optical mineralogy use of the petrographic microscope
TRANSCRIPT
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)
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
Twinning
Presence and style of twinning can be diagnostic
Twins are usually most obvious in XN (upper polarizer in)
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 - 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?
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
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%