THE EXTERNAL FEATURES OF A CRYSTAL
The
external features of a crystal comprise the following:
Faces, Edges, forms,
habits and interfacial angles.
a)
Faces:
Faces
are flat and smooth surfaces bounding or enclosing a crystal. The faces on a crystal are usually well
arranged in a regular manner which is related to the internal arrangement of
the atoms. There are two kinds of faces
which are like and unlike faces
-
Like faces,
are those which have the same size and shape in a regularly developed crystal
(that is faces which have the same properties and dimensions).Like faces are
common in minerals that crystallize in the cubic system.
-
Unlike facesare
those which do not have similar sizes and shapes. They therefore have different patterns of
atoms within them.
b)
Forms:
Form
refers to the assemblage of faces within a crystal which together may wholly or
partially constitute the exterior of a crystal.
Some crystals can be made up entirely of like faces while other crystals
can be made up of like and unlike faces.
The form of a crystal can be described as open or close.
-
Closed form or Simple
form: A form is closed if it is made up
entirely of like faces (faces which close space by themselves)for example a
cubic form has 6 like faces.
-
Open form:
A form is said to be open if it has a combination of like and unlike faces for
example Beryl has an open form made up of both pyramidal and prismatic faces.
c)
Edges:
An
edge of a crystal is a line of intersection of 2 adjacent faces. The position of an edge depends on the
position of the faces that produce the intersection.
d)
Solid
angles or Nodes:
A
solid angle or node is formed by the intersection of 3 or more faces.
e)
Habit:
This
is the characteristic shape of a crystal produced by a variation in the number,
size and shape of its faces. For example a cube would assume a cubic habit
while octahedrons assume octahedral habits.
The habit of a crystal is influenced by the environment and conditions
under which the crystal grew or was formed.
For example Calcite usually has a rhombohedron habit but may assume a
scalenohedron.
f)
Interfacial
angles
An interfacial
angle is the angle between the normals or perpendiculars of 2 adjacent
faces.
Figure 3: Interfacial Angle
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Measuring Interfacial angles
Interfacial
angles are measured using an instrument called a Goniometer. There are two
kinds of goniometers which are the Contact and Reflective goniometers. The contact goniometer (Figure 4) is used for
large crystals while the reflective goniometer is used for small crystals with
flawless or smooth faces.
Figure 4: A Contact Goniometer
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How the Contact Goniometer is used
The
contact goniometer consists of two straight edged arms which are movable on a
pivot or screw and connected to a graduated arc similar to that of a protractor.
The measurements are carried out by
bringing the 2 arms firmly and accurately into contact with the adjacent faces
of the crystal (Figure 5) and the angle between read off from the graduated
arc.
The angle that is actually measured is
the internal angle (perpendicular to the 2 faces) and must be subtracted from
180o to obtain the interfacial angle.
Figure 5: Using the Contact Goniometer
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The
Law of constancy of Interfacial angles
In
1669, [1]Steno,
after studying the interfacial angles on different samples of quartz, put
forward the law of constancy of interfacial angles which states that:
“For all crystals of a given mineral, the
interfacial angles between any two given faces are always constant (the same)”
This
law only holds if they are made up of the same chemical composition and
measurements are done at the same temperature.
Geologists
have been able to predict that crystals are built up by an orderly arrangement
of atoms forming rows which correspond to crystal faces. The faces intersect at the edges which
correspond to the intersection of the atomic rows within the crystals. This means that for all crystals of the same
mineral, the faces and edges are always the same (as in the quartz crystal). Careful studies of crystals have shown that the
atomic structure for the crystals of any one mineral is fixed; consequently,
the positions of the faces of such crystals are fixed. Thus based on measurements, the tendency for
corresponding interfacial angles to be constant for all the crystals of a given
mineral brings about the “Law of
constancy of interfacial angles”.
Zonation:
A zone is a set of faces or a collection
of faces which are parallel to a given direction. This direction is termed the zone axis.
Figure 6: Zones and Zone axes
From
above, there are three zone axes (X, Y and Z) and therefore the faces can be
divided into three zones as follows:
-
Zone
1 consists of faces A, C, A1,C1; the zone axis is X
-
Zone
2 consists of faces A, B, A1, B1; the zone axis is Y
-
Zone
3 consists of faces B, C, B1, C1; the zone axis is Z
Faces
in the same zone must not necessarily be adjacent to each other. However, their edges must be parallel to each
other.
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