Complex Topaz Crystal

WHAT IS
CRYSTAL HABIT?

Crystal Habit is a description of the shapes and aggregates that a certain mineral is likely to form. Often this is the most important characteristic to examine when identifying a mineral. Although most minerals do have different forms, they are sometimes quite distinct and common only to one or even just a few minerals. Many collectors strive to collect mineral specimens of certain typical and abnormal habits.

There are basically two major divisions to keep in mind when discussing crystal habits. First, there are crystallographic forms whose names often separate the true Rock Hounds from the amateurs. These crystal forms are controlled by the structure and therefore the symmetry of the crystal. Secondly there are more descriptive terms that quickly portray the character of the crystal or aggregate crystals. The shape and character of the aggregate may be just as distinctive as an individual crystal's shape.

The many different crystallographic forms can look rather complicated to novice collectors. However, understanding them can greatly increase the chance of correctly identifying minerals and appreciating the specimens that belong to your collection. There are either open forms or closed forms.

These open forms can NOT form a whole crystal by themselves and need other forms to finish the crystal.

The Pedion
The Pinacoid
The Dome
The Sphenoid
The Prism
The Pyramid

Closed forms can form a whole crystal completely by themselves.
The Isometric Forms.

The Cube
The Octahedron
The Rhombic Dodecahedron
The Pyritohedron
The Deltoidal Dodecahedron
The Tetartoid
The Tetrahedron
The Diploid
The Gyroid
The Tetartoid
The Trapezohedron
The Hexoctahedron
The Tetrahexahedron
The Tristetrahedron
The Trisoctahedron
The Hextetrahedron

Non-isometric Closed Forms:

The Scalahedron
The Rhombohedron
The Trapezohedrons
The Dipyramids
The Disphenoid

The Open Forms:
The Pedion: In a mineral with low symmetry, a pedion is a possible face. It is a flat face that is not parallel or geometrically linked to any other faces. It can be found on the top of prismatic crystals that lack a perpendicular mirror plane or a two fold rotational axes. Elbaite is a mineral known to form a pedion face. The pedion is possible only on minerals that lack symmetry operations parallel to the pedion face and lack a center.
The Pinacoid: The pinacoid is composed of only two parallel faces. Prominent pinacoids will form platy or tabular crystals such as in wulfenite, pictured) and can be thought of as the top and bottom of a book. Pinacoids also routinely terminate the top and bottom of prismatic crystals such as apophyllite. It is caused by either a mirror plane reflecting the face from one side of the crystal to the other, a two fold rotational axes rotating the face from one side to the other or simply a center of symmetry (see the triclinic symmetry class) where an inversion of the face takes place through the center of the crystal. It is possible in most every crystal except those lacking the required symmetry of a solitary mirror, two fold axes or a center.
The Dome and Sphenoid: These forms are found in monoclinic and orthorhombic minerals They consist of only two intersecting faces that are caused by either a mirror (the dome) or a two fold rotational axes (the sphenoid). These two forms could, more graphically, be called "The Tents" because they look like simple pup tents. The rare sulfate mineral pickeringite forms sphenoids and the silicate topaz commonly forms domes (the prominent slanting face pictured above). Domes and sphenoids can also form on the sides of crystals as well as terminations for prismatic crystals. If a dome or sphenoid is on the side of a crystal it might be confused with two of the four faces of a prism (look for similar faces on the other side of the crystal axes).
The Prism: A prism is a set of faces that run parallel (meaning they never converge) to an axes in the crystal. There can be three, four, six, eight or even twelve faces that can form a prism. All the faces of one prism must be separated form each other by a specific amount of degrees or they are not of the same prism. For the three sided prism there must be 120 degrees separating each face, for the four sided prism, 90 degrees, for the six sided prism, 60 degrees, etc, etc. Four sided orthorhombic prism faces are not necessarily separated by 90 degrees and are an exception to the rule, however two adjacent angles must equal 180 degrees. In viewing a prism lengthwise, perfect geometrical shapes should be seen such as a the cube, the hexagon, the octagon or the rhombus (monoclinic crystals). Prisms are not possible in isometric or triclinic minerals. Quartz forms two sets of three sided prisms. Prismatic habit does not mean that the mineral faces are prism faces for they could be two sets of pinacoids that are parallel to the same axes.
The Pyramid: The pyramids are easy to understand since most people are familiar with the Egyptian and Mexican pyramids. Pyramids, like prisms, are composed of either three, four, six, eight, twelve or even sixteen faces. The faces are separated by a specific amount of degrees as with the prism described above. The faces are not parallel and in fact converge around a crystallographic axes forming a multi-sided tent (compare the dome and sphenoid) with a point unless capped by a pedion or pinacoid. The steepness of the faces of a single pyramid must remain constant. Many minerals may contain a tapering termination of different inclines due to the presence of different pyramids. The mineral uvite can be terminated by a three sided pyramid. Pyramids are not possible on isometric, monoclinic or triclinic minerals.

The Closed Forms:

The Isometric Forms

The Cube

The cube is familiar to everyone as a symmetrical six sided box. Although the cube is familiar it is hard to identify its faces on a highly modified crystal that has faces from other forms. Remember it has eight points, six faces and twelve edges that are perpendicular to each other forming 90 degree angles and square cross-sections. The faces are usually square, but if modified can be other geometric shapes. The cube can only be formed by isometric minerals. Galena is a mineral that forms cubes.

The Octahedron

The octahedron is a symmetrical eight sided shape that may look like two four sided pyramids lying base to base. But closer inspection will show that each set of opposing points is exactly the same and could serve equally well as the "top" and "bottom" of the two pyramids. In fact there is no top or bottom on isometric forms. The faces are equilateral triangles unless modified. There are six points, eight faces and twelve edges. Each face is parallel to the opposite face. The octahedron is related to the cube by placing each point of an octahedron at the center of each face of a cube. The mineral diamond commonly forms octahedrons.

The Tetrahedron

The tetrahedron has only four equilateral triangular faces (unless modified), four points and six edges and when sitting on one face looks like a trigonal pyramid. But every point of the tetrahedron can serve as the top of the pyramid because all four points are identical. The tetrahedron is related to the octahedron by placing the six points of the octahedron in the middle of the six edges of the tetrahedron. In fact the four faces of the tetrahedron would be parallel to four of the eight faces of the octahedron. The tetrahedron can be thought of as having removed every other face from an octahedron and extending the four faces that are left to complete the closed tetrahedron. Tetrahedrons only form in isometric minerals that have four fold rotoinversion axes. The mineral tetrahedrite as its name infers, forms mostly tetrahedrons.

jpg- Dodecahedrons

Another isometric shape the dodecahedron is twelve sided and there are four basic types. The first dodecahedron has symmetrical pentagonal faces (five edged polygons). The second dodecahedron has delta shaped faces (four edged polygons). The third dodecahedron has asymmetrical pentagonal faces. The last dodecahedron has rhombic faces (four edged diamond-shaped polygons).

The pentagonal dodecahedron is termed a pyritohedron because it is most commonly found in the mineral pyrite. The pyritohedron is related to the cube by orienting two of the pyritohedron's faces essentially with each cube face. Two of the pentagonal faces touch bases in a line or edge that is parallel to the cube faces. This edge is responsible for the striations seen on pyrite's cubic faces when the two forms are trying to form together.

The deltoid dodecahedron has four sided delta shaped faces. It is derived from a tetrahedron with each of its four faces replaced by three delta shaped faces. The deltoid dodecahedron is a rare dodecahedron and is usually only seen modifying the tetrahedral crystals of tetrahedrite and tennantite.

The tetartoid is a 12 sided form that is very rarely seen. The faces are asymmetrical pentagons. The mineral cobaltite has been known to form this type.

The rhombic dodecahedron is found commonly in the garnets as well as other minerals including fluorite (pictured above left on rhodochrosite). Each of the faces of this dodecahedron have a parallel face across from them. The faces are rhombic or diamond-shaped, like the diamonds in a deck of cards. A rhombic dodecahedron has fourteen points, twelve faces, and twenty four edges. A rhombic dodecahedron is related to the octahedron by placing a dodecahedral face over each of the twelve octahedral edges. The generic use of the word dodecahedron is usually applied to this rhombic form.

The Gyroid

The gyroid has 24 non-symmetrical pentagonal faces and is a rare form. Cuprite is the only mineral that commonly forms gyroids and usually they are modified by other forms.

The Diploid

The diploid is also a form that is rarely seen. It has 24 four edged faces that are formed from the splitting of the pentagonal faces on the 12 sided pyritohedron. The split is from the center of the pentagons base edge to the top point of the pentagon. The mineral skutterudite will occasionally form this rare habit.

The Hexoctahedron

The hexoctahedron is a richly faceted form with a total, if fully formed, of 48 triangular faces. It is related to the octahedron by the dividing of each of the octahedron's eight faces into six triangular faces. Diamond is found in this form however the crystals tend to appear rounded.

The Tetrahexahedron

This form is composed of 24 triangular faces and is related to the cube by dividing each of the cubes six faces into four faces. The edges of the cube remain in place and form the bottom edge of each isosceles triangle. This form will still give an overall cubic look but with each cube face pushed outward to a four sided squat pyramid. The mineral fluorite has shown this form but the tetrahexahedral faces are usually just modifying the cubic faces.

The Trapezohedron

The trapezohedron has 24 trapezium or deltoid shaped faces. They can be thought of in two ways. Either they are dividing each face of an octahedron into three faces or they are dividing each face of a cube into four faces. Either way the result looks a little more like a cube than an octahedron however no edges of the cube or the octahedron remain. The mineral analcime is commonly seen in this form.

The Tristetrahedron

The tristetrahedron has 12 faces that are shaped like extremely acute isosceles triangles. It is formed by the dividing of a tetrahedral face into three faces. The original edges of the tetrahedron serve as the bases for the triangular faces of the tritetrahedon. The mineral sphalerite can have faces of the tristetrahedron.

The Trisoctahedron -jpg

The trisoctahedron has 24 faces that are shaped like extremely acute isosceles triangles. It is formed by the dividing of an octahedal face into three faces. This is different from the effect created in the trapezohedron in that these faces leave the original edges of the octahedron in place and use them as their base for their squashed triangular shapes. The mineral Diamond can have octahedral crystals modified by the trisoctahedron but rarely is this form fully developed.

The Hextetrahedron

This is also a rare form that almost is never fully developed. It is composed of 24 triangular faces and is formed from the dividing of each face of the tetrahedron into six faces. Its faces are sometimes seen on the highly modified crystals of the mineral sphalerite.

The Non-isometric Closed Forms:

The Rhombohedron: The rhombohedron is difficult to describe. The best way to visualize it is to think of a partially squashed box. In other words a square box that has been sat on and deformed in one direction only. The deformation occurs in the direction of one of the three diagonals that run through the cube from one corner to the other. The steepness of the rhombohedron is variable for different minerals and even within the same mineral a crystal may have several different rhombohedrons of different inclines. The diagonal of "deformation" becomes the prominent axes of symmetry and is trigonal with a three fold rotation about the axes. Six mirror planes protrude through the middle of the faces and edges that converge on the top and bottom points. There are six faces (like a cube) and if the "deformation" is not severe then the crystal can appear to be pseudo-cubic. However careful measurements of angles will differ from 90 degrees and the square-like faces will actually be shown to be rhombic. With severe "deformation" the crystal can appear tabular. The rhombohedron is the only non-isometric closed form with parallel faces. Rotoinversion (discussed below) is the symmetry operation that produces the rhombohedron. The minerals siderite and rhodochrosite are commonly found in rhombohedral crystals.
The Scalahedron: The scalahedron is a general term for three crystal forms differentiated by the symmetry of the prominent axis. The prominent axis can have either a four fold axis of rotoinversion (an eight faced scalahedron) or a three fold axis of rotoinversion (a twelve faced scalahedron). Rotoinversion is the symmetry operation that produces the scalahedron. A scalahedral face (a scalene triangle) if rotated by a fraction of a circle around the crystal and then inverted through the center of the crystal will form a new scalahedral face. If from this new position the face is rotated by the same amount of degrees and again inverted, a third face forms. The entire crystal can be formed in this way. Calcite is famous for forming well shaped trigonal scalahedrons and fine examples of scalahedrons of hematite are actually rare.
The Trapezohedrons: The trapezohedron is a general term for two crystal forms differentiated by the symmetry of the prominent axes. The prominent axes can have either a three, four or six fold axes of rotation. The names of the three different forms are preceded by either trigonal, tetragonal or hexagonal respectively (ie. hexagonal trapezohedron) to avoid confusion. This group of forms should not be confused with the isometric trapezohedron which is not related but has trapezium shaped faces. The forms are produced by two fold axis perpendicular to the prominent axes. There is no mirror plane perpendicular to the prominent axes and the always four sided faces above and below meet with two slanted angles. The faces of a trapezohedron are never parallel to any other faces. The mineral phosgenite is sometimes terminated with the rare tetragonal trapezohedron.
The Dipyramid: The dipyramids differ from the trapezohedrons by having a perpendicular mirror plane that reflects a simple pyramid above to form the simple pyramid below. The dipyramids are routinely seen in many prismatic minerals as the terminations of their crystals. The various dipyramids are differentiated like the trapezohedrons by the symmetry of the prominent axes and are labeled accordingly. However a few dipyramids are actually doubles and are labeled as a dihexagonal dipyramids for example. The dipyramids are distinguished from rhombohedrons by a lack of any parallel faces. The mineral hanksite can be terminated by an hexagonal dipyramid.
The Disphenoid -jpg: A disphenoid is a composition of two sphenoids with their open ends turned toward each other and one sphenoid twisted 90 degrees to the other. The result is a wedged shaped crystal with either scalene or isosceles triangular faces. The disphenoid looks like a tetrahedron but is not as symmetrical as not all axes are of equal length and the tetrahedron has equilateral triangular faces. The mineral chalcopyrite forms usually modified disphenoids.
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