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Olivine group

 

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Olivine  (Click PPL image to show labels)

Olivine

 

Olivine in a peridotite from the Ardnamurchan eroded magma chamber, Centre 3. Characteristic curved cracks containing alteration minerals. No cleavage or twinning. Pleochroism not usually noticable in thin section, Colour in thin section usually very pale green to colourless. Edge of crystal showing alteration to chlorite and other minerals. Cumulate texture in this example.

 

Width of central crystal: 4mm.

 

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Olivine XPL view

 

Olivine in a peridotite from the Ardnamurchan eroded magma chamber, Centre 3. Olivine displays bright XPL colours and good crystal shapes though somewhat rounded. Oter minerals are intersital. Cumulate texture

 

Width of central crystal: 4mm.

 

 

   

 

   

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Olivine rollover

 

Olivine in a picrite lava (or possibly sill) from Northwest coast, Isle of Skye, Scotland. Less altered than the Ardnamurchan example, with curved cracks. Note crystal shape euhedral to subhedral.

 

Length of central crystal: 2.5mm

 

 

 

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Olivine in a picrite. Bright colours in XPL, Other minerals laths or ophitic. Olivine solidified first

 

Length of central (blue) crystal: 2.5mm

 

 

 

   

 

   

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Olivine rollover

 

Olivine in a basalt lava from Ardnamurchan. Chosen to illustrate the alteration often seen in olivine. Brown secondary mineral has formed at the edges and alomg cracks leaving only cores unaltered. The brown secondary mineral is often referred to as Iddingsite, which is not a single mineral but a variable mixture of clay minerals and iron oxides/hydroxides. Often the alteration of the olivine is complete and only a pseudomorph remains. The description 'Olivine basalt' is often applied to rocks in which the olivine is completely altered with no fresh mineral to be seen. Partial or complete alteration is more common than unaltered olivine, especially on older rocks.

 

Field of view 9mm across.

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Olivinein a basalt lava from Ardnamurchan. Bright colours of the olivine in the unaltered cores, compared with dull brown for iddingsite.

 

Field of view 9mm across.

 

 

 

   

 

   

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Olivine rollover

 

Olivine in a lherzolite mantle nodule from Meerfelder Maar, Eifel, Germany. The olivine is the pale almost white mineral, quite clear and showing no cleavage.

 

Length of central crystal: 2.5mm

 

 

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Olivine in a lherzolite mantle nodule from Meerfelder Maar, Eifel, Germany. The olivine has higher birefringence than the other minerals, and in some crystals shows cloudy extinction, this is not a common feature of olivine (see movie clip (different sample)).

 

Length of central (blue) crystal: 2.5mm

 

 

 

   

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Extinction

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