Physical Properties of Minerals

Minerals are the materials that make up the Earth

• Naturally occurring,
• Homogeneous solid,
• with a definite (not fixed) chemical composition,
• and a highly ordered atomic arrangement,
• usually formed by inorganic processes.
• About 5000 Known
• 200 Common
• 20 Rock-Forming

Minerals generally named on basis of Physical property (magnetic =magnetite) ,predominant element (Cr =Chromite ,Ba =Barite),Locality (Franklin ,New Jersey =Franklinite) and, Colour (Albus (L.white)=Albite)

Physical properties of minerals have distinguishing physical properties that in most cases can be used to determine the identity of the mineral.In this article, you will develop a  ystematic approach to using the physical properties of minerals as identifying tools.If you follow this approach you should be able to identify most of the common minerals, or at the least be able to narrow the possibilities to only a few. We will first discuss each of the physical properties that can be used, then develop a methodical approach to the identification of minerals using these physical properties.Physical properties of minerals are important and useful diagnostic parameters. They are used to identify minerals macroscopically

Isotropism

Minerals are grouped according to their physical properties, which may be direction dependent.

Anisotropic

In a single crystal, the physical and mechanical properties often differ with orientation. It can be seen from looking at our models of crystalline structure that atoms should be able to slip over one another or distort in relation to one another easier in some directions than others. When the properties of a material vary with different crystallographic orientations, the material is said to be anisotropic.

Isotropic

Alternately, when the properties of a material are the same in all directions, the material is said to be isotropic. For many polycrystalline materials the grain orientations are random before any working (deformation) of the material is done. Therefore, even if the individual grains are anisotropic, the property differences tend to average out and, overall, the material is isotropic. When a material is formed, the grains are usually distorted and elongated in one or more directions which makes the material anisotropic. Material forming will be discussed later but let’s continue discussing crystalline structure at the atomic level.

Polymorphism

Physical properties of minerals are directly related to their atomic structure, bonding forces and chemical composition. Bonding forces as electrical forces exist between the atoms and ions are related to the type of elements, and the distance between them in the crystalline structure. Thus, minerals having same chemical composition may show different crystal structure (as a function of changes in P & T or both). So, being crystallized in different Symmetry Systems they exhibit different physical properties, this is called polymorphism. These minerals are said to be polymorphous. They may be Dimorphic, Trimorphic or Polymorphic according to the number of mineral species present in their group.

Physical Properties of Minerals

Characters Depending upon….

I. Cohesion and Elasticity

II. Specific Gravity

III. Light

IV. Magnetism

V. Senses

VI. Crystal and Aggregates Habits

I. Cohesion and Elasticity

Cohesion: The force of attraction existing between molecules. It shows resistance to any external influence that tends to separate them, eg., breaking or scratching the surface of a solid mineral. Cohesion force is related to bonding force.

Elasticity: The force that tends to restore the molecules of a body into their original position from which they have been disturbed.

The result of cohesion and elasticity in a mineral appears as

• cleavage,
• parting,
• fracture,
• hardness
• tenacity

Cleavage

Tendency of a crystalline mineral to break in certain directions yielding more or less smooth planar surfaces.These planes of lowest bond energy have minimum value of cohesion. An amorphous body of course has no cleavage. Cleavage planes are usually // to the crystallographic planes. Exceptions: Cal, Flu.

1. Good, distinct, perfect,
2. Fair, indistinct, imperfect,
3. Poor, in traces, difficult.

Being related to the atomic structure of the mineral, cleavage may be in several directions and depending on the force of cohesion some of them may be more developed than the others. So they are classified according to their distinction and smoothness:

Parting

Obtained when the mineral is subjected to external force. The mineral breaks along planes of structural weakness. The weakness may result from pressure, twinning or exsolution. Composition planes of twinning and glide planes are usually the direction of easy parting. Parting resembles cleavage. However, unlike cleavage, parting may not be shown by all individuals of the mineral species. Parting is not continuous on crystals.

Fracture

If the mineral contains no planes of weakness, it will break along random directions called fracture

1. Conchoidal: smooth fracture (Qua,glass )
2. Fibrous and splintery: sharp pointed fibers (Asbestos, Serpentine),
3. Uneven or irregular: rough and irregular surfaces,
4. Even: more or less smooth surfaces, may resemble cleavage,
5. Hackly: jagged fractures with very sharp edges (Mat).

Hardness

The resistance that a smooth surtace of a mineral offers to scratching (H) This is an indirect measure of the bond strength in the mineral. Hardness is determined by scratching the mineral with a mineral or substance of known hardness. Moh’s relative scale of hardness exhibited by some common minerals were used to give a numerical result. These minerals are listed below, along with the hardness of some common objects. A series of 10 common minerals were chosen by Austrian mineralogist F. Mohs in 1824 as a scale.

Mohs scale of Hardness

Talc	1
Gypsum	2
Calcite	3
Fluorite	4
Apatite	5
Orthoclase	6
Quartz	7
Topaz	8
Corundum	9
Diamond	10

Hardness of other common Objcects

Fingernail	2.5
Copper penny	3
Glass	5.5

Tenacity

The resistance that a mineral offers to breaking, crushing, bending, cutting, drawing or tearing is its tenacity. It is mineral’s cohesiveness.

• Brittle: A mineral that breaks and powders easily (Sulfides,Carbonates, Silicates and Oxides)
• Malleable: A mineral that can be hammered out without breaking, into thin sheets. They are plastic (Native metals)
• Sectile: A mineral that can be cut with a knife into thin shavings (Native metals)
• Ductile: A mineral that can be drawn into wire (Native metals)
• Flexible: A mineral that bends but retains it bent form. Does not resume its original shape, permanent deformation (Asb, clay minerals, Chl, Tal)
• Elastic: A mineral that after bending springs back and resumes its original position. (Mus).

II. Specific Gravity

Specific gravity (SG) or relative density is a unitless number that expresses the ratio between the weight of a substance and the weight of an equal volume of water at 4degree (max ρ).
Density (p) is the weight of a substance per volume= g/cm3. It is different
than SG, and varies from one locality to another (max. at poles, min. at
equator).

III. Light

Diapheneity

Diapheneity is amount of light transmitted or absorbed by a solid.Diapheneity generally used strictly for hand specimens also most minerals opaque in hand specimens and transparent in thin sections

Transparent is pass the object behind it seen clearly also size of specimen (thicker specimens may become translucent)

Translucent is light transmitted but object not seen

Opaque is light wholly absorbed

Color

Color is sometimes an extremely diagnostic property of a mineral, for
example olivine and epidote are almost always green in color. But, for some
minerals it is not at all diagnosticbecause minerals can take on a variety of
colors. These minerals are said to be allochromatic.

For example quartz can be clear, white, black, pink, blue, or purple.

Streak

Streak is the color of the mineral in powdered form. Streak shows the true color of the mineral. In large solid form, trace minerals can change the color appearance of a mineral by reflecting the light in a certain way. Trace minerals have little influence on the reflection of the small powdery particles of the streak.

The streak of metallic minerals tends to appear dark because the small particles of the streak absorb the light hitting them. Non-metallic particles tend to reflect most of the light so they appear lighter in color or almost white.

Luster

Luster is the general appearance of a mineral surface in reflected light. It is the degree of reflected light and directly related to optical properties (mainly RI) and surface conditions.

Metallic luster

Metallic luster: strictly belongs to opaque minerals, where light is completely reflected from the surface. Most of the ore minerals having high content of metals shows metallic luster. Eg., Gal, Mat, Pyt, Cpy. Imperfect metallic luster, sub metallic.

Non-metallic luster

Non-metallic luster: other luster types are collectively known as non-metallic luster. It may be brilliant or faint where reflection is poor which is due to scattering of light from the mineral surface.

Related to RI

Adamantine is like luster of diamond
Vitreous is like luster of glass

Related to special mechanical and chemical properties of mineral surfaces

Resinous is like luster of resin
Greasy is like luster of thin layer of oil
Pearly is like luster of glass
Silky is like luster of a piece of satin
Earthy is little or no reflection from surface due to the porous and finegrained nature of mineral

Crystal Form and Habit

• regular geometric shape
• external expression of internal ordered atomic arrangement
• quality of external faces

1. euhedral
2. subhedral
3. anhedral

Magnetism

• Response to a magnetic field

diamagnetic – no attraction (repulsion)

paramagnetic – m weak attraction, not

ferromagnetic –  strong attraction, permanent