Crystal Habits

The mineral habit is an important part of crystallography, but can't be confused with the crystal system (Perhaps an upcoming article - Follow us and find out!) - Take quartz as an example.

 Quartz as mineral collectors refer to it crystallizes in the Trigonal crystal system, however it might have one of many habits - Drusy or Druse Quartz is often found on Blue Lace Agate, a cryptocrystalline quartz generally referred to as the name states, or as a form of Chalcedony.

 In a nutshell, we can define a crystal habit following Dana's description as:

"The common and characteristic form, or combination of forms, in which a mineral crystallizes."

 

Common crystal habits and their descriptions

Let's take a look at some of the terminology encountered when discussing mineral habits, and what it all means. A good understanding of crystallography in general is invaluable to mineral identification!

All the possible crystal habits are not described here, just those you are most likely to encounter while out collecting.

Be aware that many books will reference certain crystal habits as belonging to certain minerals - However, under the right circumstances a mineral might show a unique or unusual crystal habit.

It's important to remember that not all references regarding crystal habits are exhaustive, and several terms might be used to describe the same thing in different references. Oftentimes the mineral too can be very difficult to place into a particular habit - This guide is meant as a reference point for collectors and mineral enthusiasts wondering what exactly that describing term used means.

 

Habits describing individual crystals (As a cluster or as a single crystal)

When the specimen shows separated, or mostly separated crystals, several terms are used. These are different from aggregated crystals, where although a similar term might be used, the crystals appear to be "touching" or have grown into each other somewhat.

 

Acicular, when the mineral in question shows needle-like crystals. The term can be modified with other descriptions, such as sprays - When the cluster of needles is spreading from a common point.

 

Stibnite

Acicular Stibnite sprays, Cape Minerals stock and photography. Stibnite falls into the Orthorhombic crystal system.

 

 

Capillary or filiform, when the mineral has distinctive threadlike or hairlike crystals. Millerite can form following a filiform habit.

 

Millerite

 Millerite with Calcite. Image from Wikipedia.

 

Prismatic is probably the most commonly described crystal habit - Elongated, pencil like crystals, thicker than those showing an acicular habit. Tourmaline is a very good example of a prismatic habit.

 

Phenakite

Prismatic Phenakite. Cape Minerals photography.

 

Bladed, when the crystals appear to be elongated, sort of like the blade of a knife. Kyanite most often shows this as a common crystal habit. The mineral can show individual blades, or an aggregate of many blades together.

 

Kyanite

Blue Kyanite. Image from Wikipedia

 

Columnar crystals are generally quite well separated from each other, and will often show parallel growth. The individual crystals are well defined - The relation to the term is not hard to see, as the crystals appear to be separate "columns."

 

Selenite

 Selenite showing a radiating columnar habit. Specimen currently resides in the Houston museum of natural science. Image from Wikipedia.

 

Tabular habits are described best as if shaped like a sheet of cardboard. This habit is fairly common in minerals like barite. 

 

Gypsum ps. Glauberite

 Gypsum psuedomorph after Glauberite. Cape Minerals stock and photography.

 

Hopper crystals are most commonly seen with Bismuth - when the outer portions of cubes grow much faster than the inner portions, they form a concavity.

Synthetic Bismuth

Synthetic Bismuth. Image from Wikipedia.

 

Wheatsheaf refers to crystals that appear to be wheat sheaves. This habit is seen in some truly spectacular minerals, like wheatsheaf Rhodochrosite.

 

Stilbite

Stilbite showing a wheatsheaf habit. Image from Wikipedia.

 

 

Habits describing aggregated crystals

 

When the individual crystals are more tightly grouped, we can use slightly different terminology. These crystals often "grow" into each other slightly, as we will see with reticulated Cerussite.

 

Dendritic refers to those plant-like trees you most commonly see of native elements, specifically copper. Other minerals also exhibit this habit. Not to be mistaken for minerals exhibiting a Plumrose habit (Described in more detail later on in this article).

 

Native Copper

Dendritic native Copper. Image from Wikipedia.

 

Reticulated mineral specimens are easily recognizable, and often stunning to behold. Characterized by intergrown, usually slender crystals, these specimens are prized if in pristine condition. One of the finest examples of this specific habit is Cerussite from Tsumeb in Namibia - Large, reticulated specimens in wonderful condition were recovered from the mine.

 

Reticulated Cerrusite

Reticulated Cerussite from Tsumeb, Namibia. Specimen currently resides in the Mineralogical Museum at Bonn in Germany. Image from Wikipedia.

 

Radiating, or divergent habits are well known, from Pyrite "suns" to Wavellite. Easily recognized by the way the crystals originate from a centre point and seem to "diverge" around it.

 

Wavellite

Wavellite showing a radiating or divergent habit. Image from Wikipedia.

 

Stellated minerals are characterized by crystals spreading outwards from a common point, much like the divergent habit, common to aggregated minerals. It's important to note that the "common point" need not be a single dot so to speak, but might be an entire crystal, whereas the divergent or radiating habit originates from a common point.

Drusy, or druse habits, are defined as a surface covered by a layer or plate of small crystals. There are many good examples of this - Perhaps the most well known are the Blue lace agate druses, with fine layers of small quartz crystals sitting on the pale blue lace agate.

 

Atacamite

Atacamite showing a drusy habit. Cape Minerals stock and photography.

 

Fibrous mineral habits are simple to spot - Think of an aggregate of fine fibres. Chrysotile, a mineral often referred to by a name we all know, Asbestos, is a fibrous mineral. Often the distinguishing factors for a fibrous habit can be taken as a mineral that a) forms part of an aggregate and b) has some flexibility to the fibres. 

 

Chrysotile

Fibrous Chrysotile. Image from Hyperphysics.

 

Globular minerals are similar to Botryoidal minerals (See below) with one important difference - They aren't grouped together to the extent that botryoidal minerals are.

 

Gyrolite

Gyrolite, a good example of a mineral with a globular habit. Image from Wikipedia.

 

Colloform is a term Dana's Mineralogy proposes to include Botryoidal minerals (Similar to globular, just more tightly packed), Reniform minerals (Similar to botryoidal, just more kidney shaped) and Mammillary, simply described as large rounded masses.

I have found that many people prefer to group these minerals under the general term botryoidal - As it's very difficult to differentiate between them.

 

Smithsonite

Botryoidal Smithsonite. Image from Wikipedia.

  

Habits describing "Platy" minerals 

These crystals are exactly as the term describes - Those minerals that are most often associated with the idea of "sheets", such as the micas. Indeed, one of the common terms used is derived from the mica minerals - Micaceous.

 

Foliated minerals can be separated fairly easily into plates - Thicker than those of the micas.  Also note that foliated crystals will overlap to a certain extent. Talc is a good example of a foliated mineral.

 

Talc

Foliated Talc. John Krygier photography, image from Wikipedia.

 

Micaceous minerals split into very thin, often flexible sheets. These are quite easy to recognize. The micas, as the term suggests, are the most common minerals showing this habit.

 

Muscovite

Muscovite showing the Micaceous habit. Image from Wikipedia.

 

Lamellar minerals are similar to both the micaceous and foliated minerals, in the respect that the "plates" overlap, although less so than with the foliated habit, and are also quite thin, but not as thin as the micaceous habit. Molybdenite is a good example of this - Look closely and you'll see the habit is different from those previously described to a certain extent.

 

Molybdenite

Molybdenite showing a lamellar habit. Cape Minerals stock and photography.

 

Plumrose habits are quite distinctive. Think of what we often mistake for fossilized ferns - Minerals that have grown through deposition from fluids travelling through cracks and fractures in the rock. The image below shows Pyrolusite that has crystallized in a plumrose habit, along a fracture in the host rock (Sandstone).

 

Pyrolusite

Pyrolusite showing a plumrose habit. Image from the US Geological Survey, bulletin 737, 1923 (Plate 9-A).

 

 

General terminology

Closing off the section of this article on the terminology used when describing crystal habits, let's look at some other terms that don't fit too easily into the other categories.

Massive refers generally to minerals with no particular crystal habit that is discernible to the eye. Almost all minerals have a massive variety - Quartz is one of the most common.

 

Chalcedony

Massive blue Chalcedony. Cape Minerals stock and photography.

 

Stalactitic refers to exactly what you'd expect - Stalactites! Formed through the deposition of minerals from fluids dripping from the roof, these are quite common in caves. Don't forget the difference between the two variants - Stalagmites grow on the floor and Stalactites on the roof.

 

Stalagtites

Stalactites. David Monniaux photography. Image from Wikipedia.

 

Geodes form when a cavity has been lined through the deposition of a mineral, however the cavity is only partially filled. When cutting these open you might observe on the edges several bands - Indicating changes to the concentrations of certain elements in the fluid during deposition. 

 

Rossasite

Rossasite and calcite lining a cavity. Cape Minerals stock and photography.

 

Banded habits are seen most often in agates - when you can clearly differentiate different bands, based either on colour or texture. Another term used fairly often is Concentric - Similar to banded, with the exception that the "bands" radiate outwards from a common centre.

 

Banded Agate

Banded agate. Cape Minerals stock and photography.

 

In closing..

Describing mineral habits is tricky. Often they are so similar distinguishing between them is all but impossible! This article also by no means covers every possible crystal habit - Just those a collector will commonly encounter. And that's ignoring the fact that many other descriptive terms are used, and will be used, that are not described in literature as time goes by.

 

The majority of the textual references are sourced from Dana's Manual of Mineralogy, 16th ed. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Comments
2019-08-03 09:53 AM
Great article and beautiful pics.   Thanks for sharing.
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