What Are Minerals?

 

figure(1) minerals

Earth Crust:

          Earth crust is the outermost thin shell (layer) of the earth. It varies in thickness, being thin under open seas and oceans (8 to 12 km) consists of basaltic rocks which is called Sima (silica and magnesium) and about 60 km under continents consists of granitic rocks which is called Sial (silica and aluminium). The crust’s rocks are igneous, sedimentary, and metamorphic.

What is rock?

Rocks are naturally occurring collections or aggregates of mineral grains 

The branch of geology that studies is called Petrology.

Some rocks consist of only one mineral. For example: limestone composed of mineral called calcite (CaCO3). However, most rocks contain several different kinds of minerals. For Example Granite consists mostly of large crystals of feldspar, quartz, and mica,. Sometimes you can see the mineral grains, and other times they are too small to see without being magnified. Basalt consists mostly of tiny crystals of feldspar and pyroxene.

ores

Rocks that contain valuable minerals are called ores. To remove the valuable minerals, the ore first must be mined. Two methods of mining include surface mining and underground mining. After being mined, the valuable mineral is then separated from the rest of the ore. This is done through processes like crushing, sieving, melting, or settling through a liquid. Most metals and many important nonmetals are refined from

ores. Valuable minerals are not found evenly in Earth’s crust. Finding new deposits is an important first step in mining. The mining company needs to understand the properties of the minerals and the ores that contain them.

In general, a rock consists of a group of minerals that have some common properties.

For example, the minerals of an igneous rock have crystallized from the magma at a

relatively small range of temperature and pressure. Granite composed of quartz, feldspar

and mica and they are all have common properties Also, minerals of a detritus sedimentary rock, transported and deposited by a river are within a limited range of grain size and its specific gravity. The best example is the cultivated land of the Nile Valley, formed of silt and clay representing the flood plain deposits of the Nile.

 

Minerals and Human History

Minerals have been important to humans for a long time.

-Early humans used red hematite and black manganese oxide to make cave paintings.

People in the Stone Age made tools out of hard, fine-grained rocks.

 In the Bronze Age, people discovered how to combine copper and tin from minerals into a metallic mixture (alloy) called bronze. Later, in the Iron Age, people made tools of iron. Iron is contained in minerals like hematite (Fe2O3) and magnetite (Fe3O4). Fe is the chemical symbol for iron.

Today, minerals are used in thousands of ways.

Feldspar is used to make porcelain.

Calcite is used to make cement.

Iron and manganese, together with small amounts of several other metals, make steel that is used to make buildings, trains, cars, and many other things.

 Gypsum is use to make plaster and wallboard.

These are just a few examples of how minerals are used in your daily lives.

Mineral definition

Mineral is the building unit of rocks

To be considered a mineral, a material must meet five criteria.

• Minerals are solid, not gas or liquid.

• Minerals are inorganic. That means they are not alive and never have been.

• Minerals occur naturally. They are not manufactured

• Minerals have definite chemical compositions. These can be expressed as chemical formulas, such as SiO2, Ag, or Fe2O3.

• Minerals have a regular three-dimensional arrangement of atoms. This is called a crystal structure.

-coal and oil are not minerals because coal is organic origin and has not special crystal form and increases oil in addition to the previous, it is liquid and has no definite chemical structure but oil adding to the previous it is liquid and has not definite chemical structure.

The Chemistry and Structure of Minerals

Minerals can be very different from one another. The reason for this is that all minerals have a specific chemical makeup. Minerals consist of atoms of one or more chemical elements.

Atoms are the smallest unit of a chemical element that has all the element’s properties. Each chemical element has different chemical and physical properties. The atoms in minerals occur in a regular three-dimensional arrangement. called crystals the atoms in almost all minerals are in the form of ions. Ions are atoms that have an electric charge. This charge is due to electrons being added to or removed from the atom. Electrons are particles that orbit around the nucleus of the atom. They have a negative charge. The ions in a mineral are packed together in a way that brings the ions as close together as possible. As a result, positively charged ions are in close contact with negatively charged ions. Objects with unlike electric charges are attracted to each other. These forces of attraction hold the mineral together as a solid.

Types of minerals:

Some minerals are formed of one element are called native-element minerals for example, gold, Sulphur , graphite, A good example is gold (Au),

Copper (Cu), iron (Fe), and silver (Ag) are other native elements

Graphite and diamond are formed of the same element but they have different crystal shape .

 However, most minerals are formed of two or more elements which unite to from a stable compound according to chemical bonds, laws.  for example. Quartz which consists of silicon dioxide and calcite consists of calcium carbonate.

 

Minerals groups:

       Mineralogists could define more than two thousands of minerals, most of them found in little amount in nature. If we count common minerals and that which have economic value, we will find they do not exceed two hundreds minerals. While the minerals which form the rocks of Earth’s crust , do not exceed tens and divided to few mineral groups, the most famous groups are silicates then carbonates after that the economic minerals as oxides, sulphides , sulphate, and native elements minerals…

 

            figure( 2 )Minerals groups

Crystal Shape:

When minerals grow in unconfined spaces, they usually have a regular crystal shape.

How the mineral internal structure formed?

        The chemical composition of a mineral is reflected internally in a regular, repeating arrangement of atoms, called the crystal structure of the mineral. Crystal is a regular three-dimensional arrangement of atoms in a repeated pattern,  For example, the crystal form of halite (sodium chloride) known as rock salt which is consists of combination between positive sodium ions with negative chlorine ions in a repeated pattern produces characterized crystal for halite in form of cubic shape Minerals have different crystal shapes, for example Quartz crystals grow as six-sided (hexagonal) columns with pointed tops.  Garnets often grow in regular twelve-sided shapes. These shapes are called dodecahedra.

figure( 3)Crystal of Halite (sodium chloride) 

Identifying Minerals

         One of the most important duties of mineralogist is to identify minerals in the places of

their existence in the field. He firstly uses the phaneritic properties and that make it easier to be observed in hand specimen to initially identify the mineral and then confirmed by laboratory to identify ways that require devices and complex analyzes.which require special, expensive equipment. These include specific gravity, crystal shape, electrical conductivity, and reaction to acid

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the most important factors in determining the properties of a mineral are Chemical composition and crystal structure including shape, density, hardness, and color. Fracture and Hardness characteristics can be easily determined in the field with a hammer and a magnifying lens, allowing for rapid identification of the mineral.to understand how The internal atomic structure determine minerals properties let’s compare between graphite and diamond  to explains why  they have different properties.

The graphite-diamond mineral pair is an example of the importance of crystal structure. These two very different minerals have the same chemical formula (C), however the crystal structure of the two minerals is very different. In graphite, carbon atoms are bonded together along a flat plane, these sheets of carbon are loosely held together by weak attractive forces. However, the attractive forces between sheets can be broken easily, allowing them to slide past one another. Thus, graphite is a soft, slippery mineral that is often used to lubricate machines When graphite is rubbed against another material, such as a piece of paper, it leaves a trail of small sheets that have broken free, so it is used in pencils.

figure (4) diamond& Graphite crystal form

 However, in diamond, every single carbon atom is bonded strongly to four surrounding carbon atoms in a 3-dimensional structure .This structure results in one of the hardest natural substances on the planet a property that contributes to its value. The structure of each one of these two minerals is crucial to determining their physical properties.

 

 Physical Properties of minerals:

 Here’s a quick discussion of the most important and distinctive physical properties, which can be classified into optical, cohesive, and magnetic properties ... etc.

Optical Properties of Minerals:

These properties depend on the interaction between the incident light on the mineral and that 

reflected from its surface. The most important optical properties are:

Luster

Luster describes the way a mineral reflects light. Luster is either metallic or nonmetallic.Minerals with metallic luster look like polished metal.Nonmetallic lusters are often described as glassy (or vitreous), waxy, pearly, earthy, or dull. Pyrite and galena have metallic luster. Quartz and calcite have a vitreous (glassy) to greasy luster. Feldspar has a pearly luster.

Color

 The color of a mineral depends of the wavelength of light waves reflected from its surface

Although Color is usually the first thing you notice about a mineral, it is the least reliable property in identifying a mineral. it may not be very useful for their identification because most of minerals change the color by changing its chemical composition or contain small portion of impurities may change the color of the same mineral.  Many minerals have different colors depending on what impurities are present. Many minerals have different colors depending on what impurities are present.  For examples are:

 Quartz

- Pure Quartz is usually transparent or colorless and known as “rock crystal”.

But Quartz can found in different colors :

- Rose quartz due to the presence of manganese impurities,

- Purple to violet quartz (amethyst) the color is due to impurities of iron oxides,

- Milky quartz is white like milk due to minute gas bubbles,

-Smoky quartz, its grey to black color due to breaking some bonds when

they are exposed to high energy radiations as from a radioactive source

 

  figure (5)Amethyst

 Sphalerite (zinc Sulphide) is yellowish transparent,

 which turns brownish color when small amounts of iron replace zinc in its atomic structure.

 

 

figure (6) Sphalerite

 Corundum (Al2O3) is sometimes

Rubies is tinted red by small amounts of chromium.

A Sapphire is the same mineral tinted blue by small amounts of titanium.

 

figure (7)Ruby

 However, it can be many other colors, depending on what impurities are present. Some minerals tarnish or change color when their surfaces are exposed to air.

 Many minerals have the same color as others. For example, Gold  and Pyrite. Pyrite is known as fool’s Gold. It has a metallic luster and a color like Gold. However, it has a lower specific gravity than Gold. It is also brittle (Gold is malleable) and leaves a black streak on a white porcelain tile (Gold has a Gold-colored streak).

On the other hand, certain minerals have a permanent and unchanging color “known as real or native color”. For example, the yellow color of Sulphur and the green color of malachite (hydrous Copper carbonate).

 figure (8) Malachite

Streak

Streak is the color of the powdered mineral. To determine the streak of a mineral, scratch it across a piece of unglazed porcelain tile (a streak plate). Many minerals have a distinctive streak color. Streak color may be different from the color of the mineral sample. For example,

-Hematite mineral has two colors dark grey and red while its streak is red,

-Pyrite, which is characterized by golden color, its streak is black

- Quartz which has different colors, its streak is white only.

 

 

figure (9)Streak

Transparency:

The amount of light able to be passed through a mineral determines its transparency

transparent minerals: Light can pass through.

translucent minerals partially let light pass through.

Opaque minerals – minerals in which light does not go through even in thin sections

 All minerals with a metallic luster are opaque.  Typically these have molecules with higher atomic density (which includes many ore minerals).  How light reflects from these minerals is used to identify them with a reflected light microscope.

 

Cohesive Properties of Minerals

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

 

Hardness 

Hardness is the resistance of a mineral to scratching.

Mineralogists use a relative scale of hardness. It is called the Mohs scale(developed by Friedrich Mohs) and is given below.

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Orthoclase

7. Quartz

8. Topaz

9. Corundum

10. Diamond

Each mineral in the scale scratches minerals earlier in the scale and is scratched by minerals later in the scale. Diamond (with a hardness of 10) is the hardest natural substance known. The mineral talc (with a hardness of 1) is one of the softest.

The way to test the hardness of an unknown mineral is to scratch it with a material of known hardness. If the mineral is scratched, it is not as hard. If the unknown mineral scratches the known material, then it is harder.

Here are the hardness, on the Mohs scale, of some common materials:

 

1.      fingernail: about 2.5

2.     copper coin  3.5

3.     iron nail: about 4.5

4.    knife blade: a little more than 5

5.     window glass, masonry nail: 5.5

6.    steel file: 6.5

 

It is easy to determine hardness in the geological fields or laboratory by using either common tools or  set of hardness Picks

 

Hardness Picks

An alternative to using the reference minerals for testing is a set of hardness picks.( alloy) These picks have sharp metal points that you can use for very accurate testing. The picks allow much more control, and their sharp points can be used to test small mineral grains in a rock. The sharp picks can be used easily and either produce a scratch if they are harder than the specimen being tested or leave behind a tiny streak of metal if they are softer. Examine the test site with a hand lens to see the results of your test.

figure ( 10)Hardness Picks

 

Fortunately, most minerals have hardness less than 6.5 which makes the determination of hardness relatively easy. As used hardness property to distinguish between natural and precious gemstone that are expensive and those industrially imitation ornamental stones of glass material or aluminum oxide. The most important gemstones and precious minerals, in addition to their attractive colors, they do not been scratched easily, therefore, most of them exceeding hardness of 7.5, while imitation ornamental stones are characterized by attractive colors but their hardness are often less than 6.

 

Cleavage

Many minerals have cleavage. This means that when they break, they tend to break along regularly oriented planes of weakness. Cleavage planes form along planes of weak atomic bonds in the mineral. For example,

- Mica splits easily into sheets because there are very weak atomic bonds between the layers of atoms in mica.

 -Galena and halite break in cubes because they have cleavage in three directions, all at right angles to one another.

-Feldspar has cleavage in two directions, at nearly right angles.

Some minerals, such as quartz, have no cleavage.

 

 

 

figure (11)  cleavage in minerals

Fracture

If the mineral contains no planes of weakness, it will break along random (irregular) directions called fracture. Fracture has many different shapes:

·         Conchoidal: smooth carved fracture. OBSIDIAN 

·         Fibrous and splintery: sharp pointed fibers (Asbestos, Serpentine),

·         Uneven or irregular: rough and irregular surfaces,

·         Even: more or less smooth surfaces, may resemble cleavage,

·         Hackly: jagged fractures with very sharp edges (Mat).

                                                        Figure (12 )Conchoidal fructure 

Specific Gravity

Specific gravity is a ratio of the weight of the mineral to the weight of an equal volume of water. Water has a specific gravity of 1. Galena is a mineral that contains lead. It has a specific gravity of about 7.5. That means it is about 7.5 times as dense as water. Most nonmetallic minerals have specific gravities less than 3. These include minerals such as quartz, feldspar, and calcite.

Malleability and ductility:

Metals have properties that make them useful in machinery. Most metals

are malleable. This means that under pressure they can be changed in

shape without breaking or ductile. This means that they can be stretched into wire A feature reflects the extent of ease or possibility of malleable and ductile a mineral to form a thinner sheet or wires such as gold, silver or copper.

 Also, metals are very useful in the production and distribution of power. Metals tend to be good conductors of electricity.   

On the other hand, the minerals are considered as brittle or breakable if they are fragmented by hammering 

Other Properties 

Some minerals have special properties. These make them easy to identify.They also make these minerals useful for specific purposes.

Some minerals have a chemical reaction to acids. Carbonate minerals are

an example. They fizz when a drop of weak hydrochloric acid is applied.

Acid breaks down the chemical bonds in the carbonate. The fizz is CO2

gas being released. Acid is a good test to identify the calcium carbonate

mineral calcite.

• A few minerals are radioactive. The atoms of radioactive materials are

not stable. They release subatomic particles and radiation as they decay.

Uranium minerals are radioactive. They can be detected with a Geiger

counter. (This an instrument that detects radiation.) 

Some minerals are magnetic. Magnetite is an important ore of iron. It is magnetic

Some minerals are fluorescent. They change ultraviolet light to other wavelengths. Fluorite is one example.

A few minerals are phosphorescent. They store light energy. Then they slowly release it. 

Thermal properties:such as the ability of mineral fusion to melt and its melting point(high or low).

Other properties: in addition to properties help identifying the minerals. The taste of  

mineral such as (salty in halite or bitter or etc ... ) and the touch and smell.