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Optical Mineralogy Use of the petrographic microscope.

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Presentation on theme: "Optical Mineralogy Use of the petrographic microscope."— Presentation transcript:

1 Optical Mineralogy Use of the petrographic microscope

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3 Pyroxenes

4 Not only pyroxenes

5 Olivine

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8 Clinopyroxenes (monoclinic ) & Orthopyroxenes (orthorhombic)

9 Clinopyroxenes (monoclinic) Aegirine (Sodium Iron Silicate) Augite (Calcium Sodium Magnesium Iron Aluminium Silicate) Clinoenstatit (Magnesium Silicate) Diopside (Calcium Magnesium Silicate, CaMgSi 2 O 6 ) Esseneite (Calcium Iron Aluminium Silicate) Hedenbergite (Calcium Iron Silicate) Jadeite (Sodium Aluminium Silicate) Johannsenite (Calcium Manganese Silicate) Kanoite (Manganese Magnesium Silicate) Kosmochlor (Sodium Chromium Silicate) Namansilite (Sodium Manganese Silicate) Natalyite (Sodium Vanadium Chromium Silicate) Omphacite (Calcium Sodium Magnesium Iron Aluminium Silicate) Petedunnite (Calcium Zinc Manganese Iron Magnesium Silicate) Pigeonite (Calcium Magnesium Iron Silicate) Spodumene (Lithium Aluminium Silicate) Jervisite (Sodium Calcium Iron Scandium Magnesium Silicate)

10 Aegirine (Sodium Iron Silicate) Augite (Calcium Sodium Magnesium Iron Aluminium Silicate) Clinoenstatit (Magnesium Silicate) Diopside (Calcium Magnesium Silicate, CaMgSi 2 O 6 ) Hedenbergite (Calcium Iron Silicate) Pigeonite (Calcium Magnesium Iron Silicate)

11 Aegirine (Sodium Iron Silicate) Augite (Calcium Sodium Magnesium Iron Aluminium Silicate) Clinoenstatit (Magnesium Silicate) Diopside (Calcium Magnesium Silicate, CaMgSi 2 O 6 ) Hedenbergite (Calcium Iron Silicate) Pigeonite (Calcium Magnesium Iron Silicate)

12 Orthopyroxenes (orthorhombic) Hypersthene (Magnesium Iron Silicate) Donpeacorite, (Mg Mn)MgSi 2 O 6 Enstatite, Mg 2 Si 2 O 6 Ferrosilite, Fe 2 Si 2 O 6 Nchwaningite (Hydrated Manganese Silicate) Bronzite ………(Bastite)

13 Orthopyroxenes (orthorhombic) Hypersthene, (Magnesium Iron Silicate) Enstatite, ( Mg 2 Si 2 O 6 )

14 A few properties, and then some review… Cleavage – number and orientation of cleavage planes Twinning – type of twinning, orientation Extinction angle – parallel or inclined? Angle? Habit – characteristic form of mineral

15 Cleavage Most easily observed in PPL (upper polarizer out), but visible in XN as well No cleavages:quartz, olivine 1 good cleavage:micas 2 good cleavages:pyroxenes, amphiboles

16 Cleavage 2 cleavages intersecting at ~90° pyroxene

17 Twinning Presence and style of twinning can be diagnostic Twins are usually most obvious in XN (upper polarizer in)

18 Twinning - some examples Clinopyroxene (augite) Simple twin on {100}

19 Extinction angle Extinction behavior is a function of the relationship between indicatrix orientation and crystallographic orientation parallel extinction inclined extinction

20 Extinction angle – parallel extinction PPL XN orthopyroxene

21 Extinction angle - inclined extinction Clino Pyroxene inclined extinction (and extinction angle helps to identify them) clinopyroxene extinction angle

22 Pyroxene Well-formed crystal seen here with 2 cleavages at 90 o : Brightly coloured under XPL XPLPPL pyroxene Enlarged pyroxene

23 Review – techniques for identifying unknown minerals Start in PPL: Color/pleochroism Relief Cleavages Habit Then go to XPL: Birefringence Twinning Extinction angle Uniaxial or biaxial? 2V if biaxial Positive or negative?

24 (a) Pyroxene (b) Amphibole. (c) Black biotite (mica). (a)(b) (c)

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26 This first photograph shows a pyroxene crystal with cleavage aligned E-W along the crosshairs. The second photograph shows the same crystal in an extinction position. This shows inclined extinction

27 Pyroxene & Amphibole

28 Muscovite & Biotite

29 Augite is the most common pyroxene, with the formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al) 2 O 6. It is the typical pyroxene in mafic igneous rocks. Augite is usually black, with stubby crystals. It is a common primary mineral in basalt, gabbro and peridotite and a metamorphic mineral in gneiss and schist.

30 Diopside is a light-green mineral with the formula CaMgSi 2 O 6 typically found in contact-metamorphosed limestone. It forms a series with hedenbergite, CaFeSi 2 O 6. Calcium Pyroxene

31 Aegirine is a green or brown pyroxene with the formula NaFe 3+ Si 2 O 6. It is no longer called acmite or aegirite. (Sodium Pyx)

32 Enstatite is a common greenish pyroxene with the formula MgSiO 3. With increasing iron content it turns dark brown; its iron version is ferrosilite. Magnesium Pyroxene

33 Bronzite - Dark brown or greenish-brown, opaque variety of Enstatie or Hypersthene with a submetalic luster. Bronzite is formed through weathering of normal Enstatite or Hypersthene. Hypersthene

34 Bronzite - Dark brown or greenish-brown, variety of Enstatie or Hypersthene with a submetalic luster. Bronzite is formed through weathering of normal Enstatite or Hypersthene. Hypersthene

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39 Mineralogical composition of the principal mantle peridotites Minerals Peridotites Olivine Enstatite Diopside Aluminum Minerals LHERZOLI TE 60-70% 20%5-I0% Plagioclase<30km 30km< Spinelle <75km Garnet>75km HARZBUR GITE 70-80%20%0-5% Spinelle 5% DUNITE 95% Spinelle (Chromite) 5%

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42 Calcite

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44 You might have seen a pattern of colored light formed by interference between white light being reflected from the surface of a thin film of diesel fuel on the surface of water, and the diesel-water interface.


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