An introduction to minerals and rocks under the microscope
An introduction to minerals and rocks under the microscope

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An introduction to minerals and rocks under the microscope

3.5.3 Other sheet silicates

Some other sheet silicates that you are likely to meet include talc, chlorite and serpentine. These minerals have structures related to biotite, with magnesium (and iron) in trioctahedral layers (as summarised in the right-hand column of Figure 49). Unlike biotite, however, their tetrahedral layers contain no aluminium, only silicon. These layers, therefore, have no net electrical charge, and so no interlayer ions are required to balance charges. Without the interlayer ions, there is little to hold the layers together.

  • How would you expect the hardness of talc to compare with that of biotite?

  • Without interlayer ions between the sandwich layers, minerals such as talc are much softer than biotite. Talc is one of the softest minerals known, with a hardness of 1 on Mohs' scale (Table 2).

All of these minerals are stable at low temperatures (and up to surprisingly high pressures) and tend to form (serpentine, especially) by the breakdown of high-temperature Mg-rich minerals, such as olivine (Mg2SiO4), under hydrous conditions. These minerals, therefore, contain hydroxyl groups and are common (serpentine and talc, especially) in basaltic and mantle rocks that have been altered by watery fluids. Chlorite is common in many low-temperature metamorphic rocks derived from sediments and from basaltic igneous rocks.

Pyrophyllite, with dioctahedral layers (Figure 49b), is a low-temperature metamorphic mineral found in aluminium-rich sedimentary rocks such as slates.

Activity 3.4 Micas in hand specimen and thin section

You should allow about 20 minutes for this activity.

Task 1

This activity will help you to recognise micas in hand specimen and thin section.

For this activity you will require the muscovite mica sheet, granite and schist in the Digital Kit [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] and the Virtual Microscope.

  1. Examine the thin flake cleaved from a larger crystal of muscovite mica in the Digital Kit. The fact that it is easy to separate such flat crystal flakes (see also the video clip of a peeling flake of muscovite mica in the Digital Kit) indicates that mica has an excellent cleavage.
  2. Examine the granite in the Digital Kit. This contains small elongate crystals of dark mica (biotite), which appear to glint in the light as the specimen is rotated. If you zoom in using the Digital Kit to look at these crystals more closely, you may be able to make out flat cleavage surfaces.
  3. Compare the granite (which contains biotite) with the schist. The silvery mineral in the schist is the white mica, muscovite.
  4. Examine the granite in thin section using the Virtual Microscope in plane-polarised light. The brown crystals (View 1, especially) are biotite.

Question 3.4.1

What happens to the colour of the biotite crystals in PPL when you rotate the stage (View 1)? What is this property called?

Answer

The colour of the biotite crystals changes from pale brown to dark reddish-brown as they are rotated. This property is pleochroism.

Task 2

  1. Note that the biotite crystals contain tiny, dark 'haloes' (often referred to as pleochroic haloes) surrounding minute specks (tiny inclusions). Now examine the biotite crystals between crossed polars and as you rotate the thin section.

Question 3.4.2

How do the haloes behave as you rotate the XP view (View 1)?

Answer

You should find that the haloes around inclusions in the biotite always appear dark between crossed polars, whereas normal biotite shows interference colours, although they are often masked by the strong colour of biotite. The haloes are actually regions of biotite that have been damaged by radiation, becoming amorphous (and optically isotropic). The radiation comes from the minute crystals at the centre of every halo: most of these are the radioactive mineral, zircon (ZrSiO4), that contains minor amounts of uranium.

Task 3

  1. Examine the schist in plane-polarised light, then between crossed polars. You should find curved, sheaf-like masses (View 2, especially) that show colourful, high-order interference colours. These are crystals of white mica (muscovite).
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