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

3.3.2 Garnet

Like olivine, the garnet group of minerals is also based on a structure of isolated SiO4 tetrahedra and, as a result, garnets are also dense minerals and are commonly formed at high pressures both in the Earth's crust and mantle. Garnets are found in many crustal metamorphic rocks, especially those formed at depths greater than ~24 km, and are ubiquitous in the upper mantle at depths greater than 85 km. Garnets may also occur in some igneous rocks.

Although the garnet crystal structure is complex, it belongs to the cubic system, so garnets are optically isotropic (Figure 44c). Garnets have the general chemical formula A3B2(SiO4)3 where A represents divalent (2+) metal ions and B represents trivalent (3+) metal ions. However, garnets have a wide range of chemical compositions; they may contain various combinations from aluminium, calcium, iron, magnesium or even rare-earth elements, such as yttrium.

Figure 44 (a) Garnet in its distinctive dodecahedral (12-sided) form, due to its cubic crystal structure (long diagonal of the crystal face is 3.4 cm across). (b) Plane-polarised light image of a garnet in garnet mica schist. Garnet is a high-relief mineral that can be faintly coloured, depending on its composition (field of view 5.5 mm across). (c) The same field of view as in (b) between crossed polars; garnet is isotropic so stays black when the thin section is rotated. The grey and transparent streaks and spots in the garnet are quartz crystals included in the garnet as it grew.

Activity 3.2 Garnet in hand specimen and thin section

You should allow about 15 minutes for this activity.

Task 1

This activity will help you to recognise garnet in hand specimen and thin section. For this activity you will require 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 schist using the Digital Kit. This rock contains several other minerals, apart from garnet, but you can identify the garnets as small, pinkish-red to deep wine-red crystals. Unlike the other minerals such as mica and quartz, however, the garnets have equidimensional shapes reflecting their crystal symmetry. Note that garnets can be different colours, which vary widely from green to red to brown and very dark reddish-brown.

Question 3.2.1

Garnet belongs to the cubic system. What prediction can you make about its optical properties?

Answer

Cubic crystals are optically isotropic. You can therefore predict that, when viewed between crossed polars, the garnet crystals will appear dark, and will remain dark as the crystal is rotated.

Task 2

  1. Examine the schist in thin section with the Virtual Microscope. Starting with plane-polarised light, note the high-relief garnet crystals, up to 4 mm across, which have almost equidimensional shapes. Note also the way that the surrounding minerals appear to 'flow' around the garnets. Now select crossed polars and verify that garnet crystals are isotropic (see Question 3.2.1). For further confirmation, select the View 1 rotation.
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