Geoscience Reference
In-Depth Information
'Lapis Porphyrites Niger' (
30
) has white feldspars in a
groundmass that appears black with a greenish tinge.
black and are popular decorative and cladding stones.
They are often incorrectly named in the stone industry as
'black granite'. In fact they contain much less silica, being
composed essentially of feldspar (mainly plagioclase
feldspar with minor alkali feldspar) and dark coloured
ferromagnesian minerals (>20%, typically pyroxene,
olivine, biotite mica, or hornblende), and only accessory
amounts of quartz. Figure
31
shows a stone of typical
hypersthene gabbro composition while Figure
32
shows a
B
ASIC IGNEOUS ROCKS
The basic rocks most used for building include the coarsely
crystalline plutonic variety known as gabbro and the
medium crystalline variety called dolerite (known as
diabase in America) that occur in dykes and sills. These
rocks have a dark appearance ranging from dark grey to
29
30
29
'Rubet Porphyrites' from Mons Porphyrites, Egypt.
Porphyritic dacite comprising phenocrysts of feldspar
(white) and augite (brown), in a groundmass of very
finely crystalline feldspar (grey). The feldspar pheno-
crysts have inclusions of piemontite (pink); XPT, ×35.
30
'Lapis Porphyrites Niger' from Mons Porphyrites,
Egypt. Porphyritic dacite comprising phenocrysts of
feldspar (grey/white) in a groundmass of very finely
crystalline feldspar (grey) and hornblende
(green/brown); XPT, ×35.
31
32
31
'Impala Black' gabbro (South Africa) consisting
chiefly of coarsely crystalline plagioclase feldspar
(grey), orthopyroxene (brown, upper right), and
clinopyroxene (brightly coloured), with minor biotite
mica (green) and magnetite (black); XPT, ×35.
32
Norite consisting mainly of coarsely crystalline
calcic plagioclase (labradorite composition) feldspar
(grey) and orthopyroxene (brown), with minor
clinopyroxene (blue). A natural joint that is tightly
filled with silica runs obliquely through the field of
view; XPT, ×35.
