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EPSC210 Laboratory #1: Physical Properties and Mineral Identification.

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Presentation on theme: "EPSC210 Laboratory #1: Physical Properties and Mineral Identification."— Presentation transcript:

1 EPSC210 Laboratory #1: Physical Properties and Mineral Identification

2 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.

3 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.

4 fluorite, CaF 2, 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! rhodonite CaMnSiO 3 ruby Al 2 O 3 rhodochrosite MnCO 3

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

8 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... IRIDESCENCE: a “play” of colours

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 “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)

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...... vitreous (= glassy)... metallic

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

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. sylvite, KCl, is distinctly more bitter... halite (NaCl): familiar taste of table salt

18 Habit: general shape of a single crystal

19 Some minerals crystallize as perfect cubes. Their habit is described as “cubic”. fluorite CaF 2 pyrite FeS 2 galena PbS If they have room to grow, most minerals develop flat faces with some symmetry.

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

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