chapter 3 Crystal systems

Material and structure

Crystal face

A plane surface on the exterior of a crystal where unrestricted crystal growth has ceased.

CRYSTAL FORM

A set of crystal faces dictated by the internal structure of a single crystal e.g. bipyramidal form.

Crystal habit

the form, or combination of forms, exhibited by any crystal that determines its characteristic shape

It reflects the arrangement of its crystal faces and internal structure. .

surface markings

etch pits growth marks fine parallel lines; surface grains or striation

Prismatic Habit

When a crystal forms in such a way that the prism form is the most dominant

Crystal shape resembling a prism.

Pinacoid

A crystal form composed of a pair of parallel faces.

Prism

A crystal form consisting of 3 or more faces parallel to a line

tabular habit

short prism faces and large pinacoid faces

rhombic

3 unequal axes at right angle to each other with the same length

Cubic

Diamand= octahedron

Spinel = octahedron

Garnet = dodecahedron

Fluorite = cubic

Tetragonal

Zircon

Trigonal

Corundum

Quartz; amethyst, Chalcedony, Citrine

Tourmaline

3-fold

Hexagonal

Beryl; emerald, aquamarine, heliodore, Morganite

6-fold

Orthorhombic

Peridot

Topaz

Chrysoberyl

Iolite

Tanzanite

Monoclinic

feldspar

Jade

Nephrite

Moonstone

Triclinic

Feldspar

crystal reference axes

represent the patterns repeat in the lattices

Indicates the relative distance at which the crystal pattern repeats in that directionand the angles between them.

Crystal system: Cubic

a=b=c

All have the same length, all angles at 90 degrees, and the symmetry is high, resulting in a variety of shapes such as cubes and octahedra.

crystal system: Tetragonal

one difference with Cubic

the C axe can be longer or shorter

a=b but not equal to C

all angles are at 90 degrees to each other

Crystals often have square prims and can exhibit a variety of forms, including elongated shapes.

crystal system: Trigonal

a=b=d ongelijk aan C

Angles a,b and b,d = 120 degrees

3-fold

The identical pattern repeats in three directions at 120 degrees

with a different pattern repeats in the fourth direction(C axe) at 90 degrees to the red axes.

C-axe is a direction of 3-fold

symmetry, with a hexagonal arrangement of lattice points.

Hexagonal system

a=b=d ongelijk aan C

Angles a,b and b,d = 120 degrees

3-fold

The identical pattern repeats in three directions at 120 degrees

with a different pattern repeats in the fourth direction(C axe) at 90 degrees to the red axes. C-axe is a direction of 3-fold

The hexagonal crystal system features two distinct lengths where a=b, but both are unequal to C. The angles between a and b are 120 degrees, and the structure exhibits a three-fold symmetry along the C axis, which is perpendicular to the a-b plane.

Orthorhombic crystal system

A niet b niet gelijk c

all angles are 90 degrees to each other

different patterns repeat in three directions

rhomb shaped prisma

basal cleavage

looks a bit like a Matchbox

The orthorhombic crystal system is characterized by three mutually perpendicular axes of unequal lengths (a, b, and c). It exhibits distinct rhomb-shaped prisms and often shows basal cleavage.

Monoclinic System

basically the same as orthorhombic

Difference angles A,C are not equal to 90 degrees

one axe is at 90 degrees to the C axe, hence the skewed shape Often, it has shapes based on a rectangle with a skewed appearance

The monoclinic crystal system has three unequal axes where two axes are perpendicular while the third is inclined, resulting in non-right angles between them. This leads to a skewed rectangular appearance in its crystal forms.

Triclinic Crystal System

angles of a,c and a,b not equal to 90 degrees

basically the same as orthorhombic

Often has shapes based on a rectangular box skewed in two directions.

The triclinic crystal system is characterized by three unequal axes with no angles equal to 90 degrees, resulting in a skewed rectangular box shape. It lacks any symmetry and exhibits the most general form of crystal structure.