Minerals - GeoScience - Marden Senior Collegegeoscience.msc.sa.edu.au/library/1-1 Minerals.pdf ·...

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Handout 1 of 14

(Topic 1.1)

Minerals

A selection of some common rock-forming minerals (natural scale: http://en.wikipedia.org/wiki/Image:Minerals.jpg)

Minerals

Key Ideas Intended Student Learning Earth is made up of minerals that are the constituents of rocks.

State five criteria for categorising a substance as a mineral.

Name the seven most important mineral groups (silicates, oxides, carbonates, sulphides, sulphates, halides, and native elements).

Explain how minerals are classified into these groups according to their chemical composition.

Describe, for mineral specimens:

colour

streak

cleavage

hardness

lustre

density

magnetism

reaction to dilute hydrochloric acid. Identify the following common rock-forming minerals, and know the mineral group to which each one belongs:

Quartz Feldspar

Biotite mica Muscovite mica

Calcite Olivine

Augite Hornblende

Clay minerals - (e.g. kaolinite). Identify the following metallic ores, know the mineral group to which each one belongs, and name the extracted metal:

Galena Chalcopyrite

Malachite Sphalerite

Bauxite Haematite.

Magnetite Explain the difference between a mineral deposit and an ore deposit.

The sections of the Intended Student Learning that are italicised must form part of the fieldwork or practical materials submitted for moderation. They will not be examined in the public examination

Topic 1.1 Minerals of 18

1.1 - MINERALS

Earth is made up of minerals that are the constituents of rocks.

Rocks of Earth's crust are made up of minerals in the same way that a cake is made of flour, eggs, sultanas, sugar etc. Therefore, minerals can be considered as the constituents of rocks. DEFINITION OF A MINERAL (See p. 23, Essentials of Geology) A mineral is a substance that is

• naturally occurring

• solid

• inorganic – i.e. not the result of any processes of life

• crystalline – i.e. having a regular arrangement of atoms

• pure – either an element or a compound

(p. 125, Chapter 6c – Crystals, Perspectives of the Earth)

CLASSIFICATION OF MINERALS

Minerals are classified according to their chemical compositions.

The majority of the common minerals in Earth's crust can be classified into seven groups, according to their chemical composition. This classification is summarised in the table below:

Name of Group Example Chemical Composition

Carbonate Calcite CaCO3

Halide Halite NaCl

Oxide Haematite Fe2O3

Sulphide Galena PbS

Sulphate Gypsum CaSO4

Silicate Quartz SiO2

Native element Gold Au The largest group is the silicates. About 95% of Earth's crust comprises approximately 1100 silicate minerals. Most of the common rock-forming minerals belong to this group.

On the other hand, native elements are very rare because most elements combine with elements in the air and water to form compounds. Only six Topic 1.1 Minerals of 18

elements occur in Earth's crust as pure elements. One of these is gold which is always found as an element because it is chemically very stable. It does not react with other elements to form compounds. DETERMINATION OF PHYSICAL PROPERTIES

Mineral specimens are usually identified by determining their physical properties and using a key1 to name the mineral which has those properties. A test kit, consisting of hand lens, magnet, streak plate (unglazed tile), copper coin, knife, bottle of dilute hydrochloric acid (HCl) and quartz crystal, is used.

The diagnostic properties used in mineral identification are:

• Colour • Lustre • Density • Magnetism

• Streak • Hardness • Cleavage • Reaction with dilute HCl

In addition, specific tests are diagnostic for certain minerals. For example, halite tastes salty, and magnetite is magnetic. In some cases, a given mineral specimen may not possess all the properties listed in the key, and a diagnosis must be made using the best available information.

Colour:

Although the colour of some minerals, such as azurite, is quite distinctive, other minerals, such as quartz, occur in a variety of colours. Also there are many white minerals. Hence colour is frequently NOT a useful diagnostic property. Streak:

Streak is the colour of the powdered mineral. It is a useful diagnostic property for many coloured minerals — especially those with a metallic lustre. It is found by rubbing the specimen on a piece of unglazed tile, or streak plate.

Lustre:

The lustre of a mineral is the way its surface shines when held up to the light. Lustre is a property distinct from colour. There are many ways of classifying and describing lustre, but the following system is adequate:

Vitreous — the mineral shines like glass — e.g. quartz, diamond

Metallic — the mineral shines like the surface of a metal — e.g. pyrite, galena

Earthy (dull) – the mineral does not shine at all — e.g. kaolinite

Hardness:

The hardness of any mineral can be assigned a number between 1 and 10, on Moh's Scale of Hardness. The instruments used to determine the hardness of a

1 Diagnostic properties: Ref. p. 107, Perspectives of the Earth; Ref. p. 30, Essentials of Geology


mineral specimen are (in order of increasing hardness) a finger-nail, copper coin, knife blade and a quartz crystal.

The table below lists the minerals that define Moh's Scale of Hardness, and gives the relative hardnesses of the test items named above.

Hardness Defining Mineral Test Item 1 Talc 2 Gypsum 3 Calcite 4 Fluorite

Finger nail ~2.5

Copper coin ~3.5

5 Apatite 6 Orthoclase

Knife blade ~5.5

7 Quartz Quartz crystal 7 8 Topaz 9 Corundum 10 Diamond

Density:

It is not usual to measure the actual densities (relative to water = 1) of specimens; however, minerals should be classified according to whether they are light, medium or heavy. This can be done by holding similar-sized specimens of two different minerals in your hands, and comparing their weights. Cleavage:

When a piece of a mineral is dropped or struck, it may tend to break so that flat, shiny surfaces are formed. This effect is shown in the adjacent diagram. The cleavage of a mineral

• is the shape which some mineral specimens tend to form when they are broken.

• is described in terms of the number of flat surfaces formed and the angles between them.


Minerals, or individual surfaces, that do not cleave to form flat faces are said to show fracture. Cleavage is a diagnostic property for identification of minerals, but the cleavage of an actual specimen is not always easy to determine. Many specimens do not show the expected cleavage characteristics. If cleavage planes are not clearly visible, you should use your hand lens to carefully examine a broken specimen. Cleavage is described in terms of the number of cleavage planes formed and the angles between these planes. 1. One Cleavage Plane

Some minerals (e.g. the micas) cleave in one direction, forming flat surfaces in one plane only. This is why mica tends to break into thin sheets. However, if one of these sheets is broken, it will have a rough, jagged edge, or fracture rather than a smooth cleavage surface. 2. Two Cleavage Directions

Minerals of the feldspar group form cleavage planes in two directions which are at right angles to each other; however, if they are broken along other directions, only irregular fracture surfaces are formed.

Hornblende tends to form two pairs of cleavage planes that are at approximately 60° and 120° to each other. Again the remaining surfaces show only fracture. 3. Three Cleavage Planes

Some minerals cleave to form flat surfaces in three directions, which may be at right angles to each other (e.g. galena).


Other minerals form cleavage planes which are not at right angles to each other e.g. calcite).

al acture surfaces, are formed. There

h crystals and cleavage fragments reflect the internal structure of the ineral, it is not surprising that cleavage fragments are often mistaken for

nd haematite are weakly magnetic. Consequently, a magnet can only ttract small particles scraped off a piece of haematite, or ilmenite sand-sized

ic acid is placed on them. This is a useful diagnostic test for calcite, a hite mineral and, in many cases, not easily distinguished from other white inerals.

e common rock-forming minerals are the major constituents of

( Crystal Faces versus Cleavage Planes DO NOT CONFUSE Crystal Faces and Cleavage Planes: Quartz forms crystal faces but not cleavage planes. Quartz has no cleavage. If a quartz crystal is broken, irregular conchoidfrwill not be any flat cleavage planes. Well-developed crystals (e.g. quartz) only form if there is room for them to grow. Since botmcrystals. Magnetism:

Some minerals that contain iron are magnetic. Magnetite is strongly magnetic, and will be attracted by a magnet. Other iron-bearing minerals such as ilmenite aaparticles. Reaction to dilute Hydrochloric Acid:

Some minerals especially carbonates, effervesce when a drop of dilute hydrochlorwm COMMON ROCK-FORMING MINERALS

Although about 4000 minerals are known to exist, only about 8 of them are common. Thesigneous, sedimentary and metamorphic rocks. They constitute more that 99% of Earth's crust.

You must be able to identify these 8 minerals, and you must know the mineral group to which each one belongs.


The following table lists the common rock-forming minerals, lists the mineral group to which they belong, and summarises their most important diagnostic

roperties. Due to factors such as weathering or presence of impurities, indivi specim n e table. I ication be ade on the balance era ties.

N NOST ROP

pdual

dentifens may

mustot show all the properties listed in th

m of sev l proper

SIGNIFICA T DIAG IC P ERTIES Mineral Group

H Lustre Cleavage Other properties Colour

Quartz Silicate 7 riations.

Vitreous None Hexagonal crystals Clear when pure. Impurities cause many colour va

Feldspar group: e

Orthoclase: pink, cream ite, grey 2 at 0° Orthoclase

PlagioclasSilicate 6 Plagioclase: wh Vitreous ~9

Biotite Silicate 2.5 Black 1

Muscovite clear

appears are flexible

Silicate 2.5 White or

Vitreous: sometimes

metallic 1

Thin sheetsand elastic

Amphibole (e.g. Silicate 5.5 Black Vitreous 2 at 120° Hornblende) Often confused with Pyroxene

Olivine Silicate 6crystals.

.5 Green Vitreous none Small green Often enclosed in a basalt volcanic 'bomb'.

Pyroxene (e.g. us 2 at 93° with Augite) Silicate 5.5 Black Vitreo 87° & Often confused

Amphibole Carbonate 3 White or clear Vitreous 3 not at 90° Effervesces with acid Calcite

Clays (e.g. Kaolinite) Silicate 2.5 White Dull None Very powdery

ORE MINERALS

Most metals react with other elements in the atmosphere and hydrosphere (water), and therefore do not exist in nature as elements. Rather they are found s compounds, usually oxides, sulphides or carbonates of metals. Compounds of

erals (known as angue). Properties such as hardness, density and cleavage are often

he compositions and significant diagnostic properties of the ore minerals sted in the SSABSA curriculum statement, that you must be able to recognise re summarised in the table below.

ametals are known as ore minerals and can be smelted to extract the metal itself. Many of the ore mineral specimens used to teach geology consist of the mineral embedded in, or covering a piece of rock that contains other mingimpossible to determine in these specimens. However, colour, lustre and streak are usually sufficient for identification of common ore minerals Tlia


SIGNIFICANT DIAG CNOSTI PROPERTIES Mineral Composition H Colour Lustre Streak Other properties

Galena Lead sulphide (PbS) Lead Very high density, 3 2.5 Grey Metallic -grey cleavage planes at 90°

Chalcopyrite Greenish-gold or Gree

re'. Copper iron sulphide(CuFeS2)

3.5 many colours (iridescent)

Metallic nishblack

Iridescent specimens are known as 'peacock o

Malachite )2] s stic.

Copper carbonate [Cu2CO3(OH 3.5 Green Vitreou

or dull Green Green colour is diagno

Sphalerite Zinc sulphide (ZnS) 3.5 Brown/black Metallic Brown Dodecahedral cleavage planes of cleavage)

(6

Bauxite Mixture of aluminium hydroxides

recognised. 2 Brown Dull Brown

Consists of round no(i.e. pisolitic). Easily

dules

Haematite Iron oxide (Fe2O3) 6 Reddish brown to black

Usually dull

Reddish brown

Appearance of mineral varies. Streak is diagnostic.

Magnetite Iron oxide (Fe3O4) 6 Black Metallic Black Strongly magnetic.

Compounds of metals are found in all rocks (e.g. iron-bearing minerals in

concentrati

A concentration of mineral deposit.

granite). However, they cannot be economically extracted unless a natural rocess has concentrated large amounts of the same mineral (e.g. the p

on of ilmenite in palaeochannels or placer deposits).

any mineral by natural processes is called a

An ore body is a mineral deposit from which one or more ore minerals can be economically extracted.

Fa ors n be divided into two gr ps

g e e rock)

• extent of the ore body (i.e. total amount of ore present)

derground water

e

• distance of the deposit from potential markets

gy for processing the ore.

Continual advances in mining and ore processing techniques has meant that lower grade deposits can be profitably mined compared with (say) at the end of the nineteenth century.

ct that may affect the profitability of a deposit caou - geological and economic.

eological factors are thSome

• grade of the ore (i.e. percentage of ore in th

• depth of the ore body

• difficulty of removing un • presence of any folds or faults.

conomic factors are the Some

• current market price of the material extracted

• availability of ener Other factors to be considered include environmental considerations and Aboriginal perspectives.


EXERCISES DEFINITION OF A MINERAL

1. Explain why mineralogy, the study of minerals, is a very important topic in geology.

2. List five properties which a substance must possess if it is to be classified as a mineral.

3. In everyday life, substances such as coal and petroleum (oil) are often referred to as minerals. Give two reasons why petroleum and one reason why coal do not meet the above criteria.

Petroleum:

Coal:

CLASSIFICATION OF MINERALS

1. Name the seven groups into which the majority of the most common minerals may be classified, and name a common mineral in each group.

Group 1: Common mineral:







2. Name the largest mineral group, and state the proportion of Earth's crust which consists of minerals belonging to this group.


3. Explain why native elements are rare, and name one element which always occurs in this form.

DETERMINATION OF PHYSICAL PROPERTIES

1. Explain how you would proceed to identify a specimen that you knew to be a mineral.

2. List the items of test equipment that you would use.

3. List the diagnostic properties which you would determine.

4. Give two reasons why the colour of a mineral specimen is not often a useful diagnostic property.

5. a. What is the streak of a mineral specimen?

b. Explain how this property is determined.

c. For what type of minerals is streak a very useful diagnostic property?


6. a. What is the lustre of a mineral specimen?

b. Name and describe three terms which may be used to describe the lustre of a specimen.

7. How is the hardness of minerals described?

8. In the table below, give the hardness of each of the commonly used test items

TEST ITEM APPROXIMATE HARDNESS

Finger nail

Copper coin

Knife blade

Quartz crystal

9. Explain how you use the above test items to determine the hardness of a mineral specimen.

10. How do you estimate the approximate density of a mineral specimen?

11. How is the cleavage of a mineral specimen described?


12. Surfaces that are not flat and smooth are said to show

13. Explain the difference between the processes of formation of crystals and of cleavage fragments.

Crystals are formed when:

Cleavage fragments are formed when:

14. Explain why crystals of ice form on the inside walls of your freezer as it frosts up, but no ice crystals form when water freezes in an ice tray.

15. You have a black mineral which is attracted to a magnet.

That mineral is:

16. You have a white mineral which effervesces (fizzes) when a drop of dilute hydrochloric acid is placed on its surface.

That mineral is:


17. The first column of the table below gives the number of mineral cleavage planes and the angles between them, the second column shows these planes diagrammatically, and the third column shows the general appearance of mineral with each type of cleavage, and the fourth lists an example.

However, the information in each row does not correspond. By means of coloured lines, or otherwise, indicate which diagram, mineral specimen and mineral name correspond to each type of cleavage.

CLEAVAGE

PLANES DIAGRAM APPEARANCE OF MINERAL EXAMPLE

One

Feldspars

Two at 90°

Calcite

Two not at 90°

Galena

Three at 90°

Micas

Three not at 90°

Hornblende


COMMON ROCK-FORMING MINERALS

1. Collect mineral specimens, one at a time from the trays provided, together with a mineral test kit. Write the names of the mineral diagnostic properties in the top row of the table below.

Then determine these properties for each of your specimens.

Mineral

Quartz

Muscovite

Biotite

A feldspar

Hornblende

Olivine

Kaolinite

Calcite

Augite

ORE MINERALS 1. Explain why most metals do not exist in nature as elements, but in the

form of ores, which are compounds of the metals.

2. To which mineral groups do most metallic ores belong?


3. For each of the following ore minerals, name the mineral group and the metal obtained:

Mineral group Metal obtained

Galena

Chalcopyrite

Magnetite

Bauxite

Sphalerite

Malachite

Haematite

Remember these are the ores listed in the SSABSA curriculum statement — the ones you must recognise. 4. Explain the difference between a mineral deposit and an ore body.

5. Discuss the geological factors which would determine whether a deposit of an ore mineral might become an ore body.

6. What economic and other factors must also be taken into consideration?


7. The diagram below shows how the profitability of mining a particular ore body changed between 1900 and 1980.

a. What does the graph tell us about the change in profitability of

mining different grades of ore during the period indicated?

b. What was the lowest grade of ore which could be profitably mined in

1920?

c. What is the prediction for the lowest grade which could be profitably

mined in the year 2000? d. During which period did the grade of ore which could be profitably

mined, decrease at the greatest rate?

e. Suggest reasons for the decrease in the grade of ore which can be profitably mined.


8. Collect ore mineral specimens, one at a time from the trays provided, together with a mineral test kit. Write the names of the mineral diagnostic properties in the top row of the table below.

Then determine these properties for each of your specimens.

Ore mineral

Bauxite

Chalcopyrite

Galena

Haematite

Magnetite

Malachite

Sphalerite

9. How do you describe the cleavage of members of the feldspar family of

minerals?

10. What is the hardness of the micas and how many cleavage planes do they possess?

11. Describe the cleavage of hornblende:

12. Describe the cleavage of calcite:

13. What is the hardness of calcite?

14. Describe a test for calcium carbonate in any form i.e. calcite, limestone or marble.


Minerals - GeoScience - Marden Senior Collegegeoscience.msc.sa.edu.au/library/1-1 Minerals.pdf ·...

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