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.

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.
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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.
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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).
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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.
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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 multisided 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.
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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 crosssections. 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.
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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.
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The Tetrahedron
jpg

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.
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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 diamondshaped 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 diamondshaped, 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.
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The Gyroid

The gyroid has 24 nonsymmetrical pentagonal faces and is a rare form.
Cuprite
is the only mineral that commonly forms gyroids and usually they are modified
by other forms.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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The Nonisometric 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 pseudocubic. However careful measurements of angles will differ
from 90 degrees and the squarelike faces will actually be shown to be
rhombic.
With severe "deformation" the crystal can appear tabular.
The rhombohedron is the only nonisometric 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.
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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.
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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.
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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.
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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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Color

Luster

Diaphaneity

Crystal Systems

Technical Crystal Habits

Descriptive Crystal Habits

Twinning

Cleavage

Fracture

Hardness

Specific Gravity

Streak

Associated Minerals

Notable Localities

Fluorescence

Phosphorescence

Triboluminescence

Thermoluminescence

Index of Refraction

Birefringence

Double Refraction

Dispersion

Pleochroism

Asterism

Chatoyancy

Parting

Striations

Radioactivity

Magnetism

Odor

Feel

Taste

Solubility

Electrical properties

Reaction to acids

Thermal properties

Phantoms

Inclusions

Pseudomorphs

Meteoric Minerals