Crystal Lattice - The way the atoms are ordered in crystals forms a 3-D lattice, which controls mineral properties.

Crystal Form - The external expression of the internal arraignment of atoms. This is related to how fast the mineral cools, and how much room the mineral has to grow.

• Slow cooling, lots of room = Large, pretty crystals
• Fast cooling, little room = crystals not visible to the naked eye
  

Crystal faces (surfaces) can be described thusly:

• Euhedral – Good crystal faces; grown in open cavity. (less common)
• Anhedral – No crystal faces; grown in tight space. (most common)
• Subhedral – Between the two.

Luster - Appearance of a mineral in reflected light. There are two basic categories:

• Metallic (looks like a metal)
• Non-metallic
  • vitreous
  • pearly
  • silky
  • waxy
  • resinous
  • dull
  • earthy
    
  
  

Color - Highly variable within minerals due to slight chemical changes. Color is the least reliable of all the mineral properties. Check out the Smithsonian's Grossular Garnet page for a range of garnet colors: http://geogallery.si.edu/index.php/10002799/grossular-garnet

Streak - The color of a mineral in its powdered form. There are two types:

• Congruent streak – Streak color same as mineral.
• Incongruent streak – Streak color different than mineral.

Cleavage - is the tendency to break along planes of weak bonding. These planes are related to the internal structure of the mineral (crystal latice). For example, bonding within layers, like within a single chain of tetrahedra or a sheet of tetrahedra, is very strong. Bonding between these layers is usually very weak. A mineral will break along these weaker areas. The result is a flat, shiny surface.

• One cleavage plane - sheet structures produce one cleavage plane.
  
  
• Two cleavage planes - chain structures produce two cleavage planes, but the angle between the intersection of these planes will depend on the chain orientation.
  • Single chain structures will produce two planes @ 90°
  • Double chain structures will produce two planes @ 120° & 60°
    
    
    
• Three planes of cleavage - minerals with three planes of cleavage usually develop in non-silicate minerals such as calcite, halite, gypsum, etc. The development of cleavage planes is dependent upon the arrangement of the atoms in the structure.
  • Three directions at 90º - halite is an example.
  • Three directions NOT at 90º (dodecahedral structure) - Calcite and gypsum are examples.
    
    

    Halite	Calcite Structure

Fracture  - Minerals with equal molecular bonds in all directions do not have any planes of weakness for breakage to easily follow. Instead, fracture develops. Minerals with a three-dimensional, single island, or double island structure have fracture. There are two types:

• irregular
• conchoidal

Other Properties:

• Taste
• Smell
• Elasticity
• Malleability
• Feel
• Magnetism
• Double Refraction
• Reaction To Hydrochloric Acid
• Luminescence Or Fluorescence

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