Sodalite – sodium aluminium silicate chloride – is a mineral of volcanic origin (chemical formula: Na8Al6Si6O24Cl2) and it comes from hydrothermal fluids in a volcanic rock’s cavity. The sodalite containing rock itself is not homogenous but consists of many different, small minerals beside the blue sodalite.
The mineral itself is very nice deep royal blue in general, the piece illustrating this article was mined at Mt. Vesuvius and bought in a mineral shop in Italy. The sodalite pieces are full of other crystals, usually well visible whitish veins which mostly consist of calcite.
When looking at the mineral with the help of some magnifying device we can see small parts of it having thin and colourful layers! These coloured parts are concentrated at the edges of the calcite veins or patches and only visible in a magnified form. Here, the translucent calcite was built on the blue sodalite mineral in a later process different from the forming of the blue crystals from the original hydrothermal solution. These places must also contain a very thin layer of air which is responsible for the colours with its interference.
What is unknown: the forming of the air layers. Are they originally there or are they created when the stone is cut from the rocks? I think the later is more possible as the sodalite rocks can more easily break where the white veins run, so the chopping of the rock might create the gaps, resulting interference patterns. The process might be the same as the ice pieces with fissures showing interference colouration too.
The pictures (1 – 2 – 3) were taken with a cheap digital microscope, the magnificiation which shows the interference colours is 200X. Smaller magnification also shows it but only in tiny coloured spots.
Author: Mónika Landy-Gyebnár, Hungary
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