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.