Leucite-Tephrite from Mt. Vesuvius, Italy

This rock sample was collected by a member of our local RIGS Group when in Italy.
Extract from: Imperial College Rock Library Author: Matt Genge
Website: https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewrecord.php?cID=821

It is a fine-grained porphyritic igneous rock dominated by phenocrysts of leucite (60% of phenocrysts), augite (40%) and minor plagioclase
 in a groundmass of leucite, plagioclase, sanidine and augite with accessory nepheline and opaques. Leucite occurs as anhedral to subhedral
rounded phenocrysts up to 1 mm in size and exhibit tartan twinning. Most leucite phenocrysts occur in glomerocrysts of up to 30 crystals measuring
 up to 6 mm in size. Augite is present as euhedral, pale brown, weakly pleochroic phenocrysts up to 4 mm in size. Most augites are present as clots
of several component crystals, some exhibit oscillatory zoning. Inclusions of oxides up to 0.1 mm in size are common, melt inclusions also occur.
Plagioclase microphenocrysts are sparse elongate laths up to 0.6 mm in size. The groundmass comprises70% of the rock. It is dominated by rounded
leucite crystals (50% of groundmass) and intergranular augite and feldspar laths. Some of the feldspars are sanidine. Intergranular nepheline occurs in places.
Leucite (KalSi2O6) belongs to the feldspathoid family of silicate minerals and occurs in potassium-rich basic extrusive rocks which are usually silica deficient.


plane polarised light: colourless glomerocrysts of leucite can be seen, especially slightly below right of centre


leucite tephrite: cross polarised light; augite phenocryst near centre has a hole in it due to section making process.

Polished surface of leucite tephrite displaying dark coloured euhedral phenocrysts of augite and white coloured glomerocrysts of leucite both set within a microcrystalline groundmass of  leucite, plagioclase, sanidine (an alkali feldspar) and augite with accessory nepheline and opaques.

 

Photo to the left shows a highly magnified photo of a glomerocryst of leucite taken in crossed polars in order to illustrate the complex cross hatching type of twinning nearly always shown by this mineral. Due to its very low birefringence it appears almost isotropic under crossed polars.

 

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