Optical Mineralogy

Use of the petrographic microscope

Pyroxenes

Not only pyroxenes

Olivine

Olivine

Olivine

Clinopyroxenes (monoclinic) & Orthopyroxenes (orthorhombic)

Clinopyroxenes (monoclinic)Aegirine (Sodium Iron Silicate) Augite (Calcium Sodium Magnesium Iron Aluminium Silicate) Clinoenstatit (Magnesium Silicate) Diopside (Calcium Magnesium Silicate, CaMgSi2O6) 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)

Aegirine (Sodium Iron Silicate)

Augite (Calcium Sodium Magnesium Iron Aluminium Silicate)

Clinoenstatit (Magnesium Silicate)

Diopside (Calcium Magnesium Silicate, CaMgSi2O6)

Hedenbergite (Calcium Iron Silicate)

Pigeonite (Calcium Magnesium Iron Silicate)

Aegirine (Sodium Iron Silicate)

Augite (Calcium Sodium Magnesium Iron Aluminium Silicate)

Clinoenstatit (Magnesium Silicate)

Diopside (Calcium Magnesium Silicate, CaMgSi2O6)

Hedenbergite (Calcium Iron Silicate)

Pigeonite (Calcium Magnesium Iron Silicate)

Orthopyroxenes (orthorhombic)

Hypersthene (Magnesium Iron Silicate)Donpeacorite, (Mg Mn)MgSi2O6 Enstatite, Mg2Si2O6 Ferrosilite, Fe2Si2O6 Nchwaningite (Hydrated Manganese

Silicate)

Bronzite ………(Bastite)

Orthopyroxenes (orthorhombic)

Hypersthene, (Magnesium Iron Silicate)

Enstatite, ( Mg2Si2O6 )

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

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

Cleavage

2 cleavages intersectingat ~90° pyroxene

Twinning

Presence and style of twinning can be diagnostic

Twins are usually most obvious in XN (upper polarizer in)

Twinning - some examples

Clinopyroxene (augite)

• Simple twin on {100}

Extinction angle

Extinction behavior is a function of the relationship between indicatrix orientation and crystallographic orientation

n

n a=X

c=Z

b=Y

c

a

b

Z

X

Y

parallel extinction inclined extinction

Extinction angle – parallel extinction

PPL XN

orthopyroxene

Extinction angle - inclined extinction

Clino Pyroxene inclined extinction (and extinction angle helps to identify

them)

clinopyroxene

extinction angle

PyroxeneWell-formed crystal seen here with 2 cleavages at 90o: Brightly coloured

under XPL

XPLPPL

pyroxene Enlarged pyroxene

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?

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

(a) (b)

(c)

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

Pyroxene & Amphibole

Muscovite & Biotite

Augite is the most common pyroxene, with the formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. 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.

Diopside is a light-green mineral with the formula CaMgSi2O6 typically found in contact-metamorphosed limestone. It forms

a series with hedenbergite, CaFeSi2O6.

Calcium Pyroxene

Aegirine is a green or brown pyroxene with the formula NaFe3+Si2O6. It is no longer called acmite or aegirite.

(Sodium Pyx)

Enstatite is a common greenish pyroxene with the formula MgSiO3. With increasing iron content it turns dark brown; its iron version is ferrosilite.

Magnesium Pyroxene

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

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

Mineralogical composition of the principal mantle peridotites

Minerals

Peridotites

Olivine Enstatite Diopside Aluminum Minerals

LHERZOLITE

60-70% 20% 5-I0% 5-I0%Plagioclase<30km30km< Spinelle <75kmGarnet>75km

HARZBURGITE

70-80% 20% 0-5% Spinelle 5%

DUNITE 95% 1-2 0-0.5 Spinelle(Chromite) 5%

Calcite

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.