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Rock
and
Mineral
Identification
for
ngineers
November 99
~
u s epartment
of Transportation
Federal ighway
dministration
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cid
bottle
8
n i f _
v /
gr nite
muscovite
8 09
g n i y
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Rock
And Mineral
Identification
for
Engineers
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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
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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
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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.
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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.
•
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Mineral Identification
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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 .
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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
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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.
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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
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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.
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• 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.
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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
•
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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).
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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 •
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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.
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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)
.
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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
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hotos
Figure
2 The
Mineral hert
Figure 3
The
Mineral Quartz
6 Rock and Mineral
Identification
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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
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hotos
Figure 6 The Mineral ornblende
Figure 7 The Mineral Calcite
8
Rock and Mineral Identification
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hotos
Figure 8 . The Mineral
Muscovite
m
Figure 9. The Mineral Biotite
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• 9
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l
a..
QI
C
E
I
o
=
2
2
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Figure
10
Mineral Identification Flowchart
Part B Not Scratched y a Knife
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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 •
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and Mineral dentification
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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
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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
ock and Mineral dentification
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hotos
Figure 11. The Rock
limestone
(coarse grained example)
Figure 12.
The
Rock
Marble (note banding)
Rock and Mineral
Identification
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Photo
Figure 13 The Rock
Dolomite
dolostone)
Figure 14 The Rock Serpentinite
8
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Mineral Identification
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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
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hotos
Figure 17 The Rock ranite
Figure 18 The Rock Syenite
3
Rock and
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Identification
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Photos
Figure 21 .
The
Rock
Diabase
Figure 22. The Rock Pyroxenite
3
Rock
and
Mineral
Identification
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Photos
]
Figure 23. The Rock Peridotite
Figure 24 . The Rock Sandstone
(note
sandy, grainy appearance)
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hotos
Figure 25. The Rock Quartzite
(note more
glassy, sharper
surface than
sandstone)
Figure 26 . The Rock Conglomerate
4 Rock and Mineral Identification
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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
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Photo
Figure 29 . The Rock Shale
Figure 30 .
The
Rock Slate (note
thin
layers that can
be
split
apart)
6 •
Rock and Mineral Identification
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hotos
Figure 31 The Rock Rhyolite
Figure 32 The Rock Andesite
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Photos
Figure 33. The Rock Basalt
Figure 34. The Rock Basalt (vesicular)
8 Rock and Mineral Identification
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Figure 35 - Rock Identification Flowchart
Rocks with
Mineral Grains/Crystals
Easily Visible to the
Naked
Eye
none
S RP NTINIT
Part-A
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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
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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.
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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
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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
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(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
ock and Mineral dentification
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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
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• 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
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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
8/19/2019 Visual Identification
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8/19/2019 Visual Identification
57/57