Physical Mineralogy (PM): The study of physical properties of minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals. Constituents of PM: Crystal Structure: The unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-‐range order and symmetry. Patterns are located upon the points of a lattice, which is an array of points repeating periodically in three dimensions. The points can be thought of as forming identical tiny boxes, called unit cells, that fill the space of the lattice. Lattice examples: Simple cubic
Crystal Habit: The typical appearance of crystals in a mineral. Ex1: Diamonds have an Octahedral Crystal Habit, as can be seen by the picture at the top left, in which 8 triangles stretch to the tip of the diamond.
Ex2: Mica has a Foliated (micaceous) Crystal Habit, meaning its habit is to form layers of thin sheets as seen below.
Twinning: When two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Exs:
Cleavage: The tendency of crystalline materials to split along definite crystallographic structural planes. Types of Cleavage: Basal: occurs parallel to the base of the crystal Cubic: occurs parallel to the faces of a cube in a crystal with cubic symmetry. (Ex: grains of table salt) Octahedral: forming octahedral shapes for a crystal with cubic symmetry. Diamond and fluorite exhibit perfect octahedral cleavage. Octahedral cleavage is seen in common semiconductors.
Dodecahedral: forms dodecahedra for a crystal with cubic symmetry. Rhombohedral: occur parallel to the faces of a rhombohedron. Calcite and other carbonate minerals exhibit perfect rhombohedral cleavage. Prismatic: cleavage parallel to a vertical prism. Luster: A description of the way light interacts with the surface of a mineral. Types of Luster include: Adamantine (Ex: diamond) Dull Greasy Metallic Pearly (Ex: Muscovite) Resinous (Ex: Amber) Silky (Ex: Minerals with extremely fine fibers) Submetallic (similar to metallic, but duller and less reflective) Vitreous (Ex: Quartz or any mineral that looks like glass) Waxy Color: color of the mineral (it really is that simple) Streak: the color of the powder produced when a mineral is dragged across an unweathered surface.
Hardness: Is based on the Mohs scale of mineral hardness. The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another. The Mohs scale simply records which mineral is harder, while the absolute hardness, measured by a sclerometer, shows the comparative ratio of hardness. In the figure below, it can be seen that on the Mohs scale, Diamond is harder than Corundum, which is harder than Topaz. However, the absolute hardness shows that Diamond is about 3.75-‐4x (this scale says diamond has absolute hardness 1500, some others say 1600) harder than Corundum, which is 2x harder than Topaz.
Specific Gravity: The ratio of the density of a mineral to the density of a reference substance. The reference substance is almost always water. SG is measured by a pycnometer.
SG= Specific Gravity; V= Volume; p(it’s actually greek letter rho, but it looks like a p)= density; g= gravitational acceleration (9.8 m/s2); wV= weight in vacuo (latin for in a vacuum ); wA= weight in air weight= mass * g