GEOLOGY - CHAPTER2(EKSECS) Flashcards
is the systematic study that extensively covers description, crystallography, physical,
chemical and environmental features of all minerals.
Mineralogy
is the branch of science concerned with the formation, structure, and properties of
crystals. Also known as study of crystals and crystal formation.
Crystallography
5 Symmetry Elements of Crystallographic Systems
- Identity (E)
- Proper Axis/Rotational axis of Symmetry (Cn)
- Plane of Symmetry/Mirror Plane (σ)
- Center of Symmetry/Inversion Center(i)
- Rotoinversion (n̅ )
- Simplest symmetry element
- Corresponds to the operation of doing nothing to the object
- Every object has identity
- Asymmetric objects are defined as having just identity.
Identity(E)
- An imaginary line passing through the center of mass around which a rotation by 360°/n leaves an
object in an orientation indistinguishable from the original. - C1=E
Proper Axis/Rotational axis of Symmetry (Cn)
An object that requires rotation of a full 360° in order to
restore it to its original appearance has no rotational symmetry.
1-Fold Rotation Axis
If an object appears identical after a rotation of 180° that
is twice in a 360° rotation, then it is said to have a 2-fold rotation axis (360/180 =
2). A filled oval shape represents the point where the 2-fold rotation axis intersects
the page.
2-fold Rotation Axis
Objects that repeat themselves upon rotation of 120° are
said to have a 3-fold axis of rotational symmetry (360/120 =3), and they will repeat
3 times in a 360° rotation. A filled triangle is used to symbolize the location of 3-fold
rotation axis.
3-Fold Rotation Axis
If an object repeats itself after 90° of rotation, it will repeat
4 times in a 360° rotation, as illustrated previously. A filled square is used to
symbolize the location of 4-fold axis of rotational symmetry.
4-Fold Rotation Axis
- If rotation of 60° about an axis causes the object to repeat
itself, then it has 6-fold axis of rotational symmetry (360/60=6). A filled hexagon is
used as the symbol for a 6-fold rotation axis.
6-Fold Rotation Axis
- An imaginary plane divides the crystal in two parts in such a way that one part is mirror of the other,
the crystal is said to have a plane of symmetry. - Reflection Operation: Two parts on object after reflection through the plane produce the
indistinguishable orientation. - σ2=E
Plane of Symmetry/Mirror Plane (σ)
is such a point that any line drawn through it touches the
surface of crystal at equal distance in both directions.
Center of Symmetry/Inversion Center(i)
A combination of rotation with a center of inversion.
Rotoinversion (n̅ )
axis is the same as a center of symmetry,
1-fold rotoinversion
The operation of 2-fold rotoinversion involves first rotating the object by
180o
then inverting it through an inversion center.
2-fold Rotoinversion
This involves rotating the object by 120
o
(360/3 = 120), and inverting through
a center.
3-fold Rotoinversion
This involves rotation of the object by 90o
then inverting through a center.
4-fold Rotoinversion
- A 6-fold rotoinversion axis ( ) involves rotating the object by 60o and
inverting through a center.
6-fold Rotoinversion
is a set of axes.
crystal system
is obtained by attaching atoms, groups of atoms or molecules. This structure occurs
from the intrinsic nature of the constituent particles to produce symmetric patterns.
Crystal Structure
SEVEN PRIMITIVE CRYSTAL SYSTEM
- Isometric (cubic)
- Tetragonal
- Orthorhombic
- Hexagonal
- Triclinic
- Monoclinic
- Rhombohedral (trigonal)
All three axes are equal in length and all are perpendicular to one another.
Isometric (cubic)
Two of the three axes are equal in length and all three axes are perpendicular to one
another.
Tetragonal
All three axes are unequal in length and all are perpendicular to one another.
Orthorhombic
Of four axes three are equal in length, are separated by equal angles and lie in the same
plane. The fourth axis is perpendicular to the plane of the other three axes. Hexagonal
cells have lattice points in each of the two six-sided faces.
Hexagonal
All three axes are unequal in length and none is perpendicular to another.
Triclinic
All three axes are unequal in length and two axes are perpendicular to each other.
Monoclinic
All three axes are of equal length and none of the axes is perpendicular to another but the
crystal faces all have the same size and shape.
Rhombohedral (trigonal)
