MINERALOGY

Mineral

A mineralis a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered characteristic chemical composition, a highly ordered atomic structure, and specific physical properties.

The International Mineralogical Association in 1995 adopted a new definition:

“A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.”

Minerals vs. Rocks

A mineral is a naturally occurring solid with a definite chemical composition and a specific crystalline structure. Some minerals, like quartz, mica or feldspar are common, while others have been found in only four or five locations worldwide

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. Other rocks contain many minerals, and the specific minerals in a rock can vary widely

Physical Properties of Minerals:

• Color - indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals (i.e. what it looks like to the naked eye).

• Luster - indicates the way a mineral's surface interacts with light and can

Distinguishing properties of Minerals2 kinds – Physical & Chemical

• Luster - indicates the way a mineral's surface interacts with light and can range from dull to glassy (vitreous). Metallic – high reflectivity like metal: galena and pyriteSub-metallic – slightly less than metallic reflectivity: magnetiteNon-metallic luster's:

+ Adamantine – brilliant, the luster of diamond also cerussite and anglesite+ Vitreous – the luster of a broken glass: quartz+ Pearly – iridescent and pearl-like: talc and apophyllite+ Resinous – the luster of resin: sphalerite and sulfur+ Silky – a soft light shown by fibrous materials: gypsum and chrysotile

+ Dull/earthy – shown by finely crystallized minerals: the kidney ore variety of hematite

• Hardness - the physical hardness of a mineral is usually measured according to the Mohs scale. This scale is relative and goes from 1 to 10. Minerals with a given Mohs hardness can scratch the surface of any mineral that has a lower hardness than itself.

• Streak - refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate. Note that this is not always the same color as the original mineral.

• Cleavage- describes the way a mineral may split apart along various planes. In thin sections, cleavage is visible as thin parallel lines across a mineral.

• Fracture - describes how a mineral breaks when broken contrary to its natural cleavage planes.natural cleavage planes.- Chonchoidal fracture is a smooth curved fracture with concentric ridges of the type shown by glass.- Hackley is jagged fracture with sharp edges.- Fibrous- Irregular

• Specific Gravity - relates the mineral mass to the mass of an equal volume of water, namely the density of the material. While most minerals, including all the common rock-forming minerals, have a specific gravity of 2.5–3.5, a few are noticeably more or less dense, e.g. several sulfide minerals have high specific gravity compared to the common rock-forming minerals.

Chemically minerals are classified into • Silicates • Carbonates• Sulfates

Chemical divisons of Minerals.

• Sulfates• Halides• Oxides• Sulfide• Phosphates• Elements class• Organic class

Silicates

• A silicate is a compound containing a silicon bearing anion.

• In the vast majority of silicates, including silicate minerals, the Si occupies a tetrahedral environment, being surrounded by 4 oxygen centers. These tetrahedra sometimes occur as isolated SiO44- centres, but most commonly, the tetrahedral are joined together in various ways, such as pairs (Si2O7) and rings (Si6O18). as pairs (Si2O7) and rings (Si6O18).

• Commonly the silicate anions are chains, double chains, sheets, and three-dimensional frameworks. All such species have negligible solubility in water at normal conditions.

• Mineralogically, silicate minerals are divided according to structure of their silicate anion into the following groups:

1)Nesosilicates (lone tetrahedron) - [SiO4]4−, e.g. olivine.2) Sorosilicates (double tetrahedra) - [Si2O7]6−, e.g. epidote, melilite group.3) Cyclosilicates (rings) - [SinO3n]2n−, e.g. tourmaline group.

4) Inosilicates (single chain) - [SinO3n]2n−, e.g. pyroxene group.

5) Inosilicates (double chain) - [Si4nO11n]6n−, e.g. amphibole group.

6) Phyllosilicates (sheets) - [Si2nO5n]2n−, e.g. micas and clays.

7) Tectosilicates (3D framework) - [AlxSiyO2(x+y)]x−, e.g. quartz, 7) Tectosilicates (3D framework) - [AlxSiyO2(x+y)]x−, e.g. quartz, feldspars, zeolites.

Olivine

The mineral olivine is a magnesium iron silicate (nesosilicate) with the formula (Mg,Fe)2SiO4. It is one of the most common minerals on Earth, and has also been identified in meteorites.

Olivine is named for its typically olive-green color (thought to be a result of traces of nickel), though it may alter to a reddish color from the oxidation of iron. oxidation of iron.

Olivine incorporates only minor amounts of elements other than oxygen, silicon, magnesium and iron. Manganese and nickel commonly are the additional elements present in highest concentrations.

Translucent olivine is sometimes used as a gemstone called peridot, the French word for olivine. It is also called chrysolite, from the Greek words for gold and stone. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad island in the Red Sea.

Pyroxenes

The pyroxenesare a group of important rock-forming inosilicateminerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems.

Pyroxenes have the general formula XY(Si,Al)2O6 (where X represents Ca, Na, Fe2+ and Mg and Y represents ions of smaller size, such as Cr, Al, Fe3+ and Mn.

Pyroxenes are divided into Orthopyroxenes and Clino pyroxenes based on the crystal system minerals crystallize.

The most common rock forming pyroxine is Augite.

Amphiboles

Amphibole defines an important group of generally dark-colored rock-forming inosilicateminerals, composed of double chain SiO4

tetrahedra, linked at the vertices and generally containing ions of Fe and/or Mg in their structures.

Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic . In chemical composition and general characteristics they are similar to the pyroxenes.

The chief differences from pyroxenes are that:

(i) amphiboles contain essential hydroxyl (OH) or halogen (F, Cl) and (ii) the basic structure is a double chain of tetrahedra (as opposed to the

single chain structure of pyroxene).

Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes (at around 120 degrees), whereas pyroxeneshave cleavage angles of approximately 90 degrees. Amphiboles are also specifically less densethan the corresponding pyroxenes.

The most common rock forming amphibole is Hornblende.

Feldspars

Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of rock-forming minerals which make up as much as 60% of the Earth's crust.

Feldspar is derived from the German Feld, "field", and Spath, "a rock that does not contain ore". "Feldspathic" refers to materials that contain does not contain ore". "Feldspathic" refers to materials that contain feldspar.

This group of minerals consists of a framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

1) Potassium-Feldspar (K-spar) endmember KAlSi3O82) Albite endmember NaAlSi3O83) Anorthite endmember CaAl2Si2O8

Solid solutions between K-feldspar and albite are called Alkali feldspar.

Solid solutions between albite and anorthite are called plagioclase, or more

properly plagioclase feldspar.

Albite is considered both a plagioclase and alkali feldspar.

In addition to albite, barium feldspars are also considered both alkali andIn addition to albite, barium feldspars are also considered both alkali and

plagioclase feldspars.

Barium feldspars form as the result of the replacement of potassium feldspar

Defn: A single, solid, homogeneous crystalline phase containing two or more chemical species.

A solid solution can be substitutional or interstitial.

A substitutional solid changed only slightly. solution is a homogeneous crystalline phase in which some of the constituent molecules are substituted by foreign molecules that possess sufficient similarity that the lattice dimensions are

In an interstitial solid solution, the foreign molecules are inserted into interstitial positions, such that the lattice dimensions are changed only slightly.

Mica

The word "mica" is derived from the Latin

word micare, meaning "to glitter", in reference to the

brilliant appearance of this mineral (especially when

in small scales).

The mica group of minerals are sheet silicates (phyllosilicate) including several closely related materials having highly perfect basal cleavage.

All are monoclinic and are similar in chemical composition. The highly

perfect cleavage, which is the most prominent characteristic of mica, is

explained by the hexagonal sheet-like arrangement of its atoms.

Chemically, micas can be given the general formula:

X2Y4–6Z8O20(OH,F)4in which X is K, Na, or Ca

Y is Al, Mg, or Fe

Z is chiefly Si or AlZ is chiefly Si or Al

Mica is widely distributed and occurs in igneous, metamorphic and

sedimentary regimes. Large crystals of mica used for various applications

are typically mined from granitic pegmatites

The most common mica mineral is the Muscovite.

Uses

Electronics - Mica has a high dielectric strength and excellent chemical stability, making it a favored material for manufacturing capacitors for radio frequency applications.

Thin transparent sheets of mica called "isinglass" were used for peepholes in boilers and lanterns because they were less likely to shatter compared to glass when exposed to extreme temperature gradients.

It has also been used as an insulator in high Mica is also birefringentand is commonly used It has also been used as an insulator in high voltage electrical equipment.

Mica is also birefringentand is commonly used to make quarter and half wave plates

Mica is used in the production of pearlescent pigments. Many metallic looking pigments are composed of a substrate of mica coated with another mineral, usually titanium dioxide (TiO2).

Some brands of toothpaste include powdered white mica.

Quartz

Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar.

It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall with each oxygen being shared between two tetrahedra, giving an overall formula SiO2.

There are many different varieties of quartz, several of which are semi-precious gemstones.

Quartz belongs to the trigonal crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive.

Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent (clear) or translucent, and has often been used for hardstone carvings.

Quartz is an essential constituent of granite and other felsic igneous rocks. ItQuartz is an essential constituent of granite and other felsic igneous rocks. It

is very common in sedimentary rocks such as sandstone and shale and is

also present in variable amounts as an accessory mineral in most carbonate

rocks.

It is also a common constituent of schist, gneiss, quartzite and other

metamorphic rocks. Because of its resistance to weathering it is very

common in stream sediments and in residual soils. Quartz, therefore,

occupies the lowest potential to weather in the Goldich dissolution series.

Garnets

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. Garnets are nesosilicateshaving the general formula X3Y2(SiO4)3.

Six common species of garnet are recognized by their chemical composition. Six common species of garnet are recognized by their chemical composition. They are pyrope, almandine, spessartine, grossular, uvarovite and andradite.

The garnets make up two solid solution series:

1. pyrope-almandine-spessarite and

2. uvarovite-grossular-andradite.

Garnets crystallize in the cubic system, having three axes that are all of

equal length and perpendicular to each other. Garnets do not show cleavage,

so when they fracture under stress, sharp irregular pieces are formed

Importance of Garnet – Geological& Industrial

Geological IndustrialKey mineral in interpreting the genesis of many igneous and metamorphic rocks via geothermobarometry.

Red garnets are used as Gemstones

Garnets are very resistant to alteration hence Garnet sand is a good abrasive, and a common Garnets are very resistant to alteration hence history of host rock can be generated by studying garnets.

Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting

Xenoliths of garnet peridotite have been carried up from depths of 100 km and greater by kimberlite, and garnets from such disaggregated xenoliths are used as a kimberlite indicator minerals in diamond prospecting

Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets

Garnet paper is favoured by cabinetmakers for finishing bare wood

Bowen’s Reaction Series

• These minerals react discontinuously to form the next mineral in the series.

• If there is enough silica in the melt, each mineral will change to the next mineral lower in the series as the temperature drops.

• As you go down Bowen’s Reaction Series, the minerals increase in the

The Discontinuous ReactionSeries

• As you go down Bowen’s Reaction Series, the minerals increase in the proportions of silica in their composition. In a basaltic melt, olivine will be the first mafic mineral to form. When the temperature is low enough to form pyroxene, all of the olivine will react with the melt to form pyroxene and pyroxene will crystallize out of the melt.

• At the crystallization temperature of amphibole, all the pyroxene will react with the melt to form amphibole and amphibole will crystallize.

• At the crystallization temperature of biotite, all of the amphibole will react to form biotite and biotite will crystallize.

•If you are crystallizing olivine and there is not enough silica to form pyroxene, then the reaction will not occur and olivine will remain.

• If you are crystallizing olivine and the temperature drops too fast for the reaction to take place then the reaction will not have time to occur, the rock will solidify quickly and the mineral will remain occur, the rock will solidify quickly and the mineral will remain olivine.

• Plagioclase minerals have the formula (Ca, Na)(Al, Si)3O8. The highest temperature plagioclase has only calcium (Ca). The lowest temperature plagioclase has only sodium (Na).

• In between, these ions mix in a continuous series from 100% Ca and 0 % Na at the highest temperature to 50% Ca and 50% Na at

The Continuous ReactionSeries

and 0 % Na at the highest temperature to 50% Ca and 50% Na at the middle temperature to 0 % Ca and 100% Na at the lowest temperature.

• In a basaltic melt, the first plagioclase to form might be 100% Ca and 0% Na plagioclase. As the temperature drops the crystal wouldreact with the melt to form 99% Ca and 1% Na plagioclase and 99% Ca and 1% Na plagioclase would crystallize. Then those would react to form 98% Ca and 2% Na and the same composition would crystallize. • All of this is happening continuously provided there is enough time for the reactions to take place and enough sodium, aluminum, and silica in the melt to form each new mineral.