Quartz

Quartz in thin section

Chemical formulae:

SiO₄

Property
Twinning	No
Extinction	Undulose
Colour	Colourless in ppl
Relief	Very low
Pleochroism	No
Habit	Tabular, Prismatic
Birefringence

In PPL quartz is barely different to Feldspar. Quartz is usually a little cleaner looking than K-Feldspar, which often has tiny grainy inclusions in it. (Sericite Alteration or fine grained Muscovite with High Alteration Colours) and Plagioclase usually has good cleavage visible in XPL and similar tiny grainy inclusions. (Which may be epidote.)

Quartz is often confused with K-Feldspar, Plagioclase, Nepheline and Cordierite. See here for a good outline of the differences

Quartz in Plain Polarised Light

Quartz in Cross Polarised Light

Quartz Extinct in Cross Polarised Light

Pyroxene

Pyroxene

Second Mineral to form in the Bowen's reaction series; Pyroxene is studied in depth at stage 2.

Chemical formulae:

XYT₂O₆

Y     Al Fe³⁺ Ti⁴⁺ Cr V Ti³⁺ Zr Sc Zn Mg Fe²⁺ Mn
X     Mg Fe²⁺ Mn Li Ca Na

T     Si Al Fe³⁺

Pyroxenes have a solid solution relationship with Mg Fe and Ca

Pyroxene in thin section.  

Orthopyroxene and Clinopyroxene.

Good relief, with very light brown to colourless in PPL

Pyroxene has two good cleavages at 90 degrees but only one may be visible.

Clino-pyroxene has a second order birefringence and inclined extinction.

Ortho-pyroxene has first order birefringence and straight extinction.

There is a slight colour difference between clino and ortho in ppl

Pyroxene in thin section PPL

There is a lot of Pyroxene in this photo. If you look very carefully you can see the Pyroxenes are different shades of colour. There is Orthopyroxene and clinopyroxene in this sample. You will notice a few dark crystals (Labled Opaques). These are dark in XPL as well and so are probably Opaques like titanium oxide. There is also a few Olivine crystals in this image (Labled Ol)

Pyroxene in Thin Section PPL

In XPL we can see birefringence, not as high as the Olivine but much higher than the grey and white minerals surrounding. We notice the dark minerals in ppl are still dark in xpl. Hence the opaque label.
To tell the difference between clino and oath pyroxene we need to watch them go extinct. Oath as the name suggests goes extinct when the crystal is inline with the cross hairs of the microscope. Clint pyroxene goes extinct at an angle to the cross hairs.

Apologies for the sharky video but I'm holding an iPad over a microscope lens. If you slow the playback down you can see a few interesting things. In PPL you might notice the small brown crystals changing colour. This is called pleochroism. Biotite is famous for this. When I switch to the cross polariser you will notice Olivine in purple and light blue shades going extinct as the stage is rotated. You will also see the pyroxenes going extinct. Some inline with the cross hairs, some later at maybe forty five degrees. You might need to step it through very slowly to notice. The most dominant  feature in this video is the three Olivine crystals all one below the other.  You can also see the plagioclase (Black and White tabulate crystals in XPL, near invisible in ppl) with its distinctive twinning and each twin going extinct at different times.

Olivine

Olivine

First in the Bowen's reaction series; Olivine is studied in depth at stage 2.

Chemical formulae:


 (Mg⁺², Fe⁺²)₂SiO₄

(This means it can be Mg^+2or Fe⁺² since both these ions have the same charge and relative atomic  size. This is called a solid solution)

Olivine is a mafic mineral (it appears in mafic and ultramafic rocks) that's usually green in colour. In thin section it is either colourless or sepia in plain polarised light (PPL) and is birefringent in cross polars (XPL). Olivine has a high relief in PPL and although often cracked, its never cleaved.

Olivine is most common in oceanic crust as Fe and Mg are common in the mantle. Olivine is the first mineral to crystallise in basalts. You would expect an Olivine rich rock to have probably come from a mid ocean ridge or hot spot volcano. 

Olivine thin section in PPL

Here is some Olivine in Plan Polarised Light (PPL). You can see it stands out against the other minerals in the sample. The dark outline around the olivine crystals is the relief. You can also see many jagged cracks int he olivine crystals. These are fractures, not twins and not cleavage. In PPL Olivine is a light green colour or sepia tone.  The Olivine crystals in the thin section are labelled Ol, NB not all the Olivine crystals are labeled.

Olivine  thin section in XPL

Here we see the same picture with the Cross Polarising Filter in. (XPL)

Notice the colours of the Olivine is XPL. The blues and greens are the birefringence. Olivine has a second order birefringence. You can also see the crystal just up and right of the centre has gone extinct in XPL. (Gone black) 

I have added a detailed walk-through identifying an igneous rock from hand sample (Posted under Pages on the right). Covering off mineral identification, mineral percentage, naming and rock history. I want to cover off sedimentary and metamorphic rocks as well. The highlight some confusing pairs. As always any questions comments or corrections welcome.

I have added some pages to cover off the first year Introduction to Geology lab. Some of the photos are a bit out of focus and I might update them one day. I will add some model answers to some of the rock id sheets. The mineral photos have hover or alt text tags to identify them. I might do the same with the rocks if there is interest.

The final practical test is challenging. However, good marks can be achieved if you put the effort in.

Still to come? The fossils and maybe some tips on mapping, if the subscriber numbers are there to warrant it. Good luck this year.