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

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.

fluorite, CaF
2
, coloured by impurities and defects

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
MnCO
3 rhodonite
CaMnSiO
3 ruby Al
2
O
3

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.

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 (TiO
2 needles in clear quartz (SiO
)
2
)

IRIDESCENCE: a “play” of colours
Bornite, Cu
5
FeS
4
, is often called “peacock’s ore” because its surface oxidizes readily and develops a characteristic iridescence.
However, iridescence can develop by surface oxidation on several other minerals...

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).

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.

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

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
.

Luster: more subtle than colour but quite useful... It refers to the way a surface reflects light.

The two most common types of luster are...
... metallic
... vitreous
(= glassy)

Other terms used to describe luster...
waxy (“turkey fat”) resinous (shiny, but neither quite vitrous nor metallic...) silky (light reflects off fibers)

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).

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...

Habit: general shape of a single crystal

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”.
fluorite CaF
2 pyrite FeS
2 galena PbS

Habit terms may describe simple geometric shapes adopted by minerals...
Octahedral (8-faced) habit of...
franklinite magnetite Fe
3
O
4