Crystals Grouped by Chemical Properties

June 3, 2022

Gems and minerals are recognized by their crystal structural symmetry, chemical, and physical properties. For crystals, they are divided into 2 categories i.e. the lattices can be primitive (only one lattice point per unit cell) or non-primitive (more than one lattice point per unit cell). In the previous article, we learned about 7 crystal structures or system which are the following:-

Cubic
Hexagonal
Tetragonal
Trigonal
Orthorhombic
Monoclinic
Triclinic

It should be kept in mind that there is no specific way to distribute crystals or divide them. Though here, we are describing 4 crystals type properties wise on the basis of chemical binding of the constituent atoms. They help us understand minerals and gemstone identification.

Group of Crystals by their Properties

According to the physical properties and chemical properties, crystals are divided into 4 main categories:;-

Covalent Crystals
Metallic Crystals
Ionic Crystals
Molecular Crystals

Covalent Crystals:

In this type of crystals, all atoms consist of true covalent bonds[caption id="attachment_2081" align="alignright" width="351"] 7 Types of Crystals Structure and Shapes. Read this also.[/caption]
between each of them. It can be called like one big molecule.
Many covalent crystals are melted at high temperature or they have a very high melting point of. Examples are diamonds and zinc sulfide crystals.
Very stable crystals

Metallic Crystals:

Metallic crystals usually have metal elements or formed by metal elements.
Their main point is that they conduct electricity.
They are malleable
Their surface usually shines.
They are ductile
Outer electrons of metal atoms float around the lattice because these metal atoms individually sit on lattice sites.
High melting point
Tend to be very dense

Ionic Crystals:

Are usually hard substance
Posses ionic bond
Relative high melting point and boiling point
oppositely charged ions are joined together by strong electrostatic forces
Conduct electricity in form of a solution and molten state both
Usually are brittle
Can not be spread on a sheet or drawn on a sheet
They are not ductile
Table salt is an example

Molecular Crystals:

Molecules are joined together by weak Van der wall force or hydrogen bonding
Noncovalent interaction let the molecular crystal held together
An example is rock candy, sulfur, table sugar. like, $H_2, O_2, CO_2, I_2, S_8$ sugar etc.
Low melting and boiling point
They evaporate soon and thus are volatile i.e $NH_3(s)$ $(kJ mol^{-1})$ : 23.55
Hardness is soft