mn)6{Bo)3Si6o¡s(OH)a

green, blue, brown, black

usually yellowish

vivid blue

colorless

aqua to green

black

iridescent blue, purple

variable; colorless if pure

brassy yellow

light green

red

variable; colorless in pure form

blue

white or pinkgreen

variable; often green or purple

silver-gray

variablel often dark red

dark gray

colorless

colorless

red or dark glay

white

blue

yellow-brcwn

black

green

dark gray

colorless

yellow-green

brown

white to gray

white; often stained pink or aqua

yetlow

usually green or black

va¡iable; commonly colo¡less or white

green to yellow

white

yellow-brown

brown

yellow

colo¡less

white to green

black, red, green

520

Hardness Other Characteristics

5 to 6 often forms needlelike crystals; two good cleavages forming l2Gdegree angle5 crystals hexagonal in cross section

3% to 4 often associated with malachite

3 to 3'Á high specific gravity, 4.5 (denser than most silicates)

1'lto 8 usually found in pegmatites

5% excellent cleavage into thin sheets

3 metallic luster

3 effervesces in weak acid

3'Á to 4

2to2k cleaves into small flakes

2'A earthy luster; may show silve¡ flecks9 most readily identiñed by its hardness

ltL lo 2 metallic luster

3'Ato 4 powdered mineral effervesces in acid6 to7 glassy luster

4 cleaves into octahedral fragments; may fluoresce in ulraviolet light2% metallic lusterl cleaves into cubes

7 glassy luster

I to2 skeaks like pencil lead

2

2Y salty taste; cleaves into cubes

5'Á to 6'A red-brown streak rcgardless of colo¡2 eafhy luster

5 to7 found in high-pressurc metamorphic rocks; often fo¡ms bladelike crystals2to3 eafhy lustet yellow-b¡own streak

6 strongly magnetic

3'A to 4 often fo¡ms in concentric rings of light and da¡k green

I fo 714 cleaves into flakes; more metallic luster than graphite

2to2k excellent cleavage into thin sheets

6Yto1 glassy luster

2t4fo3 mica closely resemblirg bioüte

6 may show hne striations on cleavage surfaces

6 two good cleavages forming a go-degree angle: no striations6 to 6'Á metallic lustel black streak

5 to 7 two good cleavages foming a 9o-degree angle

7 glassy luster; conchoidal fracture

3 to 5 waxy or silky luster; may be flbrcus (asbestos)

6to'7 occurs only in metamorphic rocks; often forms needlelike crystats

3Y to 4 glassy luster

7 to7% found in metamorphic rccks; elongated crystals may have crosslike folmltL fo 2tA sulfurous odo¡

2 cleaves into cubes; salty taste, but more bitter üan haliteI greasy or slippery to the touch'7 to1% elongated crystals, tdangular in cross section; conchoidal fracture

521

;tl APPENDIX C Mineral and Rock ldentificat¡on

TEIilITEA Key to Aid ln Rock ldentification

fneous Rocks

Ertremely coarse-grained: Rock is a pegmatite. (Most peg-

matites a¡e gra tic, with or without exotic minerals.)

Phanedtic (coa¡se enough that all grains are visibte to the naked

eye).

A. Significant quafz visible; only minor mafic minerals:granite.

B. No obvious qua¡tz; feldspar (light-colo¡ed) and mafic miner-

als (dark) in simila¡ amo].l,nts: diorite.

C. No quartz; ¡ock consists mosdy of mafic minerals: 8aá¿ro.

D. No visible quartz ¿¡ feldspar: Rock is ultamafic.

Poryhyritic with fine-grained groundmass: Go to Pa¡t IV to de-

scribe groundmass (using phenocryst compositions to as§ist);

adjective "porphyritic" will preface rock name.

Mphanitic (grains too fine to distinguish easily with the

naked eye).

A. Quartz is visible or rock is light in color (white, cream,prnk) : pr obably r hy o I íte,

B. No visible quartz; medium tone (commonly gray or green);

if phenocrysts are present, commonly plagioclase, pyroxene,or amphibole: probably andesite.

C. Rock is da¡k, commonly black any phenocrysts are olivineot pyrcxenei basah

D. Rock is glassy and massive: obsitl.ian (regardless ofcomposition).

E. Rock consists of gritty mineral grains, ash and glass shards:

i gnimb rite (w elded fiff).

Sedimentary Rocks

I. Rock consists of visible shell fragments ot of oolites; liñestone.

II. Rock co¡sists of interlocking grains with texture somewhat likethat of igneous rock and is light in color probable chemical sed-

imentary rock.

A. Tastes like table salt li¿rire.

B. No marked tastei hardness of 2 (if grains are la¡ge enough to

scratch); does not effervescet gypsum.

C. Effervesces in weak HCli l¡nestone (aalcite).

D. Effervesces weakly in HCt, only if scratcbed: dolomite.

m. Rock consists of grains apparently cemented or compacted to-gether: probable clastic sedimentary rock.

A. Coarse gmins (several millimeters or mo¡e in diameter), per-

haps with a finer matrix conglomerate if lhe graits arcrctnded, breccia if they a¡e angular.

Metamorphic Rocks

B. Sand-sized grains; gritty feel: saridstone. Ifpredominantlyqu.artz gtal].:s, quartz sandstone: if toughly equal proportionsof quartz and feldspar, arkose; if many rock fragments, andperhaps a fine-grained matix, greywacke.

C. Grains too ñne to see rcadily with the naked eyet mudsto e.

If rock shows lamination, and a tendency to part along parat-

lel planes, s&ale.

IV. Relatively dense; compact, dark, no visible grains; massive tex-

ture, conchoidal fracture: cl¡e¡, (silica).

I. Nonfoliated; compact texture with interlocking grains: identiñedby Fedominart mineral(s).

A. If quartz-rich, perhaps with a sugary appeatance'- quarfzite.

B. If calcite or dolomite (identified by effervescence, hardness):

marble.

C. Rock consists predominantly of amphiboles'. atnphibolite.

II. Foliated: classified mainly by texture.

A. very fine-g¡ained; prcnounced rock cleavage along parallel

planes, to rcsemble flagstones: .tl¿r¿.

B. Fine-grai¡ed; slatelike, but with glossy cleavage surfaces:phyllite.

C. Coa¡ser grains; obvious foliation, commonly defined byprominent mica flakes, sometimes by elongated crystals likeamphiboles: sciisr.

D. Compositional or textural banding, especially with altemat-ing light (quafz, feldspar) and dark (ferromagúesian) bands:gneiss.