Geoscience Reference
In-Depth Information
Figure
26
shows a popular, currently available larvikite
stone. Figure
27
shows a syenite that was used in
antiquity for building the remarkable monuments of the
Aksumite civilization in Ethiopia.
Another igneous rock of intermediate composition is
the 'Roman Imperial Porphyry', a dark purple stone that
was much sought after in the Roman and Byzantine
worlds (Maxfield & Peacock, 2001). Imperial Porphyry is
a dacite found only at one location in the Red Sea
Mountains of Egypt. The rock is porphyritic with
phenocrysts of plagioclase feldspar (oligoclase to
andesine) and augite, set in a very fine groundmass of
feldspar (Klemm & Klemm, 2008). The most famous
variety 'Lapis Porphyrites' (
28
) has white feldspars set in
a deep purple groundmass (coloured by the presence of
haematite). 'Rubet Porphyrites' has pink feldspars
(coloured by the presence of the pink mineral piemontite)
set in a deep purple groundmass (
29
). Another variety,
26
26
'Blue Pearl' (Norway) syenite stone consisting
chiefly of cryptoperthitic alkali feldspar (grey) with
minor clinopyroxene (lower left, brightly coloured),
biotite mica (brown/green), and opaque minerals
(centre, black); XPT; ×35.
27
28
27
Metamorphosed nepheline syenite comprising
phenocrysts of pyroxene (brightly coloured) and
amphibole (brown) in a finely crystalline groundmass
of nepheline and alkali feldspar. Ghosts of nepheline
and feldspar phenocrysts can still be seen (from The
Great Stela, Aksum, Ethiopia); XPT, ×35.
28
'Lapis Porphyrites' from Mons Porphyrites, Egypt.
Porphyritic dacite comprising phenocrysts of feldspar
(white) and augite (brown), in a groundmass of very
finely crystalline feldspar (grey); XPT, ×35.
