AgateAgate is a type of quartz (silica), chiefly chalcedony, characterised by its
fineness of grain and brightness of color. Although agates may be found in
various kinds of rock, they are especially common in metamorphic rock.
The colorful agate and other chalcedonies were obtained over 3,000 years ago
from the Achates River, now the Drillo, in Sicily.
Formation and characteristics
Most agates occur as nodules in volcanic rocks or ancient lavas where they
represent cavities originally produced by the disengagement of volatiles in the
molten mass which were then filled, wholly or partially, by siliceous matter
deposited in regular layers upon the walls. Such agates, when cut transversely,
exhibit a succession of parallel lines, often of extreme tenuity, giving a
banded appearance to the section. Such stones are known as banded agate, riband
agate and striped agate.
In the formation of an ordinary agate, it is probable that waters containing
silica in solution -- derived, perhaps, from the decomposition of some of the
silicates in the lava itself -- percolated through the rock and deposited a
siliceous coating on the interior of the vapour-vesicles. Variations in the
character of the solution or in the conditions of deposit may cause
corresponding variation in the successive layers, so that bands of chalcedony
often alternate with layers of crystalline quartz. Several vapour-vesicles may
unite while the rock is viscous, and thus form a large cavity which may become
the home of an agate of exceptional size; thus a Brazilian geode lined with
amethyst and weighing 67 tons was exhibited at the Dusseldorf Exhibition of
1902.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is
generally a dark greenish mineral substance, like celadonite, delessite or
"green earth", which are rich in iron probably derived from the decomposition of
the augite in the enclosing volcanic rock. This green silicate may give rise by
alteration to a brown iron oxide (limonite), producing a rusty appearance on the
outside of the agate-nodule. The outer surface of an agate, freed from its
matrix, is often pitted and rough, apparently in consequence of the removal of
the original coating. The first layer spread over the wall of the cavity has
been called the "priming", and upon this base zeolitic minerals may be
deposited.
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