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Home | Site Map | Mineral I.D. | Guide To Properties Metallic Minerals | Non-Metallic Minerals | Glossary Colour Streak Lustre Transparency Form Habit Cleavage Fracture Hardness Density Acid reaction Tenacity Other special Properties A mineral is a substance of known chemical composition and atomic structure (which may vary within fixed limits), therefore the physical properties of a mineral specimen are clues to it's identity. When you know how to test for a mineral's properties there are many good books you can buy with tables of the various properties to help you identify your minerals. Colour
An excellent example of the above is quartz. Six different varieties of quartz are each a different characteristic colour despite having identical chemical compositions (SiO2): Rock Crystal - colourless
It is also worth remembering that completely
different minerals may be the same colour.
Streak The streak of a mineral is the colour of it's powder when rubbed along an unglazed porcelain plate (streak-plate) and may be different from the colour of the mineral itself. Powder may also be produced by scratching the mineral with a knife. The streak of any given mineral is consistent for that mineral despite any differences in colour. The six different varieties of quartz above all have the same white streak. Back to Top Lustre The mineral's appearance due to the amount and quality of light reflected from it's surfaces. Depending on the quality of light a mineral reflects it may appear: Adamantine - the lustre of diamond
Depending on the quantity or intensity of light a mineral reflects it may appear: Splendent
Transparency If an object can be seen with a clear outline through a mineral then that mineral is transparent. If an object viewed through a mineral can be seen with a indistinct outline then the mineral is said to be subtransparent. If a mineral cannot be seen through, but is transmitting light then that mineral is said to be translucent. A mineral that does not transmit light is termed opaque. Back to Top Form The form of a crystal is dependant upon the conditions under which it grew. For example growth may have occurred outwards into a melt unhindered or it may have been restricted by the presence of other solid matter. The following terms are used to describe form: Crystallized - the mineral occurs
as well developed crystals
Habit The habit of a specimen (the shape of it's crystals) is greatly affected by the conditions under which the crystals grew. It is quite common for a mineral to have many different habits. The terms used to describe a specimen's habit are split into two groups; (1) the habit of crystals, (2) the habit of crystal aggregates. 1. Crystal Habits
Individual crystals may be described by
their shape i.e.. cubic,
hexagonal elongated (prismatic),
2. Crystal Aggregate Habits
A mineral with no crystal or aggregate
shape is a glass.
Cleavage Cleavage is the tendency of a mineral to split in certain preferred directions when struck. These directions are parallel to sheets of atoms in the mineral's atomic lattice. Cleavage is described in terms of: (1) the ease of cleavage, (2) the number and orientations of cleavage planes. For example: Gypsum has 'easy' cleavage in one
direction
Fracture The fracture of a mineral is how it breaks other than along cleavage planes. The fracture may be described as: Conchoidal - a 'shell-like', convex
or concave fracture displaying curved fracture or undulation rings concentric
to the point of impact and lines or fractures radial from the point of
impact, as in quartz, flint and obsidian.
Hardness The hardness of a mineral is measured on Moh's scale. The scale lists hardness values from 1 to 10. The numbers may be treated as relative values except for diamond; i.e. fluorite(4) is twice the hardness of gypsum(2). Diamond(10) is about ten times the hardness of corundum(9). Each value has a corresponding mineral of thathardness. Therefore the hardness of a mineral can be tested relative to the minerals on Moh's scale by scratching them with those minerals and other household items of known hardness. Moh's scale of hardness
Density The relative density of a mineral is its mass divided by it's volume. The specific gravity of a mineral is it's mass divided by the mass of an equal volume of water. In the field it is adequate to simply 'heft' a specimen to determine whether it is of low, high or moderate weight compared to it's size. Silicates and other non-metallic minerals are the least dense with SGs of 2.5 to 3.5 Metallic minerals are denser with SGs from 5 upwards (typically 5 to 8). Gold has an SG of 19 to 20. SGs of some common minerals
Acid Reaction Carbonate minerals react with dilute hydrochloric acid: Calcite effervesces strongly in
dil. HCl
Brittle - The minerals breaks or
crumbles easily, such as fluorite.
Taste. Halite NaCl, is rock-salt and therefore tastes of salt. There are other minerals with distinctive tastes, most of them somewhat less pleasant than salt. Odour. Pyrite, sphalerite and chalcopyrite give a sulphurous 'rotten egg' smell when struck or rubbed on a streak plate. haematite and limonite may give off an 'earthy' smell (the smell of damp earth) when breathed upon. Pyrite sparkes when struck with a geological hammer. I have also experienced this effect with haematite. Feel. Cryslalline minerals will feel rough. Talc and serpentine often feel unctuous (greasy) or soapy. Graphite and satin spar gypsum may feel smooth, unctous or soapy. Graphite is a good conductor of heat and will therefor feel cold. Graphite is also a good condcuctor of electricity (it is used a brushes on electric motors), but this property would not be tested in the field. |
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