How do you know it is hexagonal?
If a mineral belongs to the hexagonal crystal system, you will be able to find only one 3-fold rotation axis OR only one six-fold rotation axis on well-formed crystals of that mineral.
How do you know it is tetragonal?
If a mineral belongs to the tetragonal crystal
system, you will be able to find only one 4-fold rotation axis OR only
one four-fold rotoinversionaxis
on well-formed crystals of that mineral.
How do you know where the crystallographic axes are?
The tetragonal crystal system has three crystallographic
axes. Two of them (a1
and a2) are the same length, and they are at 90 degrees to each
other. The third axis (c)
is of a different length, and it is perpendicular to the other two.
The “c-axis” will coincide with the 4-fold or 4-fold rotoinversion axis.
You can pick the two a-axes to be however you want them, as long as they
are perpendicular to the c-axis, and they are 90 degrees apart from each
other.The a-axes should either “come
out” of an edge between faces or the middle of a face.
How do you know it is orthorhombic?
If a mineral belongs to the orthorhombic crystal
system, you will be able to find either three two-fold rotation axes (with
or without mirrors perpendicular to them) OR only one 2-fold rotation axis
that has two mutually perpendicular mirror planes parallel to it.
How do you know where the crystallographic axes are?
The orthorhombic crystal system has three crystallographic axes (a, b, and c). They are all of different lengths, and they are at 90 degrees to each other. The crystallographic axes will either coincide with the 2-fold rotation axes OR they will be perpendicular to the mirror planes.
How do you know it is monoclinic?
If a mineral belongs to the monoclinic crystal
system, you will be able to find only one 2-fold rotation axis and/or only
one mirror.
How do you know where the crystallographic axes are?
The monoclinic crystal system has three crystallographic
axes (a, b, and c), all of different lengths.
The angle between the b-axis and the a-axis is 90 degrees, and so is the
angle between the b-axis and the c-axis.
However, the angle between the a-axis and the c-axis is at an angle different
from 90 degrees. The b-axis
will either coincide with the 2-fold rotation axis OR it will be perpendicular
to the mirror plane.The other axes
will be perpendicular to the b-axis.
They will often be parallel to edges of the crystal faces.
How do you know it is triclinic?
If a mineral belongs to the triclinic crystal system, you will be able to find only an inversion or 1-fold rotation axis.
The triclinic crystal system has three crystallographic axes (a, b, and c), all of different lengths, and all at angles different than 90 degrees. The axes are often parallel to edges between crystal faces. The c-axis is the longest and the a-axis is the shortest.