Crystals

 A crystal is a solid in which the atoms are arranged in orderly, repeating patterns.  Crystalline structure can be seen either on the inside or the outside of a mineral.  Minerals that form with large amounts of space are able to arrange themselves in crystal form on the outside.  If a mineral forms with limited space, the crystal structure is seen on the inside. We cannot see this with the naked eye.

Crystals from Magma  Magma is hot melted rock. (when it reaches the earth’s surface it’s called lava)  When magma cools it forms crystals  The rate at which the magma cools determines the size of the crystals

These are all pictures of granite. Why do they all look different?

Crystal Size  The elements present in the magma will determine which minerals form  The slower the magma cools, the larger the individual crystals  The quicker the magma cools, the smaller the individual crystals

Crystals from Solution  Another way we can get crystals is from a solution  Remember, a solution is a mixture of two or more things that are not chemically combined  Sea water is a solution  When the water evaporated, crystals are left behind.

Cubic Crystals

 Cubic crystals have all 90 degree angles with all sides of equal length

Examples of cubic crystals Galena

Cubic Crystals Halite

Hexagonal Crystal  6-sided crystal  Sturmanite

Hexagonal Crystals  Quartz

Tetragonal Crystals  Much like the cubic, except one side is longer than the others, like a rectangle.  Zircon is a good example

Orthorhombic  Think of this as a brick. It’s like the last one, a rectangle that has been flattened so that its thicknesses are different.

Orthorhombic has all 90° angles still. The rectangle is flattened either a little or a lot, as seen here in a sample of barite. All 3 (pairs of) sides have different lengths.

Monoclinic  Take the ‘brick,’ orthorhombic, and slide it askew so that only one angle is left at a 90° angle  Orthoclase is an example of a monoclinic crystal shape

Monoclinic crystal: Orthoclase

Triclinic  This is an unsymmetrical as a mineral can get  No 90°angles  Rhodonite is an example

More rhodonite

Rhodonite

Overview  All minerals have crystalline structure; either on the inside or outside  With plenty of room they form on the outside  If confined the crystals are in the atomic structure  Crystal size is dependent on how quickly (small crystals) or slowly (large crystals) they form  Crystals can form from magma or from solution  There are 6 basic crystal shapes  5 of these are versions of cubes, the 6 th is a hexagon  All minerals are composed of crystals. Minerals are “naturally occurring, inorganic solids with definite chemical composition and orderly arrangement of atoms.”