Download Site 7 Aplite quarries

Site 7 Aplite Quarries
S7
Aplite Quarries
A Special Quarry
When first entering the larger south bank
quarry, it should be remembered that both
south and north quarry sites are protected as
part of a Site of Special Scientific Interest (SSSI).
No hammers or collecting of specimens is
allowed. But what is so special about these
quarries? The Meldon valley, including Red-aven Brook, lies within the zone of altered and
baked rocks which fringed the cooling granite
magma of Dartmoor (page 8). This magma
and metamorphism produced a rich range of
minerals which has been exploited by mining
and quarrying from the 18th century, but none
more famous than the aplite.
As the Dartmoor granite magma slowly
cooled so the familiar large solid crystals
which make up the granite formed.
This action concentrated unusual minerals
in the last of the liquid magma which,
under pressure, was expelled by the granite
mass and injected through fissures in the
surrounding non-granite rocks before
cooling rapidly into fine crystalline ‘dykes’
(wall-like bodies of rock) and thinner veins.
The most remarkable of these ‘injections’
is an 18 metre thick dyke of aplite (microgranite)
which crosses the Red-a-ven Brook here. The
main aplite and associated smaller off-shoot
dykes and veins of aplite have been quarried
both in this larger quarry and in a smaller
quarry on the northern side of the brook.
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Aplite
Here the aplite splits into several bands
which cut across bands of chert. The aplite
is a pale fine-grained granite but unlike
similar bodies elsewhere contains many
strange and unusual minerals - some of
which are named opposite.
Finding some aplite
Although collecting minerals is not
allowed it is interesting to look for the
few remaining veins and ‘dykes’ which are
exposed low in the quarry face. Can you
can find the steeply dipping bed at the
base of the face on the left-hand (east)
side as you look into the quarry?
Industrial uses
The Meldon aplite’s unique chemistry,
including its low iron content, made it
suitable for glass-making and enamelling.
In 1920 plans for large-scale glass
manufacture on this site were initiated
but within a year this turned into a
financial disaster. Because of its hardness,
the aplite was also quarried as an
aggregate until the early 1970s.
Aplite minerals in detail
The aplite (sometimes also known as
‘granulite’) found here is unique in Britain.
The Meldon aplite has concentrations of
elements such as lithium, caesium and
beryllium and, consequently, also has a
unique mineralogy, especially where these
minerals have crystallised out in bands.
Glass manufacture
Interest in exploiting the aplite for local
glass making was reported in 1890 but no
evidence of actual manufacture remains.
However, in 1920 a London syndicate
planned to establish ‘the centre of
the nation’s glass industry’ at Meldon.
There were plans for 12 furnaces and
a work-force of 500 was anticipated.
The reason for such optimism lay in
seemingly endless resources of the
aplite - whose chemistry was ideal
for the manufacture of glass.
In the quarry to the south of Red-a-ven
Brook, the aplite is prominent as a
band of fine-grained white rock in
dark hornfels. The present exposure is
approximately 10 metres wide, dipping
at an angle of about 70° to the south.
It occupies most of the inner face of the
quarry. Here, and in the smaller northern
quarry, occasional bands and lenses of
coarser crystals, up to 2 cm long, reveal
white albite feldspar crystals - different
from the orthoclase feldspar of the main
granite, and locally lithium-bearing
‘petalite’, lilac coloured lithium-mica
(lepidolite) and small pink and green
crystals of boron-rich tourmaline. Many
other rare minerals are also present.
A small experimental furnace and
two larger furnaces were initially built
but technical and financial difficulties
made this a short-lived venture and by
February 1921 glass-working ceased.
Little remains on site but examples of
Meldon glass bottles are on display
at the Museum of Dartmoor
Life in Okehampton.
Garnet crystal,
Carol Mullin © DNPA
The surrounding metamorphosed
Carboniferous rocks also contain unusual
minerals, especially where hot fluids
released by granite have reacted with
them. These include green garnets and
brown boron-containing axinite.
Meldon
glass bottle,
Carol Mullin
© DNPA
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Site 7 Aplite Quarries
S7