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Alkali Feldspar group

 

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Alkali Feldspar  (Click PPL image to show labels)

Olivine

 

Orthoclase in a granite from Fionnphort, Isle of Mull. Large orthoclase (alkali feldspar) crystal with string perthite visible even in PPL.

 

Width of central crystal: 7mm.

 

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Olivine XPL view

 

Orthoclase in granite. Distinctive perthite, best viewed near extinction. Crystal top right shows carlsbad twin.

 

Width of central crystal: 7mm.

 

 

   

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Perthitic

Texture

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Olivine rollover

 

Orthoclase in a monzonite, Hill of Johnston, Aberdeenshire. It is not possible to distinguish between the different feldspars in PPL view.

 

Width of field of view: 10mm

 

 

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Orthoclase in monzonite. Carlsbad twinning differentiates the orthoclase from the albite twinning in the plagioclase, which also shows some alteration to sericite. The orthoclase shows faint perthitic texture.

 

Width of field of view: 10mm

 

 

 

   

 

   

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Olivine rollover

 

Microcline in a scapolite syenite from Mt. St Hilaire, Canada. Granular texture,

 

Width of field of view: 9mm

 

 

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Microcline in a syenite. Characteristic "tartan" twinning. (a combination of albite and pericline twinning)

 

Width of field of view: 9mm

 

 

 

   

 

   

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Olivine rollover

 

Sanidine in a trachyte lava, possibly from a lava dome. Central Depression, (Malpais) Fuerteventura. Felted 'flow' texture made up of sanidine laths, known as trachytic texture. Trachytes do not always show trachytic texture, and other rocks, eg basalts, do sometimes show trachytic texture. This causes confusion, one is a rock type and the other a texture. This rock is about 97% Sanidine.

 

Trachyte lavas would be expected to be more viscous than basalts

 

Width of field of view: 8mm

 

 

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Sanidine in a trachyte. close examination reveals carlsbad twinning in some crystals.

 

Width of field of view: 8mm

 

 

 

   

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