EPSC210

1
EPSC210 Laboratory 1 Physical Properties and
Mineral Identification
2

• Acquaint yourself with the main physical
  properties useful for mineral identification in
  hand specimen
• Identify one specific mineral among the 40
  unknowns on display.
• Draft a description of the specimen you have
  identified as your mineral.

3
Colour dont let it trick you ! Many minerals
are colourless when pure, but show a wide range
of different colours if impurities are present.
Some minerals show a limited range of colours.
Relatively few have a fairly constant
colour. Texts usually list the range of colours
commonly noted for a given mineral.
4
fluorite, CaF2, coloured by impurities and defects
5
A few minerals, however, have a characteristic
colour, which is fairly reliable for
identification. But the colour is never unique to
one mineral species!
rhodochrosite MnCO3
rhodonite CaMnSiO3
ruby Al2O3
6
The porcelain streak plate (H 6)
The colour of many dark minerals is variable in
hand specimen. When rubbed against the porcelain
plate, the silicates are colourless but most
oxides and sulfides leave a streak (powder) of a
characteristic colour.
7
The presence of inclusions in a colourless
mineral may give rise to a faintly coloured
streak... Beware of streaks that change as you
rub different parts of a specimen! Likely, more
than one mineral is present.
rutile (TiO2) needles in clear quartz (SiO2 )
8
IRIDESCENCE a play of colours
Bornite, Cu5FeS4, is often called peacocks ore
because its surface oxidizes readily and develops
a characteristic iridescence.
However, iridescence can develop by surface
oxidation on several other minerals...
9
Goethite is normally brown (left). The specimen
to the right developed an iridescence, i.e. a
rainbow effect due to a thin coating of iron
oxide formed on the mineral surface (often
because of heating).
10
Another cause of iridescence....
(close up)
Rainbow quartz is a flaw (a small open
fracture) inside quartz which produces a rainbow
of colours... White light is bent as it travels
from gas to crystal... Much as it separates into
colours when it leaves a glass prism.
11
Another cause of iridescence is the diffraction
of white light by a periodic grating. Light
produces colourful interference patterns when it
bounces off structures that are regularly
spaced at distances close to the wavelength of
visible light.
Labradorescence iridescence in labradorite
12
Opalescence the shimmery reflection from the
interior of precious opal. This arises because
light is diffracted by the regularly spaced
planes formed by closely packed similarly-sized
silica spheres.
Precious opal has this quality. Common opal, a
mineraloid lacking long range order, does not
display opalescence as beautifully.
13
Luster more subtle than colour but quite
useful... It refers to the way a surface reflects
light.
14
The two most common types of luster are...
... metallic
... vitreous ( glassy)
15
Other terms used to describe luster...
waxy (turkey fat)
resinous (shiny, but neither quite vitrous nor
metallic...)
silky (light reflects off fibers)
16
However, the luster of a mineral does depend
partly on the size of individual
crystals. Luster becomes duller in aggregates of
microscopic crystals.
The earthy look of fine-grained hematite (left)
contrasts with the glistening specular metallic
luster of coarser crystals (right).
17
A few minerals are distinctly tasty... because
they dissolve readily in water.
halite (NaCl) familiar taste of table salt
sylvite, KCl, is distinctly more bitter...
18
Habit general shape of a single crystal
19
If they have room to grow, most minerals develop
flat faces with some symmetry.
Some minerals crystallize as perfect cubes. Their
habit is described as cubic.
galena PbS
fluorite CaF2
pyrite FeS2
20
Habit terms may describe simple geometric shapes
adopted by minerals... Octahedral (8-faced)
habit of...
franklinite
magnetite Fe3O4