​​Describing crystal habit

The habit of minerals

As the panel to the right indicates, mineral habits are tentative and variable things - more a tendency than an obsession. Habits are also difficult to define and like colour and lustre and other properties of minerals, one habit grades almost imperceptibly into another - something that's true of children as well, one might say. 
Over the years, though, many different mineral classifications of mineral habit have been developed, each with its own set of classes, but almost all without any structure to guide the user. The field of mineral habits is therefore something of a minefield that is overdue for some clearing and tidying up.

It probably helps to acknowledge that crystal habit may be expressed to different degrees of clarity. Thus we can recognise:

• euhedral crystals, which have a well-formed and coherent crystal shape;
• anhedral crystals, which show no recognisable or consistent shape; and
• subhedral crystals, which lie somewhere between these two, in having some elements of a coherent shape.

It also need to be recognised that some habits apply to individual crystals and others to aggregations or masses. Within the former, some habits are symmetrical, while others are not. Within the latter, some are composed of linear forms, albeit radiating or irregular; others are essentially blocky (composed of blocks or balls) and yet others are formless and more like a skin or a crust.
 
As it is, this sort of logic has not yet been applied with any degree of rigour and we're stuck with the mess that we have. Here, we list a dozen of the more recognisable and common crystal habits, and give some explanation of their form. Note that this list is far from complete and you may encounter other habits, or none at all. In this last case, the mineral would usually be described as massive. 
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Examples of crystal habits

Cubic
Six faces of roughly equal dimensions, at right angles to each other. 
Examples: pyrite (left), fluorite, halite.

Tetrahedral
Four triangular faces; relatively rare habit.
​Examples: magnetite (left), chalcopyrite

​Rhombohedral
Six faces, all rhombuses, like a squashed block pushed obliquely. 
Examples: calcite (left), siderite, rhodocrosite

Dodecahedral
Twelve equal five-sided faces 
Examples: garnet (left)

Prismatic
Multi-faced rhombs with elongated crystals in one direction and opposite faces parallel
Example: tourmaline (left), hornblende, augite

Bladed
Elongated rhombs, with one very short dimension; may end in a point
Examples: kyanite (left), actinolite, stibnite

Acicular
Long, tapering, needle-like crystals; sometimes arranged in a fan shape. 
Examples: rutile, tourmaline, gypsum (left)

Dendritic
Branching, irregular crystals, like the branches of a tree. 
​Examples: copper, gold, manganese (left)

Botryoidal (aka globular, mammilary)
Irregular, rounded globules. Applied to mineral aggregates.
Examples: hematite, malachite, some varieties of quartz

Fibrous
​Fine, fragile, fibre-like crystals, often radial - i.e. radiating from a central point. 
Examples: forms of asbestos (e.g. tremolite, actinolite, chrysotile), marcasite.

Foliated (aka micaceous)
Thin parallel sheets or layers, often splitting easily along the plane.
Examples: mica, chlorite

One of the most striking features of minerals, especially in crystal form, is their shape. Together with their lustre and colour it's often what catches the eye and gives them beauty, It's tempting to assume that, along with colour and lustre the shape is what will reveal its identity - for surely each mineral grows according to a set 'recipe' which decides its crystal form. 

To an extent, this is certainly true. One reason is that the growth of any crystal occurs through the process of nucleation, in which new atoms and molecules are attached to those that already exist. In a silicate, for example, a silicon ion joins to a oxygen to the silica, until a new layer of the lattice is built up, each piece locked in place. And because of this, each new layer mimics what went before, preserving the shape of the crystal face.

Developing from this, there's another factor that keeps crystals honest and makes them grow the  'correct' shape. This is the effect of lattice density - i.e. the number of lattice points (an exposed charge on an atom or ion) per unit area of the crystal face. If the density is high and there are a large number of lattice points that have to be filled, the  face grows slowly, simply because more ions need to be found  to fill the spaces. In the same medium, if the density is low, the face will grow rapidly, because it has fewer places to fill. 

We can state this another way, and that is that crystals develop parallel to the planes of highest lattice density. That probably seems contradictory at first but a little thought shows that it's still consistent with Bravais Law. As a low density face piles on the layers and advances, the faces on either side extend. If they didn't holes would appear!

Thus are different mineral shapes born. For example, if the faces in the mineral all have much the same lattice density, the mineral will grow more-or-less equally in all directions and what's called an equant form will be maintained (see the figure below). 

Equant habits are seen the cubic, tetrahedral and dodecahedral habits illustrated to the left.

On the other hand, if some faces have a sparser (less dense) lattice network than others, the mineral will tend to grow out from those faces as it grows, while the faces on either side are extended - and thus an increasingly elongated crystal will develop.

Examples of this are seen in the prismatic, bladed, acicular and foliated habits shown to the left.

But we aren't finished yet, because from all this, a further little twist to the logic of crystal growth ensues, which may stretch the mind even more. For it means that the shape of minerals can change as they grow, because growth causes some of the faces to be swallowed up. And it's the slow growing faces that survive, and the fast growing faces that are lost. 

Confused? Then consider the figure below. ​

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As can be seen, faster growth on the shorter, diagonal faces of this mineral, compared to that on the longer faces, would mean that as those diagonal faces grow outwards, they become shorter and shorter until they disappear. The final crystal thus has a different shape, and fewer faces, than when it started growing. This is one of the reasons why we often  observe that small crystals of a mineral  have more faces than larger ones.​

All this, it has to be said, is only true if everything else is equal - in particular that nothing else interferes with mineral growth. But nature rarely runs so smoothly, and much of the time stuff happens, and things go wrong. Amongst others, the hazards faced during crystal growth include:

• the presence of adjacent crystals that limit the space for growth;
• irregularities or variations in the composition supply of the necessary elements that impede growth; 
• the presence of impurities that become bound up in the structure of the growing crystal and distort its growth;
• changes in the temperature or fluidity of the growth medium that.

Because of things like these minerals don't always follow their instincts and grow according to plan. They have to compromise and adapt. How they grow is still constrained by those same 'laws' - by the structural arrangement that their chemistry imposes on them. But their shape can be distorted and curtailed. Which is why, like humans, few crystals are perfect.

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