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?

________________________________________________________

________________________________

Now arrange them by another criterion. What was the criterion and

what was the order?

________________________________________________________

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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

PHYSICAL GEOLOGY Minerals

Chapter 3 7

Sample Hardness Cleavage Luster Color Other Properties Mineral Name

Streak Color

PHYSICAL GEOLOGY Minerals

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