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Chalcedony is a variety of quartz.
Formula: SiO2 tectosilicate (framework silicate)
Varieties
Agate is a distinctly banded fibrous variety of chalcedony
Jasper is an opaque to slightly translucent, generally red or brown variety of chalcedony, containing inclusions of
hematite
Specific gravity: 2.6 measured
Hardness: 6½； to 7
Streak: White
Colour: Colourless, white, gray, blue, any colour due to embedded minerals; multicoloured
specimens not uncommon.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:
Volcanic igneous environments
Sedimentary environments
Metamorphic environments
Hydrothermal environments
Chalcedony is deposited from aqueous solutions, and is frequently found lining or filling cavities in volcanic
rocks, as crusts in epithermal (low temperature) to
mesothermal (medium temperature) hydrothermal veins, as
a constituent of silica-rich marine sedimentary rocks, and as nodular concretions and layers in
limestone and marl.
Localities
At Jalgaon, Maharashtra province, India, chalcedony pseudomorphs after
calcite on chalcedony have been found
(KL p255).
At Trestia, south of Cavnic, Muramures, Romania, chalcedony pseudomorphs
after fluorite have been found
(KL p257).
At Mkobola district, Mpumalanga province, South Africa, a chalcedony
pseudomorph after a quartz Japanese
twin has been found
(KL p258).
Near Fruita, Mesa county, Colorado, USA, pseudomorphs of chalcedony variety
agate after baryte have been found
(KL p261).
At Baker Egg Ranch, Carrizalillo Mining District, Luna County, New Mexico, USA, a thunderegg agate showing
vivid green fluorescence has been found
(Mindat photo).
Chalcedony from the Baker Egg Ranch -
Image
At the Big Bend area, Brewster county, Texas, USA, chalcedony pseudomorphs
after aragonite have been found
(KL p256).
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