Minerals with perfect cleavage break along a smooth, flat plane, while those with poor cleavage break in a more irregular fashion. Some minerals do not contain zones of weakness either because all of the bonds are the same strength or the weaker bonds are not aligned within a plane. If this is the case it will not have cleavage, but rather breaks in a random and irregular fashion.
Make sure to distinguish cleavage from crystal form. Crystal form occurs as a mineral grows , while cleavage only forms as a mineral breaks. See Figure 7. A mineral may have one or more cleavage planes. Planes that are parallel are considered to be in the same direction of cleavage and should only count as one.
These two-dimensional surfaces are known as cleavage planes and are caused by the alignment of weaker bonds between atoms in the crystal lattice. Cleavage planes are distinguished from fracture by being smooth and often having reflective surfaces.
Cleavage terms only use if cleavage planes can be recognised : Perfect - Produces smooth surfaces often seen as parallel sets of straight lines , e. Fracture terms use in all other cases : Conchoidal - Fracture surface is a smooth curve, bowl-shaped common in glass ; Hackly - Fracture surface has sharp, jagged edges; Uneven - Fracture surface is rough and irregular; Fibrous - Fracture surface shows fibres or splinters. Tenacity can be described by the following terms. Brittle - Breaks or powders easily.
Malleable - can be hammered into thin sheets. Sectile - can be cut into thin shavings with a knife. Ductile - bends easily and does not return to its original shape. Flexible - bends somewhat and does not return to its original shape. Elastic - bends but does return to its original shape. Density Specific Gravity.
Density refers to the mass per unit volume. Specific Gravity is the relative density, weight of substance divided by the weight of an equal volume of water. Specific gravity is often a very diagnostic property for those minerals that have high specific gravities.
In general, if a mineral has higher atomic number cations it has a higher specific gravity. For example, in the carbonate minerals the following is observed:. For comparison, examine the following table:. Color is sometimes an extremely diagnostic property of a mineral, for example olivine and epidote are almost always green in color.
But, for some minerals it is not at all diagnostic because minerals can take on a variety of colors. These minerals are s aid to be allochromatic. For example quartz can be clear, white, black, pink, blue, or purple.
Read in your textbook, pp. Streak is the color produced by a fine powder of the mineral when scratched on a streak plate. Often it is different than the color of the mineral in non- powdered form. Luster refers to the general appearance of a mineral surface to reflected light. Two general types of luster are designated as follows: M etallic - looks shiny like a metal. Usually opaque and gives black or dark colored streak. Non-metallic - Non metallic lusters are referred to as. Interference of light reflected from the surface or from within a mineral may cause the color of the mineral to change as the angle of incident light changes.
This sometimes gives the mineral an iridescent quality. Minerals that show this include: bornite Cu 5 FeS 4 , hematite Fe 2 O 3 , sphalerite ZnS , and some specimens of labradorite plagioclase. Minerals that light up when exposed to ultraviolet light, x-rays, or cathode rays are called fluorescent. If the emission of light continues after the light is cut off, they are said to be phosphorescent.
Some specimens of the same mineral show fluorescence while other don't. For example some crystals of fluorite CaF 2 show fluorescence and others do not.
Other minerals show fluorescence frequently, but not always. Magnetic minerals result from properties that are specific to a number of elements. Minerals that do not have these elements, and thus have no magnetism are called diamagnetic. Examples of diamagnetic minerals are quartz, plagioclase, calcite, and apatite. Minerals that contain these elements may be weakly magnetic and can be separated from each other by their various degrees of magnetic susceptibility.
These are called paramagnetic minerals. Paramagnetic minerals only show magnetic properties when subjected to an external magnetic field. When the magnetic field is removed, the minerals have no magnetism. Ferromagnetic minerals have permanent magnetism if the temperature is below the Curie Temperature. These materials will become magnetized when placed in a magnetic field, and will remain magnetic after the external field is removed.
Other Properties Other properties that may be diagnostic include chatoyancy, asterism, piezoelectricity, and taste. Familiarize yourself with the meanings of these terms. And watch for these properties as you examine minerals. Tables for Identification of Minerals. Beginning on page of the Text by Klein and Dutrow are determinative tables which should aid you in using physical properties of minerals to identify them. Note that the tables are broken first into two different groups based on Luster.
Within each group, the minerals are then further divided on the basis of streak, hardness, and cleavage. In the remarks column are listed other useful diagnostic property for each mineral. Again, I encourage you to develop a systematic approach to identifying minerals. Questions from this material that could be asked on an exam.
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