GEOLOGY 101:

Crystal Habits & Structure ▧

Let's Talk About Crystal Habits

The term crystal habit (not to be confused with crystal system) describes the characteristic or favoured growth pattern of the crystals of a mineral species, whether individually or in an aggregate. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits which are affected by certain factors. These shapes can be influenced by the atomic structure of the mineral, but they can also be influenced by the environment of crystal growth (i.e. heat, pressure, space).

The below terms can be used to describe the typical appearance of an ideal mineral & therefore can be incredibly useful in identification.

  • Acicular

    Long & needle-like, thinner than prismatic but thicker than fibrous. E.g. Rutile, Natrolite.

  • Banded

    Banded minerals have narrow layers or bands of different colour or texture. E.g. Rhodochrosite, Agate.

  • Bladed

    Blade-like, slender, & flattened. E.g. Actinolite, Kyanite.

  • Botryoidal (Globular)

    Botryoidal means "like grapes," referring to its rounded, hemispherical masses. E.g. Malachite, Grape Agate.

  • Columnar

    Long, slender prisms often with parallel growth. E.g. Calcite, Tourmaline, Gypsum.

  • Cubic

    Cube shape. E.g. Pyrite, Galena, Halite.

  • Doubly Terminated

    A prismatic crystal that has a natural termination on both ends. E.g. Quartz (Herkimer Diamond.)

  • Druzy

    A surface that is covered with small crystals, called druze. E.g. Uvarovite, Malachite, Azurite, Quartz.

  • Equant

    Crystals of nearly equal dimensions. E.g. Garnet, Olivine, Pyrite, Fluorite.

  • Fibrous

    Very fine fibre-like crystals, almost appearing like thin hairs. E.g. Actinolite, Chrysotile, Serpentine, Tremolite.

  • Foliated

    Layered structure, parting into thin sheets or books. Also known as micaceous or lamellar. E.g. Muscovite, Biotite.

  • Geode

    A roughly spherical rock containing a hollow cavity which is lined with bands & crystals. Usually formed by air pockets within sedimentary/ volcanic rocks. E.g. Celestite, Quartz, Calcite.

  • Granular

    A crystalline aggregate composed of many rounded or equant anhedral crystals of approximately the same size. E.g. Olivine, Bornite.

  • Hopper

    Partially formed crystals that have experienced more rapid growth on their outer edges than in the centre of the crystal. This causes a hollow, concave middle. E.g. Halite, Calcite, Bismuth.

  • Massive

    Masses of crystals that have no distinctive geometry. E.g. Limonite, Turquoise, Cinnabar, Realgar.

  • Prismatic

    Elongated crystals with opposite faces normally parallel to one another. The crystals are often striated along their length or across. E.g. Tourmaline, Quartz, Beryl, Diopside, Topaz.

  • Radiating

    Radiating outward from a central point without producing a star. Crystals are generally separated/have different lengths. E.g. Stibnite.

  • Rosette

    Clusters of tabular crystals in a radial arrangement that resemble a rose or flower. E.g. Barite, Gypsum, Pyrite, Marcasite.

  • Stalactitic

    Forming as stalactites or stalagmites; cylindrical or cone-shaped. E.g. Calcite, Goethite, Malachite.

  • Tabular

    Tabular crystals are flat & plate-like, resembling a tablet. They have lengths & widths that are much larger than their thickness. E.g. Feldspar, Topaz, Barite, Corundum.

Pseudomorph Minerals

A pseudomorph is a mineral that has an uncharacteristic crystalline form as a result of assuming the shape of another mineral that it has replaced. These specimens are admired by collectors for their interesting, complex & unusual appearance. The word pseudomorph means “false form,” which perfectly reflects its atypical nature. In descriptions, these minerals are generally noted as (replacement) after (original mineral) e.g. Malachite after Azurite. The specific conditions & process through which pseudomorph minerals form can vary greatly. Some may be formed by substitution, where in the original substance has been gradually or partially removed & simultaneously replaced by another. A common example of this is Petrified Wood, in which all the cellulose fibres have been replaced by Silica (Quartz or Opal), even those in the bark. Another variety of pseudomorphs, known as epimorphs or perimorphs, occur when a "new" mineral forms a "crust" over the original mineral. The original mineral is later removed through some geochemical process. This leaves a situation whereby the replacing mineral preserves the crystal shape of the original mineral. Pseudomorphs may also occur on a molecular level only, leaving no change to the external crystal form. A common example of this type of pseudomorph, often called a paramorph, is the change from Aragonite to Calcite. Pseudomorphs can also form from fossils. Take Pyrite after Ammonite for example. After the Ammonite is buried, the original shell material becomes converted by mineralising fluids rich in Iron & Sulfur into Pyrite.

Crystallography & the Basics of Crystal Structure

The scientific study of different crystals is called crystallography - a fascinating branch of science that uses chemistry, physics, biology, geology & everything in-between to better understand the complex process of crystal formation. To understand this broad subject further, it would be helpful to understand more advanced topics within chemistry, physics & geometry. But for now, we will dive into the absolute basics of how a crystal is structured.

What is a Crystal Structure?

A crystal structure is a unique arrangement of atoms, molecules or ions in a crystal. It is composed of a motif, which is a set of atoms arranged in a particular way, & a lattice. Motifs are located upon the points lattice, which is an array of points repeating periodically in 3-dimensions. The points can be thought of as forming identical tiny boxes, called unit cells, that fill the space of lattice. The lengths of the edges of a unit cell & the angles between them are called the lattice parameters. These parameters describe the size & shape of the unit cell, with a, b & c defining the edge lengths (crystallographic axes), & α, β & γ defining the angles.

What is a Crystal Lattice?

In a crystal lattice, each atom, molecule or ion (constituent particle) is represented by a lattice point. The points are held together by a straight line &, once all these straight lines are connected, we get a 3-dimensional view of the structure. This 3D arrangement is known as a crystal lattice or Bravias lattice.

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