Minnesota
Minerals Education Workshop
August 5 -7, 2003
Mineral
Identification
Crystal Shape: form and habit
Habit refers to the overall shape of a crystal or aggregate
of crystals. In other words, the appearance of single crystals, as well as
the manner in which crystals grow together in aggregates.
Form: to mineralogists this refers to a group of crystal faces related by
the crystal=s symmetry. For example pyrite forms in cubes. Quartz forms in
hexagonal prisms.
Both habit and form are extremely diagnostic in identifying minerals.
Habit: Terms
1. Acicular--slender, needle-like crystals: tourmaline, hornblende,
arsenopyrite, rutile, apatite, sillimanite
2. Capillary and filiform: hairlike or thread-like: native Au, Ag, Cu
3. Bladed--elongated crystals flattened like a knife blade: kyanite, tremolite
4. Dendritic: arborescent, in slender divergent branches, somewhat plantlike--native
metals, pyrolusite
5. Radiating: divergent: zeolite; tremolite; talc
6. Drusy--surface covered with a layer of small crystals--sugar like calcite,
quartz, sphalerite, pyrite
7. Fibrous: aspect ratio 10:1
8. Globular and colloform: radiating individuals forming small spherical groups
botryoidal-"bunch of grapes"
reniform C kidney-like
mammillary - very large
zeolites, quartz, malachite, goethite, pyrolusite, hematite
9. Foliated: easily separable into plates or leaves: tremolite; hematite;
10. Micaceous: similar to above but splits into very thin sheets: muscovite,
biotite, chlorite
11. Tabular or lamellar--flat and plate-like: barite, dolomite
12. Granular--composed of many individual grains of similar size
13. Prismatic or columnar elongated crystals with identical faces parallel
to a common direction
Euhedral, Subhedral, Anhedral
Twinning:
Crystals that are related to one another by some geometric relation.
Simple or Contact twins: e.g. the fish tail of gypsum
Penetrative: e.g. Carlsbad or Staurolite
Polysynthetic (multiple): Plagioclase
Cyclic: aragonite
Many minerals tend to break along twin planes. When this occurs it is called PARTING.
Cleavage is the easy breakage of a mineral along a plane surface. The reason it occurs is the geometric arrangement of atoms in a mineral grain is generally such that a force of cohesion is less along some particular plane in the crystal lattice. A good example is the perfect cleavage in mica.
(Note students generally become very confused about identifying crystal faces and cleavages--especially when some cleavages are parallel to particular crystal faces. (Galena for example--forms cubes and has cubic cleavage.))
Perfect: Very good breakage and the new surfaces
are shiny.
Distinct: New surfaces broken by frequent irregular steps.
Indistinct: Generally rough with only a few planar areas
(generally not useful).
Many minerals have more than one cleavage direction and this yields intersecting cleavage at various angles. These angles are very distinctive of minerals and are used to identify minerals. A good example is the 60/120 cleavage in hornblende and 87/93 cleavage in pyroxene. This is the only sure way to tell the minerals apart.
FRACTURE:
Fracture is the way a crystal breaks. Crystals that tend to have good cleavages, break along them first. In galena for example you may not see a fracture because the mineral has a tendency to break into cubes. Mica, however, has only one cleavage and you will see both fracture and cleavage. Terms that are used to describe fracture are:
Conchoidal: quartz
Hackly: jagged and sharp
Fibrous and splintery
Uneven or irregular: rough and irregular surfaces.
This is a mineral's resistance to scratching. It is a relative scale. Minerals with higher numbers will scratch minerals below them on the scale.
1) Talc
2) gypsum
3) calcite
4) fluorite
5) apatite
6) orthoclase
7) quartz
8) topaz
9) corundum
10) diamond
The last three are not common minerals.
Fingernail is 2 to 2.5, Cu penny is around 3.5; hammer is about 5.1, knife
is 5.2 and plate glass is 5.5.
This is the ratio of the density of the mineral to the density of water. It is obtained by weighing a piece of mineral in air and then in water. The difference in weight is equal to the volume of water displaced.
Weight Air/(Weight Air-Weight in water) = specific gravity
Specific Gravity increases with increasing atomic weight of the cation. For example in the orthorhombic carbonates:
Aragonite CaCO3 2.95
Strontianite SrCO3 3.76
Witherite BaCO3 4.29
Cerrusite PbCO3 6.55
Anhydrite CaSO4 2.98
Celestite SrSO4 3.97
Barite BaSO4 4.50
Anglesite PbSO4 6.38
Color is not a definitive property of a mineral, except when it is caused by a major constituent in the mineral. A good example of this is malachite that is green from the Cu.
The mineral sphalerite (ZnS) can be yellow, black, red, green or varieties thereof. Quartz is usually colorless, however it can have many colors depending on trace amounts of cations that substitute in the structure.
Streak: For opaque minerals, if you rub the sample across a streak plate, it will leave a colored powder. This streak is distinctive for minerals and is used to identify minerals. For example the mineral hematite can be brown, black, metallic grey and earthy. In all cases it give a red-brown streak.
Play of Colors: Iridescence--For example, labradorite-a variety of plagioclase.
Interference of light either at the surface or in the interior of a mineral--series of color--much the same as oil on water. Opal is another example.
LUSTER
Metallic or non-metallic
Non-metallic: vitreous or glassy; silky; resinous; pearly; greasy; waxy, dull; earthy
LUMINESCENCE
Luminescence is the emission of light not due to incandescence. For example, fluorescent minerals change colors under ultraviolet light. Minerals include varieties of calcite, fluorite and scheelite.
MAGNETISM
Magnetite and Pyrrhotite
ENACITY
The resistance of a mineral to breaking: brittle, malleable, sectile, ductile, flexible and elastic.
OTHERS
Reaction to HCl.
Last
Updated July 24, 2003
Penny Morton