PHYSICAL GEOLOGY Minerals
Chapter 3 1
Not counting elements and sub-atomic particles minerals are the
basic building block for the majority of earthen materials. Minerals
are the major constituent of nearly all rocks. Combinations of
minerals (mineral assemblages) in rocks can give insight into the
formation and possible alteration of the rocks in question.
Identification of minerals is a process of interpreting the
physical and other properties of the mineral. In this laboratory
assignment you will become familiar with many of the properties of
minerals and how to test these properties. Finally by utilizing this
data and identification charts you will be able to determine the
mineral species.
Mineral: Naturally occurring, inorganic, homogenous,
solid, element or compound with a characteristic chemical
composition and definite crystalline structure.
Properties
Color: The color of the mineral. Color is not the best
property to use to identify minerals. Many minerals occur in multiple
colors, impurities in the chemical composition may alter the color.
Many different minerals occur in the same colors.
Luster: The way in which light is reflected off the minerals
surface.
Metallic – reflects light like a polished metal (such as
gold, silver, lead, bronze, and iron)
Submetallic – reflects light like a corroded metal (such
as cast iron or rust)
Vitreous – reflects light like glass (might not be
transparent)
Adamantine – reflects light like a diamond
Resinous – reflects light like amber
Pearly – reflects light like mother-of-pearl
Satiny – reflects light like satin or silt cloth, more
reflective in one direction
Greasy/oily – reflects light like an oil coated substance
Waxy – reflects light like a candle or a crayon
Dull/earthy – reflects light like dirt or rust
Streak Color: The color of the mineral when powdered.
Minerals softer than a Mohs hardness of seven can be powdered by
rubbing the sample on an unglazed porcelain plate. Minerals with a
hardness seven or higher must be crushed.
Cleavage: The tendency of some minerals to break along
parallel planar surfaces (parting: pseudo cleavage).
Habit: The form (shape) the mineral will grow if
unimpeded.
Acicular – needle like
Bladed – like a knife blade
Equant – roughly box-like or ball like e.g. cubic,
octahedral
Mammillary – rounded masses
Micaceous – thin sheets
Oolitic – small spheres (~1-3mm)
Prismatic – elongated with opposite parallel
faces
Tabular – flat, plate-like
Hardness: The minerals resistance to being scratched. In
1812 Friedrich Mohs created a scale using ten common rock forming
mineral to compare the differing hardnesses of all minerals. Starting
at the softest and proceeding to the hardest. 1. talc 2. gypsum 3.
calcite 4. fluorite 5. apatite 6. feldspars 7. quartz 8. topaz 9.
corundum 10. diamond. Besides Mohs’ minerals; there are common
items we can also use to determine a minerals hardness. Your
fingernail is about 2.5, a copper penny is about 3.5, a wire nail is
about 4.5, a glass plate is about 5.5, and a scratch plate is about 7
(See table 3.1). Mohs scale is not incremental. Gypsum, number two
is not twice as hard as talc number one. Diamond, number ten is not
ten times as harder than talc. In absolute hardness diamond is 1600
times harder than talk.
Cubic: three cleavages (six faces) all at right-
angles (90°)
Octahedral: four cleavages (eight faces)
Dodecahedral: six cleavages (twelve faces)
Rhombohedral: three cleavages (six faces) not at
90°
Prismatic: two cleavages (four faces) fifth and
sixth sides are fractured
Basal: one cleavage (two faces)
PHYSICAL GEOLOGY Minerals
Chapter 3 2
To identify the hardness of the mineral in question you will attempt
to scratch the mineral with the minerals in the Mohs hardness kit
and/or the other common items. If the mineral is scratched it is
softer than that mineral or item. If the mineral is not scratched by
the item or it can scratch the item, it is harder than the item. If the
mineral and the item scratch each other they have an equivalent
hardness.
Mohs hardness Absolute hardness
talc 1 1
gypsum 2 3
fingernail 2.5
calcite 3 9
copper penny 3.5
fluorite 4 21
wire nail 4.5
apatite 5 48
glass plate 5.5
feldspars 6 72
quarts 7 100
topaz 8 200
corundum 9 400
diamond 10 1600
Table 3.1 Mineral hardness scale. Including Mohs hardness scale, alternate item and absolute hardness.
Other properties:
Magnetism – the mineral is attracted to a magnet.
Taste – the mineral has a distinct taste.
Smell – the mineral has a distinct smell.
Reaction to acid – the mineral effervesces in dilute
acid.
Feel – the mineral has a certain feel to it (such as
soapy or gritty).
Specific gravity – how dense the mineral is.
Heft – the mineral seems heavy or light relative to its
size.
Stiations – fine parallel “scratches”
Exsolution lamellae – “vain” looking pattern of lighter
and darker coloration
Pick samples 25 through 32.
Arrange the minerals by any criteria. What was the criterion and
what was the order?
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Now arrange them by another criterion. What was the criterion and
what was the order?
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0
1
2
3
4
5
6
7
8
9
10
0 200 400 600 800 1000 1200 1400 1600
Mohs vs. Absolute hardness
Mohs Absolute
diamond corundum
topaz
quartz
feldspars
apatite
fluorite
calcite
gypsum
talc
PHYSICAL GEOLOGY Minerals
Chapter 3 3
Metallic and Sub-Metallic Lusters
Streak Hardness Cleavage or Fracture
Color Other Properties Mineral Name
Dark Gray to Black 6.0-6.5 Fracture Brass-yellow to Gold; tarnished dark gray
Habit equant; occasionally striated
Pyrite
6.0-6.5 Fracture Silvery dark gray; tarnished to black
Habit octahedral; strongly attracted magnets
Magnetite
3.5-4.0 Fracture Greenish gold; tarnished to goldish brown
Habit equant Chalcopyrite
3.0 Fracture Brown bronze; Tarnishes black with blue to purple iridescence
Habit granular masses; may be weakly attracted to magnets
Bornite
2.5 Excellent; three direction (cubic)
Slivery gray; tarnishes to dark dull gray
Habit equant Galena
1.0-2.0 Perfect, one direction (basal); too small to see
Dark silvery gray to black
Habit tabular; can be scratched with fingernail, rubs off easily on surfaces
Graphite
Brown 5.5-6.0 Fracture Silvery black to black; tarnishes dark gray to black
Habit equant; may be weakly attracted to magnets
Chromite
Yellow-brown 5.0-5.5 Perfect, one direction (basal); too small to see
Yellowish brown to red- brown
Habit acicular (radiating needle like) often microscopic; also non-metallic
Goethite
White to pale yellow- brown
3.5-4.0 Perfect, six direction (dodecahedral); often too small to see
Silvery yellow-brown silvery red; tarnishes Black
Habit equant; rotten egg smell when scratched or powdered; also non-metallic
Sphalerite
Red to red-brown 5.0-6.0 (1.0-5.0 nonmetallic)
Parting Gray to silver Slivery verity is glittery; may be weakly attracted to magnets; Also non-metallic
Hematite
PHYSICAL GEOLOGY Minerals
Chapter 3 4
Nonmetallic Lusters (light colored) Hardness Cleavage or
Fracture Color Other Properties Mineral Name
9.0 Parting and conchoidal fracture
Red (ruby), Blue (sapphire)
Luster vitreous; habit prismatic; streak none Corundum
8.0 Perfect, one direction (basal)
Colorless, yellow, blue, green, violet, orange
Luster vitreous; habit prismatic; streak white Topaz
7.0 Conchoidal fracture Green (forsterite), Orange (fayalite)
Luster vitreous; granular massive; streak white to pale green
Olivine
7.0 Conchoidal fracture Pink (rose quartz), yellow (citrine), white (milky quartz)
Luster vitreous; prismatic and pyramids, amorphous; streak white
Quartz
6.0 Excellent, two directions (prismatic) nearly 90°
White, gray Luster vitreous; habit prismatic; streak white; striations on cleavage plains
Plagioclase feldspar
6.0 Excellent, two directions (prismatic) nearly 90°
Colorless, orange, brown, white, green, gray, pink
Luster vitreous; habit prismatic; streak white; exsolution lamellae
Orthoclase feldspar (potassium feldspar)
5.5-6.0 Conchoidal fracture White, orange, yellow, brown, blue, gray, green, red
Luster vitreous; amorphous masses; streak white
Opal
5.0 Conchoidal fracture Pale green, brown, blue, white, purple
Luster vitreous to sub-vitreous; habit prismatic; streak white
Apatite
4.0-7.0 Excellent, two directions (prismatic)
Blue, light gray, white, pale green
Luster vitreous to pearly ; habit bladed; streak white; hardness of 4-5 along the length of the blade 6-7 across the blade
Kyanite
4.0 Excellent, four directions (octahedral)
Colorless, green, purple, Blue, gray, yellow
Luster vitreous; habit equant; streak white; fluoresces in ultra violet light
Fluorite
3.5-4.0 Excellent, three directions (rhombohedral)
White, grey, pink, light tan Luster vitreous to pearly; Habit equant, slightly curves faces; streak white; slight reaction to dilute acid (HCl) more so if powdered
Dolomite
3.0-3.5 Good, two direction (prismatic)
White to off-white with brown, blue or black streaks
Luster vitreous to pearly to greasy; habit tabular; heavy
Barite
3.0 Excellent, three directions (rhombohedral)
Colorless, white, yellow, green, brown, pink
Luster vitreous to pearly; habit equant; streak white; reacts to dilute acid (HCl)
Calcite
2.5 Excellent, three directions (cubic)
Colorless, white, yellow, blue, brown, pink
Luster glassy to resinous; habit equant; steak white; salty taste
Halite
2.0-2.5 Excellent, one direction (basal)
Colorless to silvery white Luster vitreous to pearly; habit micaceous; streaks white; occurs in books of many sheets; books look more metallic while single sheets are colorless
Muscovite Mica
2.0 Perfect, one direction (basal), with two poor
Colorless, many pastel colors
Luster vitreous to pearly to satiny; habit tabular (sometime forms rosettes) streak white
Gypsum
1.5-2.5 Fracture Yellow Luster resinous; streak yellow; rotten egg smell, stronger when scratched
sulfur
1.0 Perfect, one direction (basal)
White to green Luster pearly, greasy, resinous; habit tabular; streak white; very soft (scratches easily with fingernail)
Talc
PHYSICAL GEOLOGY Minerals
Chapter 3 5
Nonmetallic Lusters (Dark Colored) Hardness Cleavage or
Fracture Color Other Properties Mineral Name
9.0 Parting and conchoidal fracture
Red (ruby), Blue (sapphire)
Luster vitreous; habit prismatic; streak none Corundum
7.0-7.5 Fracture Black, green, gradation from green to pink
Luster vitreous; habit prismatic; streak white; striations along the length
Tourmaline
7.0 Conchoidal fracture Green (forsterite), Orange (fayalite)
Luster vitreous; granular massive; streak white to pale green
Olivine
7.0 Conchoidal fracture Gray, brown (smoky quartz), purple (amethyst)
Luster vitreous; greasy; prismatic and pyramidal, amorphous; streak white
Quartz
7.0 Fracture Red, brown, orange, pale green, off-white
Luster vitreous; habit equant; streak white; variable crystal faces 12, 24, 36
Garnet
6.0 Excellent, two directions (prismatic) nearly 90°
Dark gray, blue, black Luster vitreous; habit prismatic; streak white; striations on cleavage plains
Plagioclase feldspar
6.0 Excellent, two directions (prismatic) nearly 90°
Translucent, brown, gray, green, red
Luster vitreous; habit prismatic; streak white; exsolution lamellae
Orthoclase feldspar (potassium feldspar)
5.5-6.0 Excellent, two directions (prismatic) ~56° and ~124°
Black, dark green Luster vitreous, pearly, dull; habit prismatic; streak white, greenish brown
Hornblende (amphibole)
5.5-6.0 Excellent, two directions (prismatic) nearly 90°
Black, dark green Luster vitreous, resinous, dull; habit columnar; streak greenish gray
Augite (pyroxene)
5.0 Conchoidal fracture Pale green, brown, blue, white, purple
Luster vitreous to sub-vitreous; habit prismatic; streak white
Apatite
4.0 Excellent, four directions (octahedral)
Colorless, green, purple, Blue, gray, yellow
Luster vitreous; habit equant; streak white; fluoresces in ultra violet light
Fluorite
3.5-4.0 Perfect, six direction (dodecahedral); often too small to see
Silvery yellow-brown silvery red; tarnishes Black
Luster resinous, greasy; Habit equant; streak yellow; rotten egg smell when scratched or powdered; also metallic luster
Sphalerite
3.5-4.0 Perfect, one direction (basal), poor one
Azure blue, blue, light blue, dark blue,
Luster vitreous, adamantine; habit tabular; streak pale blue
Azurite
3.5-4.0 Perfect, one direction (basal), fair one
Light green, green, dark green
Luster vitreous, satiny; habit columnar; streak light green; may form in mammillary masses
Malachite
2.5-3.0 Perfect, one direction (basal)
Brown, green brown, black
Luster vitreous, resinous, pearly; habit micaceous; streak gray; transparent brown as single sheets
Biotite
2.0-3.0 excellent, one direction (basal)
Light green, dark green, yellow, white
Luster vitreous, dull; habit tabular; streak white Chlorite
1.5-5.0 Parting Brick red to reddish grey Luster dull-earthy; may be weakly attracted to magnets; sometimes oolitic (little spheres); Also metallic luster
Hematite
1.0-5.5 Perfect, one direction (basal); too small to see
Yellowish brown to red- brown
Luster dull-earthy; Habit acicular (radiating needle like) often microscopic; also metallic
Goethite
1.0-3.0 Fracture Gray Luster Dull Earthy; habit spherical grains; streak white ,pale brown, gray; small pea sized orbs
Bauxite
1.0 Perfect, one direction (basal)
White to green Luster pearly, greasy, resinous; habit tabular; streak white; very soft (scratches easily with fingernail)
Talc
PHYSICAL GEOLOGY Minerals
Chapter 3 6
Sample Hardness Cleavage Luster Color Other Properties Mineral Name
Streak Color
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
PHYSICAL GEOLOGY Minerals
Chapter 3 7
Sample Hardness Cleavage Luster Color Other Properties Mineral Name
Streak Color
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32