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Occurrence,
Localities and Origins:
The principal
known occurrences of alunite are at
Tolfa, Italy; at Bulla Delah, New South Wales Australia; on Milo, Grecian
Archipelago, and at Mt. Dore, France.
In the United States it is found with quartz and kaolin in the
Rosita Hills, and the Rico Mts., Colo.; in the ore veins at Silverton
and Cripple Creek, Colo.; as a soft white kaolin-like material in the
ore veins at Goldfield, Nev.; as a crystalline constituent in the rocks
at Goldfield, Nev., and Tres Cerritos, CaL, and in the form of a great
vein of comparatively pure material at Marysville, Utah.
At
Bulahdelah in N.S.W., about 35 miles from Port Stephen, a
narrow range of hills that reaches an altitude of 900 feet consist almost
entirely of alunite over a length of about a mile. The rocks of the
Bulahdelah area are rhyolites, interbedded with Permo-Carboniferous
strata, the whole being folded and invaded by dolerite dykes of Tertiary
age. It is supposed that sulfurous vapors were emitted during the
intrusion of the dolerites, and that these vapors acted on the rhyolites,
decomposing the felspathic ingredients and forming alunite. In its
purest form the
Bulahdelah alunite is microcrystalline in texture,
pink in color, and contains about 9 percent potash. It has been
worked for many years as a source of alum, to obtain which the alunite
is first heated at the required temperature and then leached with water.
If heated more intensely it is converted into a mixture of alumina and
potassium sulfate, and in this condition, as already stated, it is useful
as a potash fertilizer.
At Tolfa, near Rome, alunite occurs as veins in a mass of trachyte
that overlies Tertiary sediments. The trachyte is very pyritic. There
are two views as to the origin of alunite in the Tolfa trachytes. According
to one of these views the alunite has been formed by the action of
solfataric vapors on the trachyte. The other view is that the alunite
has been formed by the decomposition of the felspars of the trachyte
due to the action of the acid solutions arising from the decomposition
of the pyrite by descending waters.
Alunite is often found disseminated in the rocks or in well-defined
veins. Notable occurrences in the western USA are at Goldfield, Nevada;
Marysvale,
Utah; and the Rosita Hills, Colorado.
The alunite near Marysvale in Utah occurs in veins traversing
andesites that are probably of Tertiary age. The main vein is in one
place 20 feet thick, and the outcrop has been proved to extend over a
distance of 3500 feet. At Rosita Hills in Colorado, alunite occurs
associated with quartz in a granite-like rock that contains not more than
about 2.5 percent potash.
Deposits of alunite occurring at Benahadux in Almeria, Spain, have
been described under the name of
"calafatite." There is, however, no
justification for regarding the Benahadux material as a separate species.
Its chemical composition is identical with that of ordinary alunite. It
occurs in compact, chalk-like masses, the texture of which is the same
as that of ordinary alunite. It occurs in the form of parallel beds 5 or
6 meters thick beneath Triassic limestone, and is easily and cheaply
quarried. Deposits of chalk-like alunite occur at other localities,
notably at Carrickalinga Head in South Australia.
Alunite is sometimes
associated with valuable gold deposits. In volcanic regions it is not
uncommon to find considerable areas of bleached and altered lavas which
contain more or less alunite (K20.3A12O3.4S03.6H2O), an earthy or compact,
rarely coarsely crystalline mineral of inconspicuous appearance.
Occasionally it is associated
with
diaspore or gibbsite.
In most cases this basic sulfate, which is insoluble in water, is probably
formed by the action of waters containing free sulfuric acid on aluminous
rocks. It is also found in places in the oxidized zones of veins containing
pyrite. In such altered zones in volcanic rocks alunogen, jarosite,
halotrichite, and other sulfates of iron and aluminum are often encountered
as products of solution and oxidation. Pyrite sometimes appears as a
primary constituent, its iron being probably derived from the ferromagnesian
silicates of the rock. Although the alunite itself is used, in large
deposits, for the production of alum and similar salts, sometimes gold and
other rare metals are associated with areas of alunitization. This type of
deposit is known as the epithermal
quartz-alunite
gold deposit. Gold, pyrite, and enargite are found in vuggy
veins and breccias in zones of high-alumina alteration related to volcanic
rocks such as dacite, quartz latite, rhyodacite and rhyolite. These deposits
of
gold ore are hosted by felsic volcanic rocks, generally intrusions or lava
domes, that have low acid buffering capacity. Mineralized rock from
the deepest part of these systems tended to have elevated abundances of
arsenic and copper but not as elevated as those characteristic of Goldfield,
Nevada or Summitville, Colorado. Another example of this type of
deposit is Paradise Peak in Nevada. Typical minerals include:
pyrite, enargite, covellite,
chalcocite,
chalcopyrite,
native sulfur,
marcasite,
native gold, and
barite. Less common are
sphalerite,
galena, and
siderite.
Uses:
In Australia alunite is
calcined and then heated with dilute
sulfuric acid. The mixture is then allowed to settle and the clear
solution is drawn off and cooled. Alum crystallizes. The mother liquor
which contains aluminum sulphate, after further treatment with the
calcined mineral, is evaporated and the aluminum salt separated by
crystallization. It has been utilized
as a source of both potassium and aluminum.
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