BIBLIOGRAPHY

-          (n.d.). Retrieved from http://etc.usf.edu/clipart/galleries/89-geology

-          (n.d.). Retrieved from www.webmineral.com

-          (n.d.). Retrieved from www.rockhounds.com

-          (n.d.). Retrieved from www.ceritageologi.wordpress.com

-          (n.d.). Retrieved from www.adnor.thya.blogspot.com

-          (n.d.). Retrieved from www.chemed.chem.purdue.edu

-          (n.d.). Retrieved from www.etc.usf.edu

-          (n.d.). Retrieved from www.vtuphysics.blogspot.com

-          (n.d.). Retrieved from www.bwsmigel.info

-          (n.d.). Retrieved from www.chemistryworkbookcrawl.com

-          (n.d.). Retrieved from www.chemistryworkbookcrawl.com

-          (n.d.). Retrieved from www.tulane.edu

-          (n.d.). Retrieved from www.encyclopedia2.thefreedictionary.com

-          (n.d.). Retrieved from www.ictwiki.iitk.ernet.in

-          (n.d.). Retrieved from www.commons.wikimedia.org

-          (n.d.). Retrieved from www.allaboutgemstones.com

-          (n.d.). Retrieved from www.tankongvtar.hu

-          (n.d.). Retrieved from www.geologycafe.com

-          (n.d.). Retrieved from www.slideshare.com

-          (n.d.). Retrieved from www.respitirio.utad.pt

-          (n.d.). Retrieved from www.lookfordiagnosis.com

-          (n.d.). Retrieved from www.thisoldearth.net

-          (n.d.). Retrieved from www.hdimagegallery.net

-          (n.d.). Retrieved from www.engr.usask.ca

-          (n.d.). Retrieved from www.wurstwisdom.com

-          (n.d.). Retrieved from www.britannica.com

-          (n.d.). Retrieved from www.slideplayer.fr.

-          (n.d.). Retrieved from www.dave.ucsc.edu/myrtreia

-          Gribble, R. b. (2004). Rutley's Elements of Mineralogy 27th Edition. New Delhi: CBS.

-          Plummer, M. a. (2005). PHYSICAL GEOLOGY 10th Edition. New York: McGraw - Hill.

CRYSTALLOGRAPHY PART 1

CRYSTALLOGRAPHY

Crystallography is the science that studies the geometry of crystals. That is the shape and forms that minerals assume in space in 3 dimensions (the regular patterns and interfacial angles of crystals).

Word History:

The word crystal,was derived from a Greek word krustallos meaning ‘clear ice’ formed by the freezing of water.  The ancient Greeks were amazed by quartz (a mineral and rock) which occurred in forms having a characteristic shape and being bounded by flat surfaces (faces).  From the transparency of quartz, together with the presence of inclusions in it, it was thought that quartz resulted from the freezing of water under intense cold and the name krustallos was given to it. The application of this name was later extended to all minerals that showed forms with smooth surfaces.  These forms are crystals and their study is crystallography.

Definition:

A crystal can be defined as a homogenous solid bounded by naturally formed plane (or smooth) surfaces called faces which can be related to a regular internal arrangement of atoms.

Crystals are formed by a process called crystallization during the solidification of minerals from the gaseous or liquid states or from solutions of magmatic origin

It is the regular internal arrangement of atoms within a mineral which really defines whether or not a mineral is crystalline.  The study of the outward appearance of minerals (crystals) is important to geologists because it helps them identify and recognize different minerals.  This is possible because different minerals have different chemical compositions and different internal atomic arrangements and therefore they have different crystal shapes and forms.

The Unit Cell

The structure of every crystal is a construction of atoms or groups of atoms arranged in three dimensional patterns which are repeated throughout the crystal. 

A unit cell is the smallest complete unit of pattern of a crystal.  Many unit cells combine in repetition to form a whole crystalfor example;

Sodium Chloride (rock salt), a cubic mineral is an excellent example of a mineral with a cubic unit cell.  Here, the unit cell consists of a cube with sodium ions at the corners and at the center of the faces and chlorine ions at the center of the edges of the cube as shown below:

 

 The unit cell in sodium chloride can be interpreted as having 8 small cubes with either sodium or chlorine ions at the corners.

There are 7 major types of unit cells namely:

-         The Cubic unit cell

-         The Tetragonal unit cell

-         The Hexagonal unit cell

-         The Trigonal unit cell

-         The Orthorhombic unit cell

-         The Monoclinic unit cell

-         The Triclinic unit cell

They give rise to seven systems under which crystals can be classified.  Some of the unit cells are further subdivided to give 14 in all.  These give the 14 types of space lattice that can be constructed from them sometimes called the [1]Bravais Lattices. 

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[1]Bravais lattices are named after Auguste Bravais, a French physicist in the early 18^th century who worked on crystal structure.

 

 

Figure 2:  The 14 types of space lattice (Bravais Lattices)

 The external morphology of a crystal (like faces, edges, angles and form) and the general symmetry of any crystal are determined by the internal arrangement of these unit cells since they are the building block for crystals.

Crystals can be formed by solidification from liquid or gaseous states or by precipitation from solutions saturated with ions.  All these processes are called crystallization.

Crystallisation

Crystallization is a process by which crystals are formed.  Naturally occurring crystals (mineral crystals) are usually formed from a solution or a melt.  In a liquid state or a solution, the atoms and ions are distributed haphazardly, but when the temperatures and pressure of the solution begin to drop, they quickly arrange themselves in an orderly manner forming crystalline solids called crystals.

A good example of crystallization can be seen during the formation of minerals from magma (molten matter).  In the molten state, the magma contains ions which are randomly dispersed within the melt.  As the temperature and pressure begin to drop, various ions are attracted to each other forming crystals of different minerals.  If the process of cooling is slow and gradual, the ions will have enough time to migrate and come together thereby building large but few and well-formed crystals with smooth faces.  However, if the drop in temperature is rapid, no time is allowed for the ions to move and coalesce (come together) so several centers or units of crystallization are developed around which many but irregularly oriented crystals are formed.  Such crystals lack flat surfaces.

The degree of crystallization affects the development of crystals which is reflected on the external form and shape of the crystal.

Based on the degree of crystalinity and the development of external form of crystals, the following terms can be used to describe crystals:

       i.            Euhedral:It is a term that describes crystals that are well formed with smooth faces.  Very few minerals show good forms.  This indicates that they are formed under suitable conditions and such minerals are said to be crystallized.

     ii.            Subhedral:It is a term that describes crystals with partially formed or imperfectly formed faces.  They are said to be crystalline.

  iii.            Anhedral:It is a term that that describes minerals that completely lack crystal faces.

According to the fineness of the grains of crystals, the following terms are used to describe minerals:

       i.            Microcrystalline:Describes minerals with fine grain aggregates, which can only be studied using microscopes.

     ii.            Cryptocrystalline:Describes minerals with crystal aggregates that are so fine grained that the individual grains cannot be seen using the microscope but can only be detected by the use of X-ray diffraction techniques. For example Chalcedony.

  iii.            Amorphous: These are minerals which lack any ordered internal arrangement of atoms for example Opal