Attributes of Crystals

by Ritika

The physical attributes of crystals need to be studied in detail for the categorization of crystals. This is a highly specialized field of crystallography.

There are various methods for categorizing a crystal. Generally the two common methods used for crystal categorization are:

• On the basis of crystalline structure
• On the basis of chemical or physical attributes

There are some main differences between facets and attributes of crystals in crystallography. Facets tell us about the actual classification system, i.e. the scheme and structure of crystals. But attributes of crystals in crystallography are simple values, not associated with a classification system. Also, attributes of crystals differ and are intrinsic to the nature of an object.

Crystal Groups Based on Lattices

On the basis of lattices or shape attributes of crystals, the crystals can be divided into seven crystal lattice systems. They are:

• Cubic which is also termed as Isometric: This include in it crystals of eight faces also termed octahedrons or crystals of 10 faces also termed dodecahedrons. Also, it is vital that cube faced crystals do not belong to this type of crystal lattice systems.
• Tetragonal: This has attributes same as cubic crystal lattice system with an additional attribute of being one axis longer. These form double pyramids and prisms shaped crystals.
• Orthorhombic: This has attributes similar to tetragonal crystals but do not have a square shape in cross section. These form rhombic prisms or dipyramids shaped crystals.
• Hexagonal: These include six-sided prisms with the cross section focusing a hexagon.
• Trigonal: This has single 3-fold axis of rotation.
• Triclinic: This crystal lattice system generally does not have normal shape. The reason for such shapes in this crsytal lattice system is because the crystal is not symmetrical from one side to the other.
• Monoclinic: One instance of the monoclinic crystal lattice system is skewed tetragonal crystals. These include prisms and double pyramids shaped crystals.

Crystal Groups Based on Physical and Chemical attributes

On the basis of physical and chemical attributes, crystals can be divided into four main categories. They are:

Covalent Crystals

Covalent crystals possess true covalent bonds with all atoms in the crystal thus focusing it as a big molecule. Another vital attribute possessed by crystal belonging to covalent crystals is that they have high melting points. Some crystals to name in the category of covalent crystals are diamond and zinc sulfide crystals.

Metallic Crystals

In the metallic crystals the outer electrons move freely around the lattice. This is because each metal atoms present in the metallic crystals lie on lattice sites. Another vital attribute possessed by crystal belonging to metallic crystals are they have high melting points and they are very dense.

Ionic Crystals

The ionic crystals possess ionic bonds and are bound together by electrostatic forces. One instance to name in the crystal to name in the category of ionic crystals is table salt. The vital attributes of ionic crystals are that they are hard and have high melting points.

Molecular Crystals

Molecular crystals possess non-covalent interactions also termed hydrogen bonding or van der Waals force. The main attribute of crystals belonging to molecular crystals are that they are soft and have low melting points. Some instances of molecular crystals category are rock candy which is nothing but the crystalline form of table sugar.

Let us study one example of crystal physical attribute of the diamond. Diamonds are high-pressure polymorphs of carbon and are meta stable at room temperature. Diamond focus lattice spacing of 1.54 and have closely packed carbon atoms. The hardness attribute of diamonds vary with the direction in the crystal, the hardest direction being normal to the octahedral face. Diamonds have a conchoidal fracture. The diamond can be cleaved in any plane as cleavage occurs readily parallel to the octahedral faces, giving four directions of possible cleavage. Diamonds occur in various colors like colorless (white) and pale yellow (off color or capes). However, colors of the diamond in blue, brown, red, orange, green, pink and deep-yellow are also found. Pink diamonds are due to to traces of manganese. Generally, the color of diamonds is widely thought to be due to lattice defects rather than trace elements.

A crystal named blue lace agate ia pale blue with white or darker lines. The crystal amber is Opaque or transparent resin with insects or vegetation trapped inside at times. The amber occurs in golden brown or yellow color and green amber occurs due to artificially coloring. An instance of transparent, pointed crystals is amethyst which occurs in purple to lavender color. The red-green colored bloodstone crystal appears as green quartz flecked with red or yellow jasper. The hematite crystal occurs in two colors namely silver, red and has vital properties like grounding and protecting. It appears as red or gray when unpolished, and shiny when polished.

Jade crystals that are translucent are termed jadeites and those that are creamy are termed nephrites. The colors in which jade crystals are available are green, orange, brown, blue, blue-green, cream, lavender, red and white. One instance of opaque and patterned crystal is jasper which is available in red, brown, yellow, green, blue and purple colors. The labradorite crystal which is available in colors grayish to black with blue and yellow is usually polished, dark until it catches the light and then turns iridescent blue or has gold flashes. Tiger’s Eye quartz crystal contains brown iron which makes it get the golden-yellow color. Crystals with higher degrees of symmetry tend to generate more form faces. Faces of particular forms commonly share unique physical or chemical attributes.

The absolute structure is the concept of relating some external macroscopic physical attribute of crystals. In fact, the powerful rays of color attribute of crystals speak about the frequency of the energy that they emit. Crystals have specific attributes attached to each of them as described above them. These special attributes of each crystal helps for a variety of specific usages of crystals and also for creating a stylish piece of jewelry using carefully chosen crystals.