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Geotechnical Engineering

2. Minerals

(Reference to notes: www.geology.csupomona.edu

Textbook: A Geology for Engineers by Blyth & Freitas)

https://www.youtube.com/watch?v=ZkHp_nnU9DY&index=5&list=PLrP7jh5S3OYo0kJYw_KgLtujuTSyiZ8Nv

YouTube video – Minerals & Rocks:

https://www.youtube.com/watch?v=QPKkRRdQLFo

Contents

1. Minerals

2. Mineralogy

2.1 Chemical mineralogy

2.3 Crystallography mineralogy

2.2 Physical mineralogy

2.2.1 Colour

2.2.2 Streak

2.2.3 Lustre

2.2.4 Hardness

2.2.5 Cleavage

2.2.6 Fracture

2.2.7 Tenacity

2.2.8 Specific gravity

2.2.9 Crystal shapes – forms

2.4 Other properties

3. Rock forming minerals

4. Identification of unknown minerals

1. Minerals

A mineral is a naturally occurring, inorganic (non-living) substance.
Each mineral has a particular chemical composition which can be expressed as a chemical formula.
Although most minerals are chemical compounds, a small number (e.g. sulfur, copper and gold) are elements.
However, the majority of minerals are compounds comprising two or more elements, for example NaCI which comprises sodium and chlorine, forming sodium chloride or Halite - Salt
Minerals are usually of uniform composition and minerals form the building blocks which make up rocks.
Within the rock mass, however, the proportion of minerals will vary from one sample to another.
Some rocks comprise mainly one mineral type but the majority are a mixture of a number of different minerals.
Mineralogy is that branch of geology which is concerned with the study of minerals.

2. Mineralogy

Mineralogy is the identification of minerals and the study of their properties, origin, and classification.

Approximately 3,000 mineral exist in nature.

The most important properties for identifying minerals and distinguishing them from one another are:

Chemical mineralogy
Crystallography mineralogy
Physical mineralogy

The crystalline form of individual crystal which make up minerals is related to the atomic structure of the crystals.

A very simple example of how the atomic structure influences the crystal shape is Sodium chloride NaCl (Salt)

This model show the atomic structure of salt - the balls represent the positions of the atoms within salt crystal

Note how the structural model closely resembles a real salt crystal – both look like cubes due to the packing of the atoms

2.1 Chemical mineralogy

If mineral crystals grow freely form a liquid state, they assume their own characteristic shape, with the angles between adjacent crystal faces constant for similar crystals of a particular mineral.

The faces are referenced to crystallographic axes

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2.2 Crystallography mineralogy

When minerals form, they can grow without interference from neighboring grains.
They commonly develop into regular geometric shapes, called crystals

Crystallographic axes intersect at a common origin and they form the framework on which the faces of the crystal are erected.
Crystal systems are:

Isomertic or Cubic (http://www.mindat.org/system_search.php?c=isometric):

Three axes are all equal in length and intersect at right angles (90 degrees) to each other (a1 = a2 = a3). Examples of minerals are halite, magnetite and garnet. Minerals of this system tend to produce crystals of equidimensional.

Triclinic (http://www.mindat.org/system_search.php?c=Triclinic):

The three axes are all unequal in length and intersect at three different angles (any angle but 90 degrees). Note: If any two axes crossed at 90 degrees, then we would be describing a monoclinic crystal! Examples include: plagioclase and axinite.

2.2 Physical mineralogy

The physical properties of minerals are used to identify and characterise them.
Most of the physical properties can be recognised at sight or determined by simple tests.
Colour
Streak
Lustre
Hardness
Cleavage
Fracture
Tenacity
Specific gravity
Crystal shape - forms

Not all of these properties are normally required to identify any one mineral; 2 or 3 of them taken together may be sufficient

Many other minerals show a variety of colours depending on the extent to which the base colour has been affected by trace substances.
A typical example of this is fluorite which can be brown, purple, pink, green or blue when it contains impurities.

2.2.1 Colour

Some minerals have a distinctive colour which clearly aids identification.
For example, sulphur is bright yellow.

Minerals of one colour are said to be idiochromatic whereas minerals of variable colour are allochromatic.

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Streak is usually produced by rubbing the mineral on a piece of unglazed porcelain or on a streak plate.

2.2.2 Streak

Streak is useful in clearly identifying some minerals but is not a useful diagnostic property of the many silicate minerals which are often too hard to produce streak.

A mineral in a powdered condition has a much more reliable colour than its colour in the mass.

Black – Graphite

Black – Pryite

Black – Magnetite

Black – Chalcopyrite

Grey – Galena

Limonite – Yellow-brown

Hematite – Red-brown

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2.2.3 Lustre

Lustre is the way its surface reflects light.

Non-metallic is further described as:
Vitreous – glassy – as in Obsidian
Adamantine - Sparkly such as diamonds
Resinous - Lustre of resin such as an opal
Pearly - As a pearl, talc being an example
Silky - Used to describe a fibrous structure
Greasy - Lustre produced by a mineral with many tiny irregularities on the surface
Earthy - No lustre, also called dull

It is subdivided into:
Metallic – as in Pyrite

Sub-metallic as shown by semi-opaque oxides such as haematite
Non-metallic as shown by transparent minerals – as in Kaolinite

Ref: www.geology.csupomona.edu

Metallic Lustre - Pyrite

Non-metallic Lustre - Kaolinite

Glassy Lustre - Obsidian

2.2.4 Hardness

This is a measure of resistance to abrasion or scratching.
It is measured relative to a reference set often minerals which is known as Mohs’ Scale of Hardness:

Hardness is tested by scratching the minerals of Mohs’ Scale with the specimen.
Hence, a mineral which scratches 4 Fluorite but not 5 Apatite, has a hardness H = 4-5.

This is a simple useful test and an example of its use is in distinguishing between Calcite (H=3) and Quartz (H=7).

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MINERAL	HARDNESS	COMMON TESTS
Talc	1	Scratched by fingernail
Gypsum	2
Calcite	3	Scratched by copper coin
Fluorite	4	Steel Nail
Apatite	5
Feldspar	6	Scratches a knife blade or window glass
Quartz	7
Topaz	8
Corundum	9
Diamond	10

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2.2.5 Cleavage

Many minerals will break easily in certain directions.

This feature is known as cleavage and it is dependent upon the atomic structure of the mineral.

Cleavage can occur in one or more directions.

The split will usually leave a smooth, plane surface called the cleavage plane.

Mineral cleavage is described as perfect, good, distinct or imperfect.

NaCl (salt)

Weak layers of bonds between atoms = cleavage planes

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2.2.6 Fracture

Fracture refers to the irregular, broken surface of a mineral.

Types of fracture are:

Cleavage – weak bonds

Fracture – strong bonds

Conchoidal - Breaks with a convex or concave surface

Even - Nearly flat

Uneven - rough

Rough – Small, shape, irregularities on the surface.

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2.2.7 Tenacity

This describes the response of a mineral to hitting, cutting or bending.
Terms used to describe tenacity are:
Malleable: e.g. gold and copper.
Brittle: Fracture when hit with a hammer
Sectile: Can he cut with a knife
Flexible and elastic: Mica bends and returns to a flat tabular state when left
Flexible and inelastic – Gypsum flakes are flexible but do not regain their original shape

2.2.8 Specific Gravity

Specific gravity is the ratio of the mass of a body of the solid to that of an equal volume of water.
Most minerals and rocks have a specific gravity of between 2 and 5
but the Specific Gravity can range from about one to more than 20.
Specific Gravity can be quite useful as some minerals feel unusually light or heavy.

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Specific gravity of common minerals
Halite	2.16	Muscovite	2.8–3.0	Rutile	4.2
Glauconite	2.3	Apatite	3.2	Zircon	4.7
Gypsum	2.32	Hornblende	3.2	Haematite	4.72
Feldspar	2.56–2.7	Tourmaline	3.0–3.2	Ilmenite	4.8
Clays	2.5–2.8	Sphene	3.5	Pyrite	5.01
Quartz	2.65	Topaz	3.6	Monazite	5.2
Calcite	2.71	Kyanite	3.6	Magnetite	5.2
Dolomite	2.85	Staurolite	3.7	Cassiterite	6.9
Chlorite	2.6–3.3	Garnet	3.7–4.3

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2.2.9 Crystal Shapes - Forms

Dendritic - Moss or tree like forms

Botryoidal - Spheroidal aggregations (resembling a bunch of grapes)

Reniform - Kidney shaped

Tabular – Based flat surfaces

Concretionary or nodular - Detached masses of spherical, ellipsoidal or irregular shape

Acicular - Fine needle like crystals, also known as filoform.

Terms used to describe clusters of minerals is ‘form’:

Drusv - closely packed small crystals growing into a cavity

Radiated - Needle like crystals radiating from a central point

Fibrous - Thread like strands

Granular - In coarse or fine grains

Reticulated - A mesh of crossed crystals

Typical commonly occurring crystal shapes are:

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2.4 Other Properties (CAUTION)

Magnetism – a few minerals are attracted to a magnet or are themselves capable of acting as magnets (e.g. magnetite)
Feel – e.g. talc and graphite feel greasy or slippery when you rub your fingers over them – the pressure of your finger is sufficient break bonds and to slide planes of atoms past each other.
Taste – e.g. salt, however some minerals can are poisonous!

Reaction with Dilute Hydrochloric Acid –
Minerals containing the carbonate anion (CO3)2- effervesce (fizz) when a drop of dilute hydrochloric acid is placed on them.
Carbon dioxide is released from the mineral & bubbles out through the acid – creating a fizz

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3. Rock Forming Minerals

The minerals within a rock sample are described as:
Essential constituents whose presence is implied by the rock name
Accessory which are present in smaller quantities than the essential minerals.
Secondary minerals are those present in the rock which have resulted from the decomposition of previous minerals.

Minerals in hand specimens of rock are usually observed and identified using a hand lens.
The characteristics most often used in identifying minerals in rocks are:
General shape of grains
Colour / transparency
Cleavage
Twinning (two crystals that have grown together)
Hardness

Minerals combine with each other to form rocks
e.g. Granite (rock) = feldspar + quartz + mica + amphibole (minerals in varying ratio)

4. Identification of Unknown Minerals
www.geology.csupomona.edu/alert/mineral/id1.htm

Step 1 – Lustre (metallic or non-metallic)

Sheen of metal

Glassy (allow light to pass), dull or waxy

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Step 2 – Hardness

Lustre = metallic

Fingernail (hardness = 1-2)

Steel Nail (Hardness = 5)

Knife blade (Hardness = >5)

Lustre = metallic; Hardness >5

Step 3 – Streak test

Black

Red-brown

Lustre = metallic; Hardness >5; Streak = Red-brown

Step 3 – Cleavage

= no apparent cleavage

Hematite Fe2O3

Identification of Minerals

Step 2 – Hardness

Metallic (Step 1 – Lustre )

Fingernail (hardness 1-2)

Steel Nail (hardness 5)

Knife blade (hardness >5)

GRAPHITE

One direction

3 direction 900

GALENA

6 directions

SPHALERITE

Magnetic

Non-magnetic

No Cleavage

LIMONITE

CHALCOPYRITE

GOLD

HEMATITE

Step 4 – Cleavage

Step 3 – Streak

No Cleavage

MAGNETITE

PYRITE

Black

Yellow

Black

Yellow-Brown

Red-Brown

3 directions 900

GALENA

Nonmetallic (Step 1 – Lustre )

Dull, Vitreous, Adamantine,Resinous, Pearly, Silky, Greasy & Earthy

Step 2 – Hardness

Fingernail (hardness 1-2)

Steel Nail (hardness 5)

Knife blade (hardness >5)

Step 4 – Cleavage

TALC

No Cleavage (earthy smell)

Step 4 – Cleavage

Step 3 – Streak

Yellow

Green Brown

White

Small flakes slippery feel

Small green flakes

Larger, silvery or transparent flakes

Colourless to chalky white, poor cleavage (blocky)

CHLORITE

MOSCOVITE

GYPSUM

KAOLINITE

No Cleavage

SULFUR

CALCITE

White

Yellow Brown

3 directions not 900

Fizzes vigorously in hydrochloric acid

Only slight fizzes in hydrochloric acid - fizzes more when mineral is crushed into a power

BIOTITE

DOLOMITE

3 directions 900

HALITE

4 directions

FLUORITE

No Cleavage

LIMONITE

White colourless

Red Brown

Pale Green - Colourless

2 directions 900

PLAGIOCLASE FELDPAR

ORTHOCLASE FELDPAR

No Cleavage

QUARTZ

GARNET

No Cleavage

HEMATITE

One direction

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http://geology.com/minerals/mineral-identification.shtml

FRACTURE	STREAK	COLOR	HARDNESS	FRACTURE	LUSTER	DIAPHANEITY	OTHER	SPECIFIC	MINERAL
CLEAVAGE				CLEAVAGE			PROPERTIES	GRAVITY	NAME
	yellow	yellow,		one			silky,
	or	brown,	5 - 5.5	direction	submetallic	translucent	fibrous	3.3 -4.3	GOETHITE
	brown	black		indistinct			appearance
	white,	white, red		perfect			brittle,
	yellow,	yellow, brown,	3.5 - 4	cleavage in	submetallic	translucent	looks like	3.9 - 4.1	SPHALERITE
	or brown	green, black		6 directions			resin
CLEAVAGE		dark green,		perfect			thin flakes,
	colorless	dark brown,	2.5 - 3	cleavage in	submetallic	translucent	tough,	2.8 - 3.2	BIOTITE
		or black		one direction			flexible
		black,		cleavage	metallic		marks paper,
	black	silver,	1 - 2	sometimes	or	opaque	soils fingers,	2.23	GRAPHITE
		or gray		indistinct	submetallic		slippery
				conchoidal			sometimes
	black	brassy yellow	6 - 6.5	fracture	metallic	opaque	in crystal	5.02	PYRITE
							shapes
		red -brown,			metallic		sometimes
	reddish	black,	5 - 6.5	fracture	or	opaque	oolitic or	5.56	HEMATITE
		silver			submetallic		magnetic
		black			metallic		strongly
	black	or	6	fracture	or	opaque	magnetic	5.18	MAGNETITE
		silver			submetallic
FRACTURE
	black	brownish	4	fracture	metallic	opaque	weakly magnetic	4.58 - 4.65	PYRRHOTITE

	greenish
	black	brassy yellow	3.5 - 4	fracture	metallic	opaque	brittle	4.1 - 4.3	CHALCOPYRITE

		brassy with		indistinct			iridescent
	black	iridescent	3	cleavage	metallic	opaque	peacock	5.0 -5.1	BORNITE
		colors					colors

Isomertic or Cubic (http://www.mindat.org/system_search.php?c=isometric):

Three axes are all equal in length and intersect at right angles (90

degrees) to each other (a1 = a2 = a3). Examples of minerals are

halite, magnetite and garnet. Minerals of this system tend to

produce crystals of equidimensional.

Triclinic (http://www.mindat.org/system_search.php?c=Triclinic):

The three axes are all unequal in length and intersect at three

different angles (any angle but 90 degrees). Note: If any two axes

crossed at 90 degrees, then we would be describing a monoclinic

crystal! Examples include: plagioclase and axinite.

MINERAL HARDNESS COMMON TESTS Talc 1 Gypsum 2 Calcite 3 Scratched by copper coin Fluorite 4 Apatite 5 Feldspar 6 Quartz 7 Topaz 8 Corundum 9 Diamond 10

Scratched by fingernail

Steel Nail

Scratches a knife blade or window glass

MINERALHARDNESS COMMON TESTS

Talc 1

Gypsum 2

Calcite 3Scratched by copper coin

Fluorite 4

Apatite 5

Feldspar6

Quartz 7

Topaz 8

Corundum9

Diamond10

Scratched by fingernail

Steel Nail

Scratches a knife blade or window glass

Halite 2.16 Muscovite 2.8–3.0 Rutile 4.2 Glauconite 2.3 Apatite 3.2 Zircon 4.7 Gypsum 2.32 Hornblende 3.2 Haematite 4.72 Feldspar 2.56–2.7 Tourmaline 3.0–3.2 Ilmenite 4.8 Clays 2.5–2.8 Sphene 3.5 Pyrite 5.01 Quartz 2.65 Topaz 3.6 Monazite 5.2 Calcite 2.71 Kyanite 3.6 Magnetite 5.2 Dolomite 2.85 Staurolite 3.7 Cassiterite 6.9 Chlorite 2.6–3.3 Garnet 3.7–4.3

Specific gravity of common minerals

Halite2.16Muscovite2.8–3.0Rutile4.2

Glauconite2.3Apatite3.2Zircon4.7

Gypsum2.32Hornblende3.2Haematite4.72

Feldspar2.56–2.7Tourmaline3.0–3.2Ilmenite4.8

Clays2.5–2.8Sphene3.5Pyrite5.01

Quartz2.65Topaz3.6Monazite5.2

Calcite2.71Kyanite3.6Magnetite5.2

Dolomite2.85Staurolite3.7Cassiterite6.9

Chlorite2.6–3.3Garnet3.7–4.3

Specific gravity of common minerals