EPSC210

Laboratory #1:

Physical Properties and Mineral Identification

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.

Colour: don’t 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.

fluorite, CaF2, 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!

rhodoniteCaMnSiO3

ruby Al2O3

rhodochrosite MnCO3

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 (TiO2) needles in clear quartz (SiO2 )

Bornite, Cu5FeS4, 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...

IRIDESCENCE: a “play” of colours

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).

“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....

(close up)

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...

... vitreous (= glassy)

... metallic

Other terms used to describe luster...

resinous (shiny, but neither quite vitrous nor metallic...)

silky (light reflects off fibers)

waxy (“turkey fat”)

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.

sylvite, KCl, is distinctly more bitter...

halite (NaCl): familiar taste of table salt

Habit: general shape of a single crystal

Some minerals crystallize as perfect cubes. Their habit is described as “cubic”.

fluorite CaF2 pyrite FeS2galena PbS

If they have room to grow, most minerals develop flat faces with some symmetry.

Habit terms may describe simple geometric shapes adopted by minerals...

Octahedral (8-faced) habit of...

franklinite magnetite Fe3O4