“PHYSICAL PROPERTIES OF MINERALS & MINERAL IDENTIFICATION”

INTRODUCTION:

Minerals are naturally occurring, usually inorganic, solids that possess a definite chemical composition and a specific, orderly arrangement of atoms. This lab will help you to develop the ability to identify common minerals found at the earth's surface. Although there are literally thousands of minerals, the small number of the most common rock forming, ore, and industrial minerals studied here constitute a large part of the earth's crust. Identification is accomplished by testing and observing the physical properties.

OBJECTIVES:

•   Recognize and describe the physical properties of minerals.
•  Identify minerals using physical properties.

ACTIVITIES FOCUSING ON PHYSICAL PROPERTIES:

Minerals exhibit certain diagnostic properties, called physical properties, which can be tested and observed, thereby leading to the correct identification of the mineral. Many (but not all) of these properties are unique to a given mineral. One of the keys to identifying minerals is observing a combination of physical properties displayed by a mineral. Therefore, you must be sure of the meaning of each of the physical properties. On the following  pages, you will define and study these properties: luster, color, streak, luster, heft, hardness, cleavage, fracture and crystal form, and others, such as, magnetism, taste, effervescence  in HCl.

Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms. The study of minerals is called mineralogy.

CRYSTAL STRUCTURE:

A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral.

Crystal structure greatly influences a mineral's physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals. This happens because the carbon atoms in graphite are arranged into sheets which can slide easily past each other, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.

There are currently more than 4,000 known minerals, according to the International Mineralogical Association, which is responsible for the approval of and naming of new mineral species found in nature. Of these, perhaps 100 can be called "common", 50 are "occasional", and the rest are "rare" to "extremely rare".

Commercially valuable minerals and rocks are referred to as industrial minerals. Rocks from which minerals are mined for economic purposes are referred to as ores (the rocks and minerals that remain, after the desired mineral has been separated from the ore, are referred to as tailings).

IDENTIFICATION OF MINERALS:

Common minerals can be identified readily, if their ordinary physical properties are known. These include such properties as colour, streak luster, hardness, cleavage fracture, form, specific gravity, tenacity, feel, fluorescence, phosphorescence, magnetism and crystal form.

PHYSICAL PROPERTIES OF MINERALS

1.      Colour

Colour in minerals is caused by the absorption, or lack of absorption, of various wavelengths of light.

Some minerals posses a characteristic colour e.g. the lead grey of galena, the black of magnetite, and green of chlorite. But in other case such as quartz, the colour is variable and cannot be relied on as a guide to identify minerals. The variations in the colour of a mineral may be due to

•          Isomorphous variations in the colour of a mineral
•          Minute colour inclusions, and
•         A small amount of some substance in solid solution

•   Copper, Cu, produces the azure blue colour of azurite,
•   Iron, Fe, produces the red colour of limonite,
•   Cobalt, Co, produces the violet-red colour in erythrite,

2.      Streak

Streak is the colour of mineral in powder form.

The minerals are rubbed against unglazed porcelain plate (called streak plate) to obtain the streak. In some cases the colour of the streak differs remarkably from the colour of the mineral. Streak is useful, for example, in distinguishing the various oxides of iron; hematite (Fe₂O₃) gives a red streak, limonite (hydrated Fe_2­O₃) a brown, and magnetite (Fe₃O_4­) a grey streak.

3.      Luster

Luster is a description of the way light interacts with the surface of a crystal.

Or It is the appearance of a mineral surface in reflected light.

It is described according to the degree of brightness from splendent to dull. It may be described as metallic, as in pyrite or galena; glassy or vitreous, as in quartz; resinous or greasy, as in opal; pearly, as in talc; or silky, as in fibrous minerals such as asbestos and satin-spar (fibrous gypsum). Minerals with no luster are described as dull.

4.      Crystal Habit or Form

The development of an individual crystal or an aggregate of crystals, to produce a particular external shape depends on the temperature and pressure during their formation. The shape or crystal habit is of use in identification of some minerals.

         Individual crystals

•   Acicular --- in fine needle-like crystals, e.g. schorl, natrolite
•   Platy --- broad, flat crystal e.g. wulfenite
•   Tabular --- elongate crystal which is also flat e.g. feldspar
•   Prismatic --- crystal is elongated in one direction e.g.  indicolite
•   Fibrous --- longcrystals-like fibers e.g. asbestos, okenite

        Crystal aggregates (amorphous minerals often assume this form)

•   Dendritic --- crystal diverge from each other like branches e.g. denritic deposits of manganese oxide
•   Raniform --- kidney-shaped e.g. kidney iron ore, a variety of hematite
•   Botryoidal --- like a bunch of grapes e.g. chalcedony
•   Amygdaloidal --- infilling of steam vesicles or holes in lavas by salts carried in solution

5.      Cleavage

Cleavage is the tendency of a mineral to split in certain regular directions. These directions depend on the arrangement of the atoms in a mineral are parallel to definite crystal faces.

 Perfect, good, distinct, and imperfect are terms used to describe the quality of mineral cleavage. Mica, for example, has a perfect cleavage; feldspars have two sets of good cleavage planes. Calcite has three directions of cleavage.

6.      Fracture

The nature of a broken surface of a mineral is known as fracture.

This breaking should be in any other direction than the cleavage. Unlike cleavage, fracture does not produce smooth planes. The various types of fractures are as follows:

•          Uneven fractures: when the broken surface is rough or irregular, e.g. apatite.
•          Even fractures: when the mineral breaks with a flat surface e.g. flint.
•         Conchoidal fractures: when the mineral breaks with curved surfaces often with concentric markings like a shell e.g. quartz.
•        Hackly fractures: when the broken surface has a small irregularities like broken metal, e.g. native copper.

7.      Hardness

Hardness is measure of the strength of the structure of the mineral relative to the strength of its chemical bonds.

 Hardness can be accessed through scratching. Hardness of a mineral is described as its resistance to the scratching. It is determined by comparison with the standard minerals of the Mohs Hardness scale which is given follow:

   

MOHS HARDNESS SCALE

1.       Talc
2.       Gypsum                             Fingernail at 2.5
3.       Calcite                               Copper coin at 3.5
4.       Fluorite
5.       Apatite                              Window glass or typical knife blade at 5.5 
6.       Orthoclase                          Streak plate or good steel file at over 6.5
7.       Quartz
8.       Topaz
9.       Corundum
10. .  Diamond

Moh’s hardness scale is only relative. An absolute hardness scale is given as:

Mineral Name                         Hardness

1. Talc Mg₃Si₄O₁₀(OH)₂                          1
2. Gypsum CaSO₄·2H₂O                          3
3. Calcite CaCO₃                                                   9
4. Fluorite CaF₂                                                   21
5. Apatite Ca₅(PO₄)₃(OH,Cl,F)               48
6. Orthoclase KAlSi₃O₈                                   72
7. Quartz SiO₂                                                      100
8. Topaz Al₂SiO₄(OH,F)₂                               200
9. Corundum Al₂O₃                                           400
10. Diamond C (pure carbon)                   1600

8.      Specific Gravity

Specific gravity is the weight of the mineral compared with the weight of an equal volume of water.

Thus a mineral with specific gravity 4 is four times as heavy as water. Minerals range from 1 to over 20 in sp. gravity, but most lie between 2 and 7. Sp. gravity of some common minerals found in sediments are given as:

Glauconite: 2.3            Feldspar: 2.56 – 2.7    Quartz: 2.65    Muscovite: 2.8 – 3.0              

Apatite: 3.2                 Hornblende: 3.2          Topaz: 3.6       Zircon: 4.7

Magnetite: 5.2             Cassiterite: 6.9

9.      Taste

Some of the more common minerals that have a significantly distinct taste:

•          Borax (sweet alkaline)
•          Chalcanthite (metallic & slightly poisonous)    
•          Epsomite (bitter)
•          Glauberite (bitter salty)
•          Halite (salty)
•          Hanksite (salty)

10.   Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), piezoelectricity and reactivity to dilute acids.