Crystal Properties

Introduction

Crystals, with their mesmerizing structures and precision, have enchanted scientists and enthusiasts for centuries. This exploration delves into the fascinating world of crystal properties, unravelling the concepts of crystal systems, crystal habits, and twinning. Understanding these fundamental aspects provides a deeper appreciation for the diverse and captivating forms that crystals, including well-known gemstones, can exhibit.

Crystal System

The crystal system classifies crystals based on their symmetry and geometric arrangement. Here are examples from common crystal systems, using well-known gemstones:

Cubic System

Symmetry: Four threefold axes
Example: Diamond

Tetragonal System

Symmetry: One fourfold axis
Example: Zircon

Orthorhombic System

Symmetry: Three mutually perpendicular twofold axes
Example: Topaz

Monoclinic System

Symmetry: One twofold axis
Example: Orthoclase (e.g. Moonstone)

Triclinic System

Symmetry: No perpendicular axes
Example: Oligoclase (e.g. Sunstone)

Hexagonal System

Symmetry: One sixfold axis
Example: Beryl (e.g. Emerald)

Trigonal System

Symmetry: One threefold axis
Example: Tourmaline

Understanding the crystal system aids in predicting the external forms of crystals, contributing to the allure of gemstones.

Crystal Habit

Crystal habit refers to the characteristic external shape a crystal assumes during growth. Gemstones commonly exhibit the following crystal habits:

Prismatic

Crystals are elongated in one direction like a prism
Example: Quartz (e.g. Amethyst)

Tabular

Flat, plate-like crystals
Example: Beryl (e.g. Aquamarine)

Equant

Blocky or cubic crystals
Example: Almandine

Striated

Crystals have parallel lines or grooves on their surface
Example: Tourmaline

Acicular

Needle-like crystals
Example: Corundum (rarely)

Bladed

Blade-like crystals
Example: Kyanite

Colloform

Spherical, bulbous or globular lumps
Example: Azurite

Botryoidal

Resembling a bunch of grapes
Example: Malachite

Reniform

Displays the shape of a kidney
Example: Malachite

Mammillary

Rounded crystals with a bumpy or knobby surface
Example: Chalcedony

Drusy

Small, tightly-packed crystals that form a crust
Example: Quartz (e.g. Amethyst)

Columnar

Crystals grow in the direction parallel to the longest dimension of the crystal
Example: Cubic Zirconia (synthetic crystals)

Massive

Large, lumpy mass which has no apparent crystal form
Example: Jadeite

Crystal habits that may be exhibited but are less commonly observed in gemstones include: lammellar, dendritic, micaceous, hopper, plumose, filiform, fibrous, reticulated, stellated, oolitic, pisolitic, stalactitic, concretionary.

Twinning

Twinning occurs when crystals share a portion of their atomic lattice. Here are twin types using examples from well-known gemstones:

Contact Twinning

Contact twinning occurs when two crystals share a common plane or surface. In this twinning type, the crystals grow adjacent to each other and are oriented in such a way that they appear to be in direct contact along a specific plane. This shared plane is known as the composition plane. Contact twinning can create distinct visible lines or markings on the surface of the crystal, and the crystals may appear to be joined together.

Example: Quartz (e.g. Amethyst) often exhibits contact twinning, forming distinctive twinned crystals with visible twinning planes.

Penetration Twinning

Penetration twinning occurs when two or more individual crystals interpenetrate each other. In this twinning type, one crystal penetrates the other along a specific axis, creating a complex interlocking structure. Penetration twins can display symmetry elements that are not apparent in individual crystals and often result in intricate patterns.

Example: Peridot can exhibit penetration twinning, forming twinned crystals with a distinctive X-shaped pattern.

Re-entrant Twinning

Re-entrant twinning, also known as reticulated twinning, occurs when crystals form inward-facing V-shapes or re-entrant angles. In this twinning type, the crystals grow in a manner that creates concave features, resembling the shape of a V when viewed from certain angles. Re-entrant twinning can add complexity to the crystal's overall structure.

Example: Zircon is known to display re-entrant twinning, forming twinned crystals with inward-facing angles.

Multiple Twinning

Multiple twinning refers to a situation where more than two individual crystals are twinned together, creating a complex intergrowth of crystals. In multiple twinning, several individuals share common crystallographic orientations and may be oriented differently from one another. This type of twinning can result in a variety of patterns and symmetries within the overall crystal structure.

Example: Tanzanite is an example of a gemstone that can exhibit multiple twinning, often forming intricate and highly prized crystal structures.

Understanding these twinning types provides insights into the internal arrangements and symmetries of crystals, contributing to the overall diversity and beauty observed in gemstones and minerals.