Accessory Minerals

7/29/2019 Accessory Minerals

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22/5/13 Accessory minerals

web2.ges.gla.ac.uk/~minerals/Accessory_minerals.htm#Zircon 1/7

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.htm


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Zircons in granite (note darker biotite adjacent to zircon)


Zircon in schist Zircon in metasedimentary gneiss


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#Shape


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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


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#Colour


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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


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

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#Relief


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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


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#Relief


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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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Accessory Minerals