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

3.4.2 Amphibole

The amphibole group of minerals is chemically more complex than the pyroxenes. The general formula can be written as AB2C5Si8O22(OH)2, where A represents a large cation such as Na, and B and C represent smaller cations, such as Ca or Mg. Amphiboles also contain the hydroxyl group, (OH), and are therefore hydrous minerals, in contrast to pyroxenes, which are anhydrous minerals.

Amphibole minerals contain double silicate chains (Figure 42) resembling the pyroxene structure, and likewise have two distinct cleavages. However, because these chains are wider than the pyroxene single chains, the cleavages intersect at about 60° or 120° (strictly 56° (or 124°)), as illustrated in Figures 42 and 46b.

Figure 46 (a) An amphibole with a prismatic crystal form (shorter crystal is 3 cm long). (b) A plane-polarised light image of an amphibole in a metamorphic rock called amphibolite. This shows a basal section in which two cleavage planes intersect at about 120°. This amphibole has a strong pale greyish-green turning to dark greyish-green pleochroism (field of view 2 mm across). (c) The same field of view as in (b) between crossed polars. Although the amphibole has second-order interference colours, they can be masked by the strong body colour of the mineral - i.e. its colour in plane-polarised light.

One way to distinguish amphibole from pyroxene in thin section is by its roughly 60° (or 120°) cleavage in basal sections (Figure 46). Most often the cleavage is parallel and amphiboles have inclined extinction. Some common varieties are strongly pleochroic (some with yellowish-green turning to brown colours; some are deep blue turning to colourless).

Amphiboles are found in many metamorphic rocks and crystallise from hydrous magmas (containing water) that have moderate to high SiO2 and Na2O contents, such as andesites.

Activity 3.3 Pyroxene and amphibole in hand specimen and thin section

You should allow about 20 minutes for this activity.

Task 1

This activity focuses on the features by which pyroxene and amphibole can be distinguished in hand specimen and thin section. For this activity you will require the samples of gabbro and amphibolite 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. Pyroxene is a mafic mineral, and often forms green to black crystals (see Digital Kit examples). Gabbro contains olivine crystals (greenish-grey), feldspar (grey to white), and pyroxene (black). See if you can identify any pyroxene crystals in the gabbro or peridotite (the labels available in the Digital Kit can help you identify pyroxene) - they are visible in the close-up view in the Digital Kit.

Question 3.3.1

Considering the structure of pyroxene, would you expect this mineral to be optically isotropic or anisotropic?

Answer

Pyroxene has a chain structure, with the chains aligned in the same direction. Its optical properties are therefore different in different directions and the transmission of light is dependent on the direction of vibration. Consequently, you would expect it to be anisotropic.

Task 2

  1. Examine the thin section of gabbro using the Virtual Microscope. This rock contains a very low-relief mineral (seen in plane-polarised light), which is typically striped (due to twinning) when viewed between crossed polars. As you saw in Activity 3.1, it also contains olivine, which is distinctive with its very high relief (and characterised by its curved cracks), and pyroxene, with moderately high relief. Many of the pyroxene crystals show cleavage traces, and are only faintly coloured.
  2. Amphibole is another mafic mineral and typically forms dark-green to black prismatic crystals. An amphibolite, contains abundant amphibole.

Question 3.3.2

How is cleavage used to distinguish pyroxenes from amphiboles?

Answer

The pyroxene structure has weak bonding between the chains, giving rise to two sets of cleavage planes at about 90° (actually 87° (or 93°)) to each other (Figure 47a). In thin section, most often you can see only a single set of cleavage traces, e.g. View 1 rotation. In a basal section, if visible, two sets of traces (coloured blue and red in Figure 47a) can be seen at about 90° to each other. Basal cleavages at about 90° can be seen at one end of a pyroxene prism in the Digital Kit. Viewed from the side, both sets of cleavage planes are parallel to the length of the prism.Amphibole, like pyroxene, has a chain structure, with weak bonding between the chains. The chains are wider than in pyroxene (they are double chains), and the cleavage angle in basal section of an amphibole is about 60° (actually 56°) (or about 120° (actually 124°)) (Figure 47b).

Figure 47 Cleavages in (a) pyroxene and (b) amphibole. Sets of cleavage planes are shown in different colours.

Task 3

  1. Look at the thin section of the amphibolite using the Virtual Microscope.

Question 3.3.3

What colour are the amphibole crystals in plane-polarised light? What happens to the colour of the amphibole crystals as you rotate the thin section in View 2? What is this property called?

Answer

The amphibole crystals appear mainly grey (sometimes bluish, sometimes greenish) in plane-polarised light. When rotated, the colours change from pale grey to darker (bluish/greenish) grey: this property is pleochroism.

Question 3.3.4

See if you can locate any cleavages at about 60°, as typical of amphibole in this section. You may find this rather difficult. Can you suggest a reason for this? Look at View 1.

Answer

It can be difficult to find cleavage relationships at about 60° in thin sections when many of the amphibole crystals are in the wrong orientation (i.e. elongate in the plane of the section). To see two sets of cleavages at about 60°, it is necessary to look down the long axis of the amphibole crystal, i.e. at a basal section (see Figure 47b). In the Virtual Microscope thin section most of the amphibole crystals are aligned parallel to their long axes and both sets of cleavage planes are parallel (see also Figure 47b), but View 1 shows a typical basal section of amphibole with cleavages at about 60° (or about 120°).

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