Visual Identification

8/19/2019 Visual Identification

1/57

Rock

and

Mineral

Identification

for

ngineers

November 99

~

u s epartment

of Transportation

Federal ighway

dministration


2/57

cid

bottle

8

n i f _

v /

gr nite

muscovite

8 09

g n i y


3/57

Rock

And Mineral

Identification

for

Engineers


4/57

T BLE

O

CONTENTS

Introduction ................................................................................ 1

Minerals ..................................................................... ................. 2

Rocks ........................................................................................... 6

Mineral Identification Procedure ............................................ 8

Rock Identification Procedure ............................................... 22

Engineering Properties of Rock Types .................................42

Summary

...................................................................................

49

Appendix: References ............................................................. 50

FIGUR S

1. Moh s Hardness

Scale ......................................................... 10

2. The Mineral

Chert

............................................................... 16

3.

The Mineral

Quartz

............................................................. 16

4.

The Mineral Plagioclase ...................................................... 17

5.

The Minerals Orthoclase .....................................................17

\

6. The Mineral Hornblende ....................................................18

7.

The Mineral Calcite .............................................................18

8.

The Mineral Muscovite ....................................................... 19

9. The Mineral Biotite ..............................................................19

10. Mineral Identification Flowchart .................................... 20

11. The Rock Limestone ..........................................................27

12. The Rock Marble ................................................................27

13. The Rock Dolomite ............................................................28

14. The Rock Serpentine .........................................................28

15. The Rock Gneiss ................................................................. 29

16. The Rock Schist ..................................................................

29

17. The Rock Granite ...............................................................30


5/57

FIGURES cont.)

18. The Rock Syenite ............................................................... 30

19.

The Rock Granodiorite.........

.....

.

...

:

..

... .. ...........................31

20. The Rock Gabbro ...................................................... ......... 31

21.

The Rock Diabase

.....

.. .... ............ ........ ............................... 32

22.

The Rock Pyroenite ........................................................... 32

23.

The Rock Peridotite ..

.... .....

......... ....................................... 33

24. The Rock Sandstone .......................................................... 33

25. The Rock Quartzite ............................................................34

26.

The Rock Conglomerate ...................................................34

27.

The Rock Limestone (fine grain) .....................................

35

28. The Rock Dolomite (fine grain) .......................................35

29.

The Rock Shale ...................................................................36

30. The Rock Slate ... .. ........... ........................... .........................36

31. The Rock Rhyolite .............................................................37

32. The Rock Andesite .............. .

...

............ .......

.....

.......... ......... 37

33. The Rock Basalt .................................................................. 38

34. The Rock Basalt (vesicular) .............................................. 38

35.

Rock Identification Flowchart, Part A ............................39

36. Rock Identification Flowchart, Part B.............................40

T BLES

1.

Mineral Groups and Their Common Minerals .................3

2. Rock Classes and the Common Rock Types ...................... 7

3. Selected Properties of the Common Minerals .................11

4. Mineral Identification Procedure ......................................

14

5.

Rock Identification Procedure ...........................................

19


6/57


7/57

ntroduction

Civil engineers routinely use rocks as aggregate material in

their construction projects. However, many engineers do

not have extensive training in rock and mineral identifica-

tion. This guide, which expands on an article and subse-

quent publications (Woolf, 1950, 1951,

1960)

written for the

Bureau of Public Roads, can help practicing civil engineers

to identify rocks and minerals and to better understand

their characteristics and performances in certain

applications.

This guide will not turn engineers into geologists or

petrographers, but it can help engineers to make basic

distinctions among various natural rock and mineral types.

The guide can also help engineers better understand why

certain types of rocks and minerals have desirable or

undesirable characteristics as potential aggregates.

The equipment needed for the procedures in this guide is

inexpensive and eas.ily obtained. The samples that are to be

identified are assumed, for our purposes,

to

be large-sized

coarse aggregate pieces or hand samples coming directly

from the quarry or gravel pit.

To

judge the hardness of

various minerals, the user will need a pocket knife with a

good steel blade and a copper penny. Other useful items are

a small bottle (with eyedropper) of dilute (O.lN) hydrochlo-

ric acid (HC ), a magnifying glass, and a magnet.

To keep the identification process simple, this guide

outlines procedures that rely as much as possible on the

visual appearance of rocks and minerals.

Basic

tests for

hardness and reactivity with dilute hydrochloric acid are

included for help in classifying a sample.

Those interested in further information may consult the list

of references at the back of this manual.

For questions or comments on this manual or the proce-

dures discussed, please contact Dr. Stephen

W.

Forster,

Pavements Division, (703) 285-2073.

ock and Mineral Identification •


8/57

inerals

Minerals are strictly defined as naturally occurring chemical

elements or compounds formed as a product of inorganic

processes (Hurlbut, 1963). Rocks are composed of an assem-

blage of one or more distinct minerals. This definition of

minerals excludes shells, coral, and other organically formed

matter which nonetheless are important constituents of some

limestones. For the purposes of this guide, these components

are also considered to be minerals.

Mineral Types. Minerals can be separated into groups on the

basis of chemical composition. Although incomplete, the

following list of groups includes those minerals which would

normally be encountered by a practicing engineer. These

groups, including their common minerals, are shown

in

table

l

Elements. This group consists of chemical elements that

occur in nature in an uncombined state. Examples are

sulfur, graphite, and gold.

Sulfides. Included in this group are combinations of

various metallic elements with sulfur. An example is pyrite.

Oxides. The minerals in this group contain a metal element

in combination with oxygen. The iron mineral hematite is

an example. A subgroup within the oxides is the hydrox-

ides, which include oxygen in the form of the hydroxyl

radical or water. Limonite

is an example of a hydroxide.

Halides. Halides are naturally occurring chlorides,

fluorides, bromides, and iodides. Examples are halite

(rock salt) and fluorite.

Carbonates. The carbonate group of minerals contains the

carbonate radical. The common minerals calcite and

dolomite are included here.

Phosphates. Minerals whose composition includes the

phosphate radical are included in this group. One example

is

apatite.

Sulfates. These minerals include the sulfate radical.

Gypsum is an example of a common sulfate mineral.

Silicates. Silicates form the largest group of minerals. They

contain various elements in combination with silicon and

oxygen. Examples are quartz and feldspar.

Although there are literally hundreds of minerals, a practicing

engineer really only needs

to

be familiar with and be able to

identify about twenty.

To

classify an aggregate sample as a

given rock type, it is usually necessary to identify only its two

to three main mineral components.

•

Rock

and

Mineral Identification


9/57

Table 1 Mineral Groups

and

their ommon

Minerals

Group

Minerals

omments

Elements

sulfur

May e seen as trace

graphite

min erals

in so

me rocks.

go

ld

s

il

ver

co

pper

iron

S

ul

fid es

PYRITE, iron di sulfide

Co

mm on accessory min eral

in

a

ll

3 rock classes.

ga lena, lead s

ul

fid e

So ur

ce

of lead.

sphalerite, zinc s

ul

fide

Source of zinc.

Ox

id

es HEMATI

TE

, ferric ox

id

e

Co

mmon mineral in a

ll

3

rock types; source of rust-

red

co

lor in many rocks.

MAG NE

TIT E, fe

rrous

Magne tic;

co mm

on

ox ide

accessory min eral in a

ll

3

roc k classes.

LIMONITE, hydrous iron

Ye ll

ow-

brow n; formed by

oxide

altering of other iron

minerals.

Halides halite, sod ium c

hl

oride

Co

mm

on r

oc

k

sa

lt.

FLOURITE, ca lcium

Co mmon accessory minera l.

f10uride

Ca rbonates

CA

LC ITE,

ca

lcium One of the

co mm

on

carbonate min erals; major co mponent

of limes tone.

DO LO MITE, ca lcium Comm on mineral; main

mag nesi um ca rbonate min eral in the rock dolomite

(also ca lled dolostone .)

Phosphatcs

APATITE, calc ium

Wid

cly distr ibutcd acces-

(f1 uoro- , chl oro-) phosphate

so ry mineral in the 3 rock

classes.

S

ul

fa tes

G YP

SUM

, hydrous

ca

lc

iu

m

Co

mmon mineral,

sulfate

espec ia

ll

y

in

limes tone and

shale.

barite, barium sulfate

Co mm on accessory

mineral, es pec ia ll y in

se

dimentary r

oc

ks.

Note: Those minerals listed in capital letters are most likely to be en

co

untered .



10/57

Table 1 Mineral Groups and their ommon Minerals

Group

Minerals

omments

Sili

ca

tes

QUARTZ, sili co n di ox ide

C HERT, s ili

co

n dioxide

FE

LDSP

ARS:

OR

THOCLASE,

potas ium aluminum

sili

ca

te

PLAGIOCLASE,

so

dium

/ca lcium

aluminum s ili

ca

te

OLIVINE, mag nes ium/iron

silica te

GA

RNET, ca lcium, iron,

magnes ium. manganese/

aluminum, titanium , iron,

chromium silicate

zir

co

n, zir

co

nium silicate

PYROX

ENES, magnesium,

iron,

ca

lcium, so

dium

,

lithium/magnes ium , iron,

aluminum sili

ca

te

AMPHIB

OLES, mag ne-

sium , iron, calcium,

sodium

/magnes ium, iron,

aluminum sili ca te

One of the

co mmon

min erals; hard and very

res istant to c hemical and

phy si

ca

l b

rea

kdown.

Cryptoc rystalline (micro-

sco pi c crystal size) variety

of qu artz.

Famil y of min erals common

in a ll 3 roc k classes.

Very

co

mm on mineral.

In

cludes a

se

ries with

co mpos

iti

ons rang in g from

the sod ium end-me

mb

er

(a lbite) to the

ca

lc ium end-

member (anorthite); these

min erals a re very co mmon .

Fairly comm on; mos t often

in darker igneo us rocks.

Co mm

on accessory mineral

in many igneo us roc ks; may

also occur in the 2 other

roc k classes.

Co

mm

on accessory mineral.

Common in many igne

ou

s

rocks; a family of minerals.

Co

mm

on

in

many igneous

rocks; a family of minerals

that includes

HORN

-

BLEND E.

Note: Those minerals listed in capital letters are most likely to be encountered.

4

Rock and M iner l Identification


11/57

Table 1 Mineral Groups and their ommon Minerals

Group

i

Minerals

i omments

Silicates (cont.)

CLAY MINERALS:

K

AO

LI

NITE, hydrous

aluminum silicate

talc, hydrous magne-

sium silicate

SERPENTINE, hydrous

magnesium sili

ca

te

MICA MI NERALS:

MUSCOVITE, hydrous

potasium aluminum

silicate

BI

OTI

TE

, hydrous

potasium, magnes ium/

iron, aluminum silicate

CHLORITE, hydrous

magnes ium/iron alumi-

num silicate

A group of usually fine-

gr

ai ned soft mineral

s.

Common clay mineral in

so il and sedimentary rocks

that includes montmorillo-

nite.

Common in metamorphic

rocks.

Common mineral

in

metamorphic rocks.

Very common mineral

in

metamorphic and ign

eo

us

rocks.

Very common mineral in

metamorphic and igneous

rock

s.

Co

mmon mineral

in

metamorphic rocks.

Note: Those minerals listed in

ca

pital letters are most likely to be encountered.



12/57


13/57

Ta

ble

2 Rock Class

es and

the Common Rock Types

Class

Rock Type

Comments

Igneous

Formed from molten rock.

subcl ass Extrus

iv

e

Fine graine

d.

fe lsite

General name which

includes th e rocks: rhyolite;

trachyte; latite; andes ite.

basa lt

Dark co lo

r.

obsidian

Glassy.

pumice

Froth y; lightweight.

subclass Intrusive

Medium to course-gra ined.

granite

sye

ni

te

grano

di

orite

monzonite

di

orite

gabbro

Gabbro and

di

abase have

di

abase

the same compos ition;

grabbo is co urse gra

in

ed,

diabase is medium gra

in

ed.

pyroxenite

pe

ri

dotite

Sedimentary

limestone

Formed by particle

do lostone (dolomite)

depos ition or chemica l

sandstone

prec ipita

ti

on.

shale

chert

conglomerate

Metamorphic slate

Formed by hi gh hea t and/o r

schi st

pressure acting on ex isting

gneiss

rock.

qua

rt

z

it

e

marble

dolomitic marble

serpentinite

ock and M ineral dentifica t ion _


14/57

Mineral Identification Procedure

Since minerals are the

co

mponents of rocks, their identifica -

tion is an integral part of proper r

oc

k identification. For this

identifica tion procedure, three characteristics of minerals

will be of major importance: hardness, reactivity with dilute

hydrochlo

ri

c

ac

id , and cleavage.

Hardness

Fig

ur

e 1 is a graph of the Mohs hardness scale applied

to

minerals. Also shown on the gra

ph

is the relative ha

rdn

ess

of several common items that can be used to separate the

minerals. The knife blade is particularly useful in separating

the

co

mmon harder minerals (quartz and the feldspars)

from the

co

mmon so

ft

er minerals (ca lcite and dolomit

e)

. To

test fo r har

dn

ess with any of these items, two approaches

may be used:

• Use the knife blade (or

co

pper penny, etc.) as a tool to

attempt to scratch the mineral

• Use the mi neral to attempt to scratch the testing

material.

Doing it both ways w

ill

often give a clearer indication of the

rela

ti

ve hardness of the two materia ls being

co

mpared.

eI Reactivity

This test serves to differentiate the carbonate minerals

(which react with He\)

fr

om other mineral types. The acid

used is dilute He

l.

The dilute ac id is obtained by mixing

water with full-strength acid. By noting the normality of the

acid being diluted, an appropriate volume of water ca n be

used to reach the target of OI N. Fo r instance , if the original

ac

id

is

1.0N, increasing the volume of water tenfo ld will

result in a O.1N . When diluting, always add theacid

to

the

water

to

avoid splashing

fu

ll strength acid

Cleavage. f a mineral breaks so it yields definite plane

surfaces, the mineral is said to possess cleavage. A mineral

ca n possess one or more direc tions of cleavage, or none:

• The micas

(e

.g.

mu

scovite and biotit

e)

are

ex

amples of

minerals with distinct cleavage in one direction.

8 • ock and Mineral

dentification


15/57

• All the feldspars have two cleavage directions, which are

at almost right angles .

• Quartz has no cleavage. This fact helps in the distinction

between quartz and the feldspars. When quartz and chert

are broken, the resulting surfaces often have a typical

concave shape called

conchoidal fracture

because of its

shell-like appearance. While not a cleavage, this distinc-

tive fracture habit can be useful in identification.

Other Characteristics. Some minerals have a distinct,

definitive color. However, because the color of most

minerals can vary significantly, color should normally be

used as supportive rather than primary evidence. Another

useful characteristic

is

a mineral's ability

to

transmit light.

Depending on the composition, crystallography, and other

factors, a mineral may be

tran

parent

translucent

or

opaque.

Table 3 lists the hardness, dilute HCl reactivity, cleavage,

and other characteristics of common minerals listed in

table 1.

See Table 4 for step-by-step mineral identification proce-

dures. The figure references within the table are to photo-

graphs of the more common minerals.

ock and Mineral dentification •


16/57

Figure 1 - Relative Hardness of Minerals in Mohs Scale

numbers

in parentheses .

- Diamond

(10)

file

win

dow

\ - Co

rundum (9)

kmfe \ - Topaz (8)

pe

nn

y \ _

Qu

artz (7)

finger \ - .ortho

cl

ase

(6)

nail Apa

tIt

e (5)

\

- Fluorite (4)

- Calcite (3)

- Gyps

um

(2)

10

•

ock and Mineral dentification


17/57

Table

3 Selected Properties of the ommon Minerals

Mineral

ardness

Cleavage

Other

Pyrite

6 - 6 1/2

None

Br

assy; foo

l

s gold; weath ers

easil y to give iron stain ;

co

mmon accessory mineral in

many rock types.

Hematite 5

1/2 - 6 1/2

None (in

Red-brown; common accessory

massive

fo

rm) in many rock

s; ce

ment in many

sandstones.

Magnetite 6

None (

in

Black; magne tic; common

granular fo rm ) accessory mineral in many rock

types.

Limonite 5 - 5 1/2

None

Ye

llow

-brown; earthy; may

appear softer than 5; formed by

a lterati on of other iron minerals.

Flu orite

4 I pl ane Common accessory mineral in

limestones and dolostones;

trans lucent

to

transparent.

Ca

lc ite

3

3

pl

anes at 75 0

Very common; occurs in many

rock types ; chi ef mineral in

limestone; vigorous reac ti on

with dilute HC .

Dolomite

3 1/2 - 4 3 pl anes at 74

0

Common; with ca lc ite in

dolomitic limestone or dolostone

(>

50

% dolomite); vigo rous

reaction with dilute HCI y

when powdered.

Apatite

5 I pl ane, poor

Common minor accessory

mineral in a

ll

rock classes.

Gypsum

2

4 planes;

Common mineral, espec iall y in

I perfec t limestones and shales; may

occ ur in laye r

s.

Quartz

7

None Very common; may occur

in

many rock types; glassy;

trans

lu

cent to transparent ; may

be co lored; very res istant to

weathering; chi ef mineral in

sandstones.

Chert

7 None Cryptoc rystalline (microsco

pi

c

crystals) va

ri

ety of quartz;

appears massive to naked eye;

common in limestones or in

co

mplete laye rs assoc iated with

limestones ;

li

ght tan to

li

ght

brown; similar mineral

s:

flint

(dark brown to bl

ack); jasper

(red); chalcedony (waxy look,

tan to brown).

Rock and Mineral dentification •


18/57

Table

3

Selected

Properties

of

the

Common

inerals

ineral

Hardness

Cleavage Other

Orthoclase 6

2 planes

A feldspar; very common in

at 90°

many rock types; white to grey

to red-pin k; translucent to

transparent; cleavage separates it

from quartz.

Plagioclase 6

2 planes

A feldspar; very common in

at 94°

many rock types; appears s

im

ilar

to orthoclase - distingui shed by

the presence

of

thin, parallel

lines on cleavage faces due to

crys tal structure.

Olivin e

6 1/2 - 7

None

Transparent to translu cent; o live

green; glassy; common

accessory mineral

in

the darker

igneous rocks.

Gamet

6 1/2 - 7 1/2

None

Red to red-brown; translucent to

transparent; common accessory

mineral in metamorhic and some

igneous rock

s;

also in sands and

sandstones.

Zircon

7 1/2

None

Usually colorl ess

to

brown;

us ually translucent; common

accessory mineral in igneous

rocks and some metamorphic

rocks; also in sands and

sandstones.

Pyroxene

5 7

2

pl

anes

Most common in th e darker

(mineral group)

at 87°

igneous rock

s;

usually green to

and 93°

bl ack; translucent to transparent;

'most common mineral: augite.

Amphibole

5 6

2 pl anes

Most common in metamorp

hi

c

(mineral group )

at 56°

rocks and the darke r igneous

and 124°

rocks; usually dark green to

brown to black; trans

lu

cent to

transparent; most common

mineral: horn blende; di stin-

guished from pyroxenes by

cleavage.

Clay

2 - 2 1/2

1

pl

ane

Usua

ll

y fine grained; earth

y;

Minerals

of

ten de

ri

ved from wea thering

(a group)

of fe ldspars; montmori llonite is

the swelling clay that expands

with th e absorption of water;

illite is the common clay mineral

in many shales.

2 •



19/57

Table

3 Selected

Properties of the Common Minerals

ineral

Hardness

leavage

Other

Talc

1 plane

Very soft, greasy; cleavage may

be hard to see because of fineness

of particles; commonl y white to

pale green; usually

in

metamor-

phic or altered igneous rock

s.

Serpentine

2 - 5 none

Massive to

fi

brous; greasy to

(usually 4)

waxy; various shades of green;

fo und in altered igneous or

metamorphic rocks; fibrous

variety is

th

e source of asbestos.

Muscovite

2 - 2 1/2

plane

A mica; perfect cleavage a

ll

ows

splitting into thin, clear transpar-

ent sheets; usua

ll

y

li

ght ye

ll

ow to

light brown; common in

li

ght

colored igneous rocks and

metamorphic rocks.

Biotite

2 1/2 - 3

plane

A

mi

ca; perfect cleavage allows

splitting into thin smoky

transparent sheets; usually dark

green to brown to bl ack; fo

un

d

in

light to medium colored igneous

rocks and metamorphic rocks.

Chl orite

2 - 2/12

pl

ane

Si

milar to

th

e micas ; usually

occurs

in

sma

ll

particles so

cleavage produces flake; fl akes

are flexible but not el

as

tic

as

are

the

mi

cas; usually some shade of

green.

Rock and Mineral Identification •

3


20/57

Table

4

Mineral

Identification

Procedure

Is it harder than a knife?

I f YES, what is its overall appearance?

A.

Dull and ea

rth

y, waxy, or metalli

c.

1.

Mag

netic (small fr

ag

me

nt

s s tick to

th

e

kn

ife blade)

-

magnetite.

2. No nm

ag

neti

c,

how does it

br

e

ak

(frac

tu r

e)?

a. Sharp edges; conchoidal (concave, like

th

e inside of

an oyster shell) sur face -

chert

(figure

2).

b. Rough, uneven surface

red-brown to black - hematite;

brown to dark brown - limonite .

NO

T

E:

both hema

t

ite

and

lim

on

it

e

can

app

e

ar

s

ofte

r

than

a

knife if

not

t ted

on

a fre

sh

unwea thered s

urface

c. Pale to medium brass color, often in cubic crystals

- pyrite.

B.

Vitreous (glassy), transpare

nt

to translucent.

1. No cleavage.

a. Co

lorless to w hite to pale

pink

-

quartz

(

fi

g

ur

e

3).

b. O

li

ve

green -

olivine

.

c.

Red-b

row

n - garnet.

2.

T

wo

clea

vage

planes,

in t

ersecting a t a

ppr

oximately a

90

-d

eg

ree angle.

a.

Good to perfect cl

eavage

surfaces

(feldspar group , one surface w ith parallel striations)

- plagioclase (figure 4); no striations present

-

orthoclase

(figure 5) .

b. Poor to fair

cleavage

s

urf

aces -

pyroxene.

c. Two cleavage

pl

anes,

in t

ersectin g a t 120 and

60

degrees - amphibole (includes

hornblende

)

(fig

ur

e

6)

.

4 • ock and Mineral dentification


21/57

Table

4 Mineral Ident ification Procedure

cont.)

II f NO, will it scratch a copper penny?

A. f yes, will it react with dilute HCl?

1. vigorous reaction - calcite (figure

7).

2. minor reaction when whole, vigorous

wh

en powdered

- dolomite.

3. no reaction .

a.

One plane of perfect cleavage - fluorite.

b.

plane of poor

cl

eavage - apatite.

c. Non-crystalline; waxy to greasy or fibrous appearance

- serpentine.

B. f no, does it have perfect

cl

eavage which allows splitting into

thin sheets?

1. Yes - mica group.

a. Pale, light color

s,

sheets are fl

ex

ible

and

elastic

- muscovite (figure 8 .

Usually in very small flakes; sheets are flexible

but

not elastic - chlorite.

b.

Dark colors, green to brown

to

black -

bi

otite

(figure

9)

.

2.

No.

a.

Opaque, very fine grained - clay minerals.

b.

Translucent to transparent - gypsum.

NOTE:

Color has b

ee

n u

se

d in the latter s

ges of somedecis

ion

s d

es

pite

th

e c

aution

g

iv

en

on

th

e u

se

of

c

olor In th

e

in

s

tanc

es

where c

olor

is u

se

d,

it

s u

se

is

jud

ge

d

appropriate for the

min

erals in

vo

lved and the low likelihood of encountering

ex

amples out

s

ide th

e color

ranges

gi

ve

n.

Fi

g

ur

e

1 is

a d

ec

is

ion

tree diag

ram of

th

e outline g

iv

en abo

ve

Two copies of this diagram are also included on weatherproof cards of

pocket size for handy reference in the field.

Rock and Mineral Identification

•

5


22/57

hotos

Figure

2 The

Mineral hert

Figure 3

The

Mineral Quartz

6 Rock and Mineral

Identification


23/57

Figure 4. The Mineral Plagioclase

(note

striations

due

to crystal structure)

Figure 5. The Mineral Orthoclase (pinkish-tan) and Quartz (white)

Rock and Mineral Identification 7


24/57

hotos

Figure 6 The Mineral ornblende

Figure 7 The Mineral Calcite

8



25/57

hotos

Figure 8 . The Mineral

Muscovite

m

Figure 9. The Mineral Biotite


• 9


26/57

l

a..

QI

C

E

I

o

=

2

2



27/57

Figure

10

Mineral Identification Flowchart

Part B Not Scratched y a Knife


28/57


29/57


30/57

Table

5. Rock Identification Procedure

cont

.

b. Minerals not in distinct layers .(massive).

1) Chief minerals are the feldspars, with orthoclase

>plagioclase;

fi

ve percent or more quartz; mica

minerals or hornblende, or both in small amounts are

co

mmon - granite (figure

17)

.

(2) Like granite, except little or no quartz - syenite

(f

igure 1

8)

.

(3) Chief minerals are the fe ldspars, with plagioclase

>orthoclase; five perce nt or more quartz; mi ca

minerals and / or hornblende in small amounts are

co mmon - granodiorite

(fig

ure 1

9).

(4) Like granodiorite, except little or no quartz

- monzonite.

(5) Mainly plagioclase, with hornblende and some

biotite; no quartz; medium to dark co lor - diorite.

(6)

Lik

e diorite,

exce

pt pyroxene and possibly oliv

in

e

present instead of hornblende and biotite; co lor is

usua

ll

y dark - gabbro (figure

20).

NO

TE:

Fi

nergrained gabbros areoften

ref

erred

to

as

d

i

se

f

igu re21). The ratherambiguous term t

raprock

is

also us

ed

for

thi

s

rock type

.

(7) Chief minerals are pyroxenes and olivine; pyroxenes

>olivine; dark co lor - pyroxenite (f

ig

ure 22).

(8)

Chief minerals are pyroxen

es

and olivine; olivine

>pyroxenes; dark co lor - peridotite (figure 23) .

2. M

in

erals are in distinct grains that are

ce

mented together

rather than intergrown.

a.

Sa

nd-sized particles,

ce

mented by sili

ca, cl

ay, calcite, or

hematit

e;

chi ef minerals are usually quartz and fe

ld

spar;

breaks around rather than through the sa nd grain s

- sandstone (figure 24).

b. Similar to sandstone in general appearance; quartz is the

chi

ef

mineral; grain bounda

ri es

range from parti

al

to total

intergrowth due to secondary quartz cr

ys

tallization

dur

ing metamorphism; due to this intergrowth, the rock

breaks through rather than around the mineral grains

- quartzite

(fig

ure

25)

.

NOT E: Under higher levels of metamorphism quartzitewou ld f ll under

category

B.1

.b. above due to

near

ly complete mine

r

l intergrowth. t c

an

24 •

Rock

and Mineral dentification


31/57

Table 5 . Rock Identification

Procedure

cont .)

eas

il

y be sep

ara

ted from the ot

her ro

cks listed under B.1 .

b

bec

au

se the

on

ly

mineral present in signi

ficant

amounts is quartz.

c.

Grave

l-

sized particles of rocks and minerals, cemented by

s

ilic

a,

clay, calcite or hematite - conglomerate

(

fi

gure

26

).

II. Very fin e mineral grains, not visible to the naked eye.

A. Glassy.

1. Looks like gla ss; may have a few inclusions or bubbles; dark

brown to black - obsidian.

2. Contains many bubbles , frothy - pumice.

B. Dull, earthy or stony.

1.

Can be

sc

ratched with a knif

e.

a. R

ea

cts vigorously with dilute HCl - limestone

(fig

ure 27).

b. R

ea

cts slowly when whole or vigorously when powdered

with dilute HCI - dolomite (figure 28) .

c.

Reacts s

lo

wly or not at all with dilute HCl, whether whole

or powdered .

(1) Tends to break into flaky pieces - shale (figure 29 ).

(2) Layered; breaks into thin,

fl

at sheets - slate

(figure 30).

2. Can' t be

sc

ratched with a knife.

a. Very hard; fractured surface is smooth (may be conchoi-

dal) with sharp edges; surfa

ce

may app

ea

r waxy; tan to

bl ack co lor - chert.

b. Massive; dull-appea ring fractured surface; may have small

inclusions of glass or crystal

s.

(1)

Light to medium colors - felsite.

N

OT

E:

Felsi

te

includes the ext

rus

i

ve

igneous

rock

typ

es:

rhyolite figure 31 , trachyte, latite, and and esite figu re

32) which usua lly can t be dist

ingu

ished by then

aked eye.

They

differ basically on ly in the rel

ati

ve amou nts of the two felds

pa

rs

(their p

rima

ry

cons

t

itue

nts) and the

pr

esence or absenceof

quartz.

(2) Dark to black

co

lor - basalt

(f

igures

33

and 3

4)

.

Rock and

Mineral dentification 5


32/57

Because this ra

ti

o between ortho

cl

ase and plagio

cl

ase is a

co ntinuum, rocks may be

fo

und on the borderline between

the two gro

up

s. f in doubt, class

if

ying a rock as a granit

e/

granodiorite or syenit

e/

monzonite is su

ff

icient b

eca

use of

their similar performance in co nstruction. As the percentage

of plagioclase in this

fa

mily of r

oc

ks increases, the percent-

age of so-ca lled dark minerals (mainly amphiboles and

pyroxenes) also inc reases, giving the rock a darker overall

appearan

ce.

This often a

ll

ows the

di

stinc

ti

on of diorite

from the other members of the gro

up

.

The two dark igneous rocks - pyroxenite and peridotite -

may be ind is tinguisha

bl

e in hand sp

ec

imen if the mineral

crys tal size is too small to distinguish

cl

eavage. As was the

case above, identi

fy

ing a rock as being one of the members

of this co ntinuum is helpful s

in ce

they behave similarly in

co

nstruction uses.

Although often even-tex tured (mineral gra

in

s are a

ll

about

the same sizc), all igneous rocks may ex hibit what is

ca ll

ed

porphyritic t

ex

tur

e.

A por

ph

yritic t

ex

ture is defin ed as one

in which one or more minerals occ ur in crystals

mu

ch larger

than the sur rounding minerals in the rock.

t

is the size

diffe rence rather than absolute size which de fin es the

tex tur

e. Th

erefore

it ca

n ex ist as 1

mm

crystals dispersed

through a basalt or

10

mm crystals in a granit

e.

The

presence of this texture does not affect the rock's classifi-

cation, although it may affect some of its engineering

properties, which will be discussed here.

Fig

ur

es

35

and

36

are two d

ec

ision tree diagrams of the

outline given above. Th ey are also included on the weather-

proof ca rd for use in

th

e field.

6



33/57

hotos

Figure 11. The Rock

limestone

(coarse grained example)

Figure 12.

The

Rock

Marble (note banding)

Rock and Mineral

Identification

7


34/57

Photo

Figure 13 The Rock

Dolomite

dolostone)

Figure 14 The Rock Serpentinite

8

• Rock and

Mineral Identification


35/57

hotos

Figure 15. The Rock Gneiss (note foliation

of minerals)

Figure 16 . The Rock Schist (note distinct foliation

of minerals)

Rock and Mineral

Identification

•

29


36/57

hotos

Figure 17 The Rock ranite

Figure 18 The Rock Syenite

3

Rock and

Mineral

Identification


37/57


38/57

Photos

Figure 21 .

The

Rock

Diabase

Figure 22. The Rock Pyroxenite

3

Rock

and

Mineral

Identification


39/57

Photos

]

Figure 23. The Rock Peridotite

Figure 24 . The Rock Sandstone

(note

sandy, grainy appearance)



40/57

hotos

Figure 25. The Rock Quartzite

(note more

glassy, sharper

surface than

sandstone)

Figure 26 . The Rock Conglomerate

4 Rock and Mineral Identification


41/57

Photos

1

m

Figure 27 The Rock limestone fine grained example)

Figure

28

The Rock

Dolomite

dolostone)

very

fine grained

example)

Rock

and

Mineral

Identification 5


42/57

Photo

Figure 29 . The Rock Shale

Figure 30 .

The

Rock Slate (note

thin

layers that can

be

split

apart)

6 •



43/57

hotos

Figure 31 The Rock Rhyolite

Figure 32 The Rock Andesite

Rock and Mineral dentification 7


44/57

Photos

Figure 33. The Rock Basalt

Figure 34. The Rock Basalt (vesicular)

8 Rock and Mineral Identification


45/57

Figure 35 - Rock Identification Flowchart

Rocks with

Mineral Grains/Crystals

Easily Visible to the

Naked

Eye

none

S RP NTINIT

Part-A


46/57

Figure

6

Rock Identification Flowchart Part 8

Rocks With Very Fine M ineral Grains/ Crystals

Not Easily Visible to the Naked Eye

4

ock

and

Mineral

dentification


47/57

Role of Aggregate Source in Identification A

natural

aggregate may come

fro

m a sand and gravel deposit or

from a quarry in the fo rm of crushed stone. For two

important reasons, identifying the rock type(s) in a crushed

stone is usua lly easier than identi

fy

ing those in a gravel

so

urce

. First, a crushed stone is normally

co

mposed of o

nl

y

one or two closely related rock types, even though some

changes in mineralogy and t

ex

ture may

oc

cur with vertica l

or horizontal separa tion, or bot

h,

in the quarry. Second ,

since it is a crushed material, sample surfaces are freshly

broken and clean, which

is

the ideal

co

ndition fo r identify -

ing minerals and determining t

ex

tur

e.

A sand a

nd

gravel source, on the other ha

nd

, has several

potential difficulties for rock and mineral identifica

ti

o

n.

Since sa

nd

a

nd

gravel are water transported a

nd

deposited,

rock fragments

co

ntained therein ca n have many different

original bedrock sources. As a result, a single gravel deposit

can contain many, often unrelated, rock types. Sa mples

must therefore be taken espec ially carefully to ensure that

they are representative of the whole deposi

t.

n addition,

particles in a sa

nd

a

nd

gravel deposit are o

ft

en worn or

coated with secondary minerals or both, making identifica-

ti

on of the minerals more difficult. This

ca

n o

ft

en be

by wetting the particles before examina

ti

on or by

breaking the particles to ex pose a fresh surface.

ock nd Mineral

dentification

• 4


48/57

Engineering Properties of ock Types

Although there ca n be some varia tion in engineering

properties within a given rock type, knowing rock type or

even rock class fo r the aggregate will often provide insight

about its physica l, mechanica l, and / or chemical properties.

These general rela

ti

onships between rock type and proper-

ties can help in sel

ec

ting a proper aggregate material for a

given applica tion.

AS

TM C-2

94

"S tandard Descriptive

Nomenclature fo r Constituents of Natural Mineral Aggre-

gates" and

STP

9Bon tests and properties of concrete

making materials

(AST

M, 1986a, 1978) provide additional

info rmation on this subj

ec

t.

Va

rious enginee ring properties

are discussed below.

Absorption. Absorption

is

closely related to the poro:.ity

(pore space) in the rock and its permeability (ability to

transmit water). Sedimentary rocks (which in general are

co

mposed of ind ividual rock fragments or mineral grains,

or both, packed to var

yi

ng degrees) tend to have mort

space between grains and therefore higher absorpt

io

n than

igneous and metamorphic rocks. The pore space in igneous

and metamorph

ic

rocks

is

generally less due to an interlock-

ing grain structure created by the mineral crystallization (or

recrystallization) in place.

Based on tests

co

nducted on thousands of aggregate

samples from across the

co

untry by the FHW A's forerun-

ner, the Bureau of Public Road

s,

absorption of sedimentary

rocks was found to be in the 1 to 2 percent range, while

igneous and metamorph

ic

rocks were usually well below 1

percent (Woolf, 1

953)

. Of the sedimentary rocks, sandstone

and chert tended to be on the high end of the range, and

limestone and dolomite at the lower end. Keep in mind that

while these trends are useful, these values are average

results, and a sp

ec

ific rock so

urce ca

n be sign

if

i

ca

ntly above

or below these levels.

FreezelThaw Durability and D-Cracking. The ability of an

aggregate to withstand the rigors of freeze/thaw (F

IT

cycling in the presen

ce

of moisture often has a complex

relationship with its porosity and permeability. Rocks can

have fairly high absorption and still be durable under F/T

co

nditions if their perm

ea

bility

is

such that any water

4 •

ck

nd

Mineral dentification


49/57

present within the rock can migrate as freezing takes place

to accommodate the volume change. Many sedimentary

rocks fit this description. Igneous and metamorphic rocks,

on the other hand, are durable under IT conditions

because their absorption is typically low.

F

T

durability tends to be lower for rocks having certain

combinations of porosity, pore size distribution, and

permeability. These rocks can absorb critical amounts of

water over long periods. However, the water cannot escape

rapidly enough during freezing, and the pressure buildup

due to the water migration and expansion fractures the

aggregate. Certain carbonate rocks found in the Central

United States are particularly susceptible

to

this

phenomenon.

When an aggregate is used in concrete, its effect on the IT

durability of the paste must also be considered. The

nondurable rock described above, when used in concrete,

not only cracks itself but may form cracks in the surround-

ing concrete. In slabs on grade, these cracks are typically

seen at the surface as a series of cracks parallel to each other

and a free edge or joint in the concrete. This is the phenom-

enon known as D-cracking.

As noted above, rocks with high absorption are durable in

an unbound condition, because their permeability allows

internal pressures to be relieved

to

their outside as water

freezing takes place. When these same rocks are enclosed in

concrete, water in the aggregate may develop disruptive

forces when the aggregate is frozen in a critically saturated

condition. This is because the lower permeability of the

concrete paste may not be able to accommodate (at a rate

sufficient to prevent pressure buildup) the water being

forced from the aggregate due to expansion during freez-

ing. The resulting pressure may be sufficient

to

cause tensile

cracking in the concrete paste (or "popouts"

if

the aggregate

particle

is

near the surface). For these IT related distresses

to occur, it is assumed that the aggregate

is

either

0

critically saturated (that is, there

is

enough water

in the pores so the remaining pore space will not

accommodate the expansion

due

to freezing) or

ock

and Mineral dentification •

4


50/57

(2) it has a pore system that w

ill

not allow a rapid

enough migration of water during freezing to prevent

the buildup of destructive tensile stresses.

Wear a

nd Polish Resistance . The wear a

nd

polish resis-

tan

ce of an aggregate ex posed to traffic at the pavement

surface is highly re lated to the absolute and rela

ti

ve

hardness of the minerals making up the aggregat

e.

These

character

is

tics are especially critica l for the coarse aggregate

when it

is

to be used in an asphalt pavemen

t.

Aggregates

co mposed of so

ft

minerals (co mmon ones are calcite and

dolomite) or aggregates whose mineral grains are wea

kl

y

cemented togethe r w

ill

quic

kl

y wear away (low-wear

resistance), leaving little or no aggregate protruding above

the general s

ur

face

of the pavement. Witho

ut

this protrud-

ing aggregate, the pavement has no drainage channels for

water to escape fro m beneath vehicle tires during rain, and

therefore high-speed skid resistance tends to be low.

The other important aspec t of skid resistance to which the

aggregate contributes is tire a

dh

esion to the surface. The

pavement surface t

ex

ture necessary fo r this adhesion is

provided by the ex posed fine aggregate and the small scale

(less than O.5

mm

) surface t

ex

ture of the

ex

posed coarse

aggregate. To be effective, a coarse aggregate must not only

have this tex ture initially, but also be of a co mposition

which resists the smoothing or polishing of this texture

under traff

ic.

Pure limestone coarse aggregates may have

this t

ex

ture after crushin

g,

but since the soft minerals of

which they are composed are easily polished, these aggre-

gates will not maintain this tex ture very long under traffic

(low-polish resistance). Sa

nd

stones often have this tex ture,

and will maintain it quite well under traffic because quartz

(a hard mineral) is usually a major mineral co mponent.

As noted above, the strength of the ce ment holding the

mineral grains together is also an important fac tor in the

performance of a sandstone. f the grains aren' t adequately

bound together, the aggregate will be worn away too

rapidly to be acceptable

(l

ow-wear resistance), even though

its surface tex ture is maintained.

Igneous and metamorphic rocks generally have the pote

n-

ti

al to provide adequate pavement fric

ti

on; however, each



51/57


52/57


53/57

stable mix due

to

interlocking of the corners of adjacent

pieces. In contrast, round particles have more tendency to

roll or slide past each other. This same tendency may affect

the stability and load bearing capacity of unbound bases. In

Portland cement concrete, this tendency for angular

particles

to

interlock may result in a harsher mix that

is

difficult to mix and place. To compensate for this difficulty,

additional natural fines may need to be included in the mix.

Excessive amounts of flat or elongate particles are not

desirable in asphalt or Portland cement mixes. In both mix

types these particles make mixing and placement more

difficult, and they are subject

to

breakage, particularly

during compaction of asphalt pavements.

Alkali Aggregate Reactivity. All aggregates react, to some

degree, when incorporated in Portland cement concrete.

This only becomes a problem when the reaction products

are of a certain composition and extensive enough so that

their uptake of moisture exerts destructive expansion forces

within the concrete. This reaction process involves the

alkalies present in the concrete (usually derived from the

cement), water, and certain siliceous or carbonate aggre-

gates. Deterioration due to this reaction is usually mani-

fested at the surface of the affected structure by a regular

system of cracks called map or pattern cracking. This

pattern may be influenced by the size and shape of the

affected structure.

Siliceous minerals identified as potentially reactive include

opal, chalcedony, microcrystalline to cryptocrystalline

quartz, crystalline quartz which is intensely fractured or

strained, and certain volcanic glasses. This group basically

includes most types of very finely divided, highly siliceous

minerals. Consequently, the list of rock types which may

contain these minerals is quite extensive.

Of the carbonate rocks, many are somewhat reactive, but

only a very limited range appear to cause deleterious

expansion. The known problem aggregates have the

following characteristics:

• They are dolomitic, but contain significant amounts of

calcite.

ock and Mineral dentification •

47


54/57

• Th eir mineralogical t

ex

ture

co

nsists of small dolomitic

crystals within a matrix of clay or silt and finely di

vi

ded

ca lcite or both; and

• The matrix is ex tremely fine-graine·d .

When dealing with the reac

ti

vity

pr

oblem, the simplest

solution is to avoid the use of reactive aggregate. With the

shortage of ava ilable aggregates in so me areas, this is not

always possible.

Us

ing a low alka

li

ce ment (less than 0.6

percent) or certain admixtures has been a

co

mmon practice

when known reac tive aggregates are used in co ncrete. The

use of low alkali cement prevents deleterious ex pansion in

most instan

ces.

Mo re recentl

y,

some aggregates have been

foun

d to be deleteriously rea ctive, even with the low alkali

cement

s.

This situation reinforces the need to test aggregate-

cement

co

mbinations before using them on

th

e job wher-

ever a reac

ti

vity problem is suspec ted . Definitive

sc

reening

tests may take six months to a year to run, but more rapid

test methods are being developed.

Weathering. All of the above discussions about aggregate

assume that the mate

ri

al is in a "fresh" unweathered

co

ndition. The most

dur

able roc k, if

ex

posed for a su

ffi

-

ciently long period to the natural environment, w

ill

degrade

into a soil. For mos t aggregates, this time frame is geologic

rather than histori

c,

and therefore, for engineering pur-

poses, need not be of concern. That is, it nee d not be of

conce rn if initially unweathered aggregate is used.

In the case of crushed ston

e,

the production of unweathered

aggregate material ca n usually be assured by stripping

su

ffic

ient overburden fro m the site before opening the

quarry, and during horizontal

ex

pansion of the quarry at

the surface . Sa nd and gravel deposits tend to be more

variable than crushed stone quarries in both the horizontal

and ver tica l direction, so variation in the wea thering state of

the material may need to be more closely monitored during

operations than with a crushed stone quarry. In either case,

the develo

pm

ent and operation of an aggregate source

should be

ca

refully monitored by a geologist, mater

ia

ls

engineer, or other qualified person.

8 Rock

n

d M iner l

de

nti f

ic tion


55/57

Summary

This manual provides a brief discussion of the formation,

co

mpos ition and classifica tion of rocks and minerals.

Identi

fica ti

on pro

cedur

es are presented fo r differentiating

the most

co

mmon

roc

k and mineral types the practicing

engineer is likely to encounter. The procedures may be

ca

rried out in the field or the laboratory using simple tools

and the

fl

owcharts provided . The ability to identify the

rocks and minerals of which an aggregate is composed will

give the engineer additional insight into the potential

performan

ce

of that aggregate in a particular

co

nstruction

applica tion. This ability will also help the engineer to

understand the aggregate's impact on the performan

ce

(good or bad) of pas t

co

nstruction projects that are being

reviewed.

Fo r

co

mprehensive aggregate identifi

ca ti

on, testin

g,

and

analysis, a qualified geologist, petrographer, or materia ls

engineer should be consulted as appropriate.

ock and Mineral dentification • 9


56/57


57/57

Visual Identification

Documents

Transcript of Visual Identification