Science, Maths & Technology

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An introduction to minerals and rocks under the microscope

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# 3.3.1 Olivine

The name 'olivine' refers to a group of minerals with a continuous spread of chemical compositions between forsterite (Mg2SiO4) and fayalite (Fe2SiO4), and the formula is written as (Mg,Fe)2SiO4, indicating that atoms (in fact, ions) of magnesium and iron can substitute for each other. This kind of chemical mixture is called a solid solution and involves ions having similar sizes and similar charges (e.g. Mg2+ and Fe2+). For a complete solid solution, as developed between forsterite and fayalite, the substituting ions must have similar ionic radii (a measure of their size), so that either ion can fit into the same interstitial site in the crystal structure.

Olivine is an important example of a structure with isolated tetrahedra. Every SiO4 group has a net charge of 4 (Figure 41b), and because the crystal has to be electrically neutral, olivine contains two doubly charged (2+) cations per SiO4 unit. Silicon atoms reside in tetrahedral interstices (i.e. at the centres of SiO4 tetrahedra), whereas octahedral interstices contain the larger magnesium and iron atoms (Section 1.4.2). These are close-packed, largely ionic structures, which explains why, compared with other silicates, olivine has a relatively high density. The structure is also strong in three dimensions, making olivine a hard mineral, without any good cleavage. Olivine has a characteristic olive-green colour (Figure 43a) that becomes darker with increasing iron content.

Figure 43 (a) Olivine 'pebbles' with distinctive olive-green colour (dish is 3 cm across). (b) Plane-polarised light image of olivine in a gabbro. The many irregular cracks containing iron oxide (optically opaque) are characteristic of olivine, as is an absence of cleavage. This olivine has partially altered to a green- or brown-coloured mineral, in this case, serpentine. Where the olivine is not altered, it is normally colourless and exhibits no pleochroism (field of view 2.6 mm across). (c) The same field of view as in (b) between crossed polars, the olivine having characteristic second-order interference colours.

Olivine is stable at high temperatures and pressures, and forms about 60% of the Earth's upper mantle. It crystallises at high temperatures from magmas rich in iron and magnesium and relatively poor in silica (SiO2). In such rocks, where olivine is one of the first minerals to crystallise as the magma cools, it may develop well-formed crystals, surrounded by later-crystallising minerals (which tend to be smaller, and lack such ideal crystal shapes).

Olivine is prone to alteration at lower temperatures under hydrous conditions, especially during weathering. Under the microscope, evidence of this alteration in olivine crystals takes the form of new minerals, such as greenish or brownish serpentine, and curved, irregular cracks filled with iron oxide (opaque) (Figure 43b and c).

## Activity 3.1 Olivine in hand specimen and thin section

Timing: You should allow about 20 minutes for this activity.

This activity will help you to recognise olivine in hand specimen and in 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 peridotite sample in the Digital Kit. If you could feel it, you'd notice that it feels much denser ('heavier') than other rocks of a similar size. The peridotite sample in the Digital Kit contains olivine that appears brownish-grey and translucent under lights (for photography) and the pyroxene is creamy bronze. The pyroxene can be recognised by its cleavage; but cleavage is lacking in olivine.
2. Now use the Digital Kit to examine the gabbro. This rock contains pyroxene and plagioclase as well as olivine.

### Question 3.1.1

Describe the textures you see in the gabbro and briefly try to explain the characteristic features that help to identify the minerals.