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.
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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. |
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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. |
