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7/29/2019 Accessory Minerals
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Minerals down the Microscope on Moodle(back to intro)
Minerals down the Microscope on Moodle.
Part 2: Accessory minerals
These are typically very stable and commonly present as small crystals in a variety of igneous, metamorphic and
sedimentary rocks but never present in large quantities. Hence they are not rock-forming minerals (i.e. the ca. 10
minerals that form the vast majority of rocks).
Minerals covered on these pages:
Zircon
Apatite
Titanite
Tourmaline
Opaques(Fe-oxides, Fe-sulphides)
Zircon
Rock types in which zircon is commonly present: Granites, Intermediateigneousrocks , Sandstones, Schists,
Gneisses
(Generally present in all silicate rocks except ultrabasic igneous rocks)
Zircon in mylonite
Analyser out Analyser in
Optical properties of zircon
High relief
High orderinterference colours
Shape: Typically euhedral (igneous) or well rounded
Formspleochroic haloes in biotite
Minerals that might be confused with zircon
Titanite (higher interference colours)
Zircon shape, interference colours andpleochroic haloes
Zircon in metasedimentary gneiss
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Pleochroichttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Pleochroichttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Intro.htm7/29/2019 Accessory Minerals
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Analyser out Analyser in
Zircons in granite (note darker biotite adjacent to zircon)
Analyser out Analyser in
Zircon in schist Zircon in metasedimentary gneiss
Analyser out Analyser in
Note: Dark spots (pleochroic haloes) in biotite caused by radiation damage from small inclusions of
zircon in the biotite. At high magnifications, some larger zircons may display internal zoning either picked
out by small differences in relief or interference colours. In igneous rocks zircon typically has good crystal
shape, in clastic sedimentary rocks and metasediments it tends to be well rounded.
Apatite
Rock types in which apatite is commonly present: Granites, Intermediate igneous rocks, Sandstones,
Schists, Gneisses
(Generally present in most silicate rocks except ultrabasic and basic igneous rocks)
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shape7/29/2019 Accessory Minerals
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Apatite inclusions in amphibole Apatite in orthogneiss
Analyser out Analyser out
Optical properties of apatite
Colourless
Moderate reliefLow orderinterference colours
Shape: Typically euhedral slightly elongate prisms (hexagonal x-section)
Minerals that might be confused with apatite
Quartzand feldspar (low relief and typically abundant)
Apatite shape and interference colours
Apatite in granite
Analyser out Analyser in
Apatite in gneiss
Analyser out
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colour7/29/2019 Accessory Minerals
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Note: Higher relief that adjacent quartz, typical hexagonal; cross section and low interference colours.
Hexagonal sections are typically completely dark in crossed polars.
Titanite
Rock types in which titanite is commonly present: Intermediate igneous rocks, Metabasic rocks, Calc-
silicates
Titanite in granite
Analyser out Analyser in
Optical properties of titanite
High relief
V. high orderinterference colours
Shape: Diamond shaped in igneous rocks
Minerals that might be confused with titanite
Garnet(isotopic), Calcite (lower relief),Zircon (lower interference colours)
Titanite shape and interference colours
Titanites in metabasic rock
Analyser out Analyser in
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Calcite.htmhttp://web2.ges.gla.ac.uk/~minerals/Garnet.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Relief7/29/2019 Accessory Minerals
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Note: Little change in colour of titanite when analyzer inserted due to very high interference colours.
Metamorphic titanite tends to be less well shaped and may form a reaction rim around Fe-Ti oxides
Tourmaline
Rock types in which tourmaline is commonly present: Schists, Gneisses, some Granites
Tourmaline in mica schist
Analyser out Analyser in
Optical properties of tourmaline
Moderate relief
Strongly coloured (often zoned)
Moderate orderinterference colours
Shape: Generally euhedral elongate with curved triangular cross section
Elongate sections arepleochroic
Minerals that might be confused with tourmaline
Amphibole (Cleavage)Biotite (Cleavage, pleochroic scheme)
Tourmaline shape and interference colours
http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Biotite.htmhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Pleochroichttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colourhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Relief7/29/2019 Accessory Minerals
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Tourmaline in mica schists
Analyser out Analyser out
Note: Colour zoning indicative of compositional zoning. Both biotite and tourmaline are strongly
pleochroic but elongate sections of tourmaline are dark N-S, biotite is dark E-W
Opaques
(Oxides and sulphides tend to be opaque minerals e.g. Magnetite, Ilmenite, Pyrite)
Most rocks contain some opaque minerals.
Ilmenite tends to form slightly tabular crystals, Pyrite often forms cubes.
Optical properties... dark with analyser out.
All images with analyser out
Ilmenite in biotite schist Fine grained magnetite in basalt
Magnetite in peridotite Fe-oxide cement in sandstone
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Take the Accessory Minerals test
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