Chemical Property of Gem Stones
We know gemstones are the result of inorganic processes. These all have their definite Chemical composition, crystalline structure & physical properties. The chemical properties of gemstones study these chemical composition, crystal system & chemical bonding of molecules in a gemstone.
Chemical composition is the presence of one or more element in a mineral.
An Element is the smallest substance which cannot be divided or broken into more elementary substances.
There are more than 110 elements has been discovered so far. All gemstones has a set amount of chemical composition and the quantity & quality of this composition in a gemstone decide their standard.
crystallography a branch of Mineralogy is important to study the chemical properties of gemstones. It is the scientific study of the crystals. Gemstones are found in the form of Crystals.
The properties like - forms of crystals, different crystal systems, habits, twins, symmetry operations and other features like inclusion within the crystal, surface structure on the habit faces etc. are closely related to the crystal structure & helps in identifying a mineral.
What is Crystal
A substance in which the constituent atoms, ions or molecules are arranged in accordance with a definite periodic and repeated regular structure throughout is called crystal. So a crystal will have symmetrical arrangement of atoms and a definite shape.
What is Amorphous
Any mineral or rock, which lacks definite arrangements Of atom & shape are called amorphous.
Now we will focus on Crystals & their properties
Elements of Symmetry
In the arrangement of crystals the symmetry is a basic property of crystals. The species vary in the symmetrical arrangement of their faces. Crystal symmetry is directly related with the symmetry of space lattice above.
There are three basic elements of symmetry as
- Centre of symmetry
- Plane of symmetry
- Axis of symmetry
Centre of Symmetry
If an imaginary line from any point of crystals surface is passed through its centre and an identical point can be found on the line on the opposite side of the crystal then a crystal is said to have a centre of symmetry. For example – Cube.
Plane of Symmetry
A symmetry plane is an imaginary plane that divides the crystal into two halves which are perfect mirror images of one another. The number of possible planes of symmetry in a crystal varies from nine to none.
Axis of Symmetry
If a crystal is rotated about an imaginary line through the centre or the origin of the crystal and the same face comes two, three, four or six times in one complete rotation of 360?, the crystal is said to have ‘n’ fold axis of symmetry.
A form is composed of all the faces of a given shape and size occurring on a crystal. A crystal form made up entirely of like faces is termed a simple form. Example – Cube.
Crystal forms are of two types:
Open form – Forms which can not on their own completely enclose space but always have other forms combined with them are called open forms.
Following are the important open forms -
This means parallel pairs of similar faces, cutting one axis and parallel to other two or three three axis. The faces which forms the top and bottom of a crystal, are called as Basal Pinacoid.
It is a form with two intersecting faces which which cut the vertical axis and one horizontal axis and are parallel to other horizontal axis.
A form with 3,4,6,8,12 intersecting faces which are parallel to the vertical axis and cut the horizontal axis.
A form with 3,4,6,8 or 12 non-parallel triangular faces that meet at a point. Each face cuts all the three axes.
The forms which totally enclose a volume of spaces and can form solid crystals by themselves are known as closed forms.
Following are the important closed forms
A form with 6,8,12,16 or 24 faces, consisting of two pyramids on either side of the horizontal plane. Each half being a mirror image of other
A form with 6 square faces, each face cuts one axis and are parallel to other two.
This form is made up of 8 faces and each face is an equilateral triangle and cuts all the three axes at equal distances.
A form is made up of 12 faces, each face is Rhomb shaped. Each face cut two axes at equal distances and is parallel to the third