Chemistry Lecture 27

MINERALS

A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not have a specific chemical composition. 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.

Differences between minerals and rocks

A mineral is a naturally occurring solid with a definite chemical composition and a specific crystalline structure. A rock is an aggregate of one or more minerals. (A rock may also include organic remains and mineraloids.) Some rocks are predominantly composed of just one mineral. For example, limestone is a sedimentary rock composed almost entirely of the mineral calcite. Other rocks contain many minerals, and the specific minerals in a rock can vary widely. Some minerals, like quartz, mica or feldspar are common, while others have been found in only four or five locations worldwide. The vast majority of the rocks of the Earth's crust consist of quartz, feldspar, mica, chlorite, kaolin, calcite, olivine, hornblende, magnetite, hematite, limonite and a few other minerals. Over half of the mineral species known are so rare that they have only been found in a handful of samples, and many are known from only one or two small grains.

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).

Physical properties of minerals

Classifying minerals can range from simple to very difficult. A mineral can be identified by several physical properties, some of them being sufficient for full identification without equivocation. In other cases, minerals can only be classified by more complex optical, chemical or X-ray diffraction analysis; these methods, however, can be costly and time-consuming.

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

How Rocks & Minerals Are Formed

THE EARTH'S CRUST

The whole earth is made of rocks & minerals. Inside the earth there is a liquid core of molten rock and on the outside there is a hard crust. If you compare the earth to an egg, the shell on an egg is like the crust on the earth. The crust is made up of rocks and minerals. Much of the crust is covered by water, sand, soil and ice.
If you dig deep enough, you will always hit rocks.

Below the loose layer of soil, sand & crumbled rocks found on Earth is bedrock, which is a solid rock.

• The Crust makes up less than 1% of the Earth’s mass (0.4%). It is made of oxygen, magnesium aluminum, silicon calcium, sodium potassium, iron. There are 8 elements that make up 99% of the Earth’s crust. The continents are about 35 km thick and the ocean floors are about 7 lm thick.
• The Mantle is the solid casing of the Earth and is about 2900 km thick. It makes up about 70% of the Earth’s mass (68.1%). It is made up of silicon, oxygen, aluminum and iron.

The Core is mainly made of iron and nickel and makes up about 30% of the Earth’s mass (31.5%).
The Outer Core is 2200 km thick and is liquid and the Inner core is 1270 km thick and is solid.

ROCKS

The rocks you see around you - the mountains, canyons & riverbeds, are all made of minerals. A rock is made up of 2 or more minerals. Think of a chocolate chip cookie as a rock. The cookie is made of flour, butter, sugar & chocolate. The cookie is like a rock and the flour, butter, sugar & chocolate are like minerals. You need minerals to make rocks, but you don't need rocks to make minerals. All rocks are made of minerals.

MINERALS

A mineral is composed of the same substance throughout. If you were to cut a mineral sample, it would look the same throughout. There are about 3000 different minerals in the world. Minerals are made of chemicals - either a single chemical or a combination of chemicals. There are 103 known chemical elements. Minerals are sorted into groups. Some common examples have been listed for each.

• Native Elements ~ copper, silver, gold, nickel-iron, graphite, diamond
• Sulfides ~ sphalerite, chalcopyrite, galena, pyrite
• Halides ~ halite, fluorite
• Oxides & Hydroxides ~ corundum, hematite
• Nitrates, Carbonates, Borates ~ calcite, dolomite, malachite
• Sulfates, Chromates, Molybdates, Tungstates ~ celestite, barite, gypsum
• Phosphates, Arsenates, Vanadates ~ apatite, turquoise
• Silicates ~ quartz, almandine garnet, topaz, jadeite, talc, biotite mica 

CRYSTALS

Crystals are minerals that have had the chance to grow in the shape that they were meant to be.
Just like your DNA determines the colour of your eyes, how tall you will get to be and the shape of your bones, the chemicals that a mineral is made of determines what shape it gets to be. We can tell different minerals apart by what crystal shape they are.

Sometimes minerals form in spaces where there is not a lot of room, so they don't have a crystal shape. When there is just a big hunk of a mineral, it is called a massive mineral. If there is a definite shape with easy to see flat sides, it is called a mineral crystal. Most of the earth's crystals were formed millions of years ago. Crystals form when the liquid rock from inside the earth cool and harden. Sometimes crystals form when liquids underground find their way into cracks and slowly deposit minerals. Most mineral crystals take thousands of years to "grow" but some like salt (halite) can form so quickly that you can watch them grow at home!

Some people think of crystals as clear pretty rocks that are used for jewelry. Amethyst is a very common quartz crystal. Crystals do not have to be clear, but those are the kinds you will usually see in the stores.

SOIL, SAND & DIRT

When rocks break down into smaller & smaller pieces, they turn into sand. If you look at the sand under a microscope, you will see that sand is made up of the same minerals as the rocks that the sand came from.
When plants start to sprout up in sand, it is turning from being just small bits of rock to being soil.  

Soil is very important to life on earth. It supports plant life. We could not live without plants. Soil is made up of sand and decomposing plants and animals. Soil has many names including: clay, silt, mud, dirt, topsoil, dust, potting soil and humus

 

THE ROCK CYCLE

Rocks are constantly being formed, worn down and then formed again. This is known as the Rock Cycle.
It is like the water cycle but it takes a lot longer. It takes thousands and millions of years for rocks to change.
Rocks are divided into 3 Types. They are classified by how they were formed.

• IGNEOUS
• SEDIMENTARY
• METAMORPHIC

CHEMICAL PROPERTIES OF MINERALS

Minerals may be classified according to chemical composition. The list below is in approximate order of their abundance in the Earth's crust. The list follows the Dana classification system which closely parallels the Strunz classification.

Silicate class

Quartz

The largest group of minerals by far are the silicates (most rocks are ≥95% silicates), which are composed largely of silicon and oxygen, with the addition of ions such as aluminium, magnesium, iron, and calcium. Some important rock-forming silicates include the feldspars, quartz, olivines, pyroxenes, amphiboles, garnets, and micas.

Carbonate class

The carbonate minerals consist of those minerals containing the anion (CO₃)²⁻ and include calcite and aragonite (both calcium carbonate), dolomite (magnesium/calcium carbonate) and siderite (iron carbonate). Carbonates are commonly deposited in marine settings when the shells of dead planktonic life settle and accumulate on the sea floor. The carbonate class also includes the nitrate and borate minerals.

Sulfate class

Sulfate minerals all contain the sulfate anion, SO₄²⁻. Sulfates commonly form in evaporitic settings where highly saline waters slowly evaporate, allowing the formation of both sulfates and halides at the water-sediment interface. Sulfates also occur in hydrothermal vein systems as gangue minerals along with sulfide ore minerals. Another occurrence is as secondary oxidation products of original sulfide minerals. Common sulfates include anhydrite (calcium sulfate), celestine (strontium sulfate), barite (barium sulfate), and gypsum (hydrated calcium sulfate). The sulfate class also includes the chromate, molybdate, selenate, sulfite, tellurate, and tungstate minerals.

Halide class

The halide minerals are the group of minerals forming the natural salts and include fluorite (calcium fluoride), halite (sodium chloride), sylvite (potassium chloride), and sal ammoniac (ammonium chloride). Halides, like sulfates, are commonly found in evaporite settings such as salt lakes and landlocked seas such as the Dead Sea and Great Salt Lake. The halide class includes the fluoride, chloride, bromide and iodide minerals.

Oxide class

Oxide minerals are extremely important in mining as they form many of the ores from which valuable metals can be extracted. They also carry the best record of changes in the Earth's magnetic field. They commonly occur as precipitates close to the Earth's surface, oxidation products of other minerals in the near surface weathering zone, and as accessory minerals in igneous rocks of the crust and mantle. Common oxides include hematite (iron oxide), magnetite (iron oxide), chromite (iron chromium oxide), spinel (magnesium aluminium oxide – a common component of the mantle), ilmenite (iron titanium oxide), rutile (titanium dioxide), and ice (hydrogen oxide). The oxide class includes the oxide and the hydroxide minerals.

Sulfide class

Many sulfide minerals are economically important as metal ores. Common sulfides include pyrite (iron sulfide – commonly known as fools' gold), chalcopyrite (copper iron sulfide), pentlandite (nickel iron sulfide), and galena (lead sulfide). The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, and the sulfosalts (sulfur and a second anion such as arsenic).

Phosphate class

The phosphate mineral group actually includes any mineral with a tetrahedral unit AO₄ where A can be phosphorus, antimony, arsenic or vanadium. By far the most common phosphate is apatite which is an important biological mineral found in teeth and bones of many animals. The phosphate class includes the phosphate, arsenate, vanadate, and antimonate minerals.

Element class

The elemental group includes native metals and intermetallic elements (gold, silver, copper), semi-metals and non-metals (antimony, bismuth, graphite, sulfur). This group also includes natural alloys, such as electrum (a natural alloy of gold and silver), phosphides, silicides, nitrides and carbides (which are usually only found naturally in a few rare meteorites).

Organic class

The organic mineral class includes biogenic substances in which geological processes have been a part of the genesis or origin of the existing compound. Minerals of the organic class include various oxalates, mellitates, citrates, cyanates, acetates, formates, hydrocarbons and other miscellaneous species.^[3] Examples include whewellite, moolooite, mellite, fichtelite, carpathite, evenkite and abelsonite.