Huntite

minerals

deweylite

brucite

hydromagnesite

Images
Formula: CaMg3(CO3)4
Anhydrous normal carbonate
Specific gravity: 2.696
Hardness: 1 to 2
Streak: White
Colour: White
Environments

Sedimentary environments
Cave deposits

Huntite occurs in caves where rocks are magnesium-rich, as a near-surface weathering product of rocks containing brucite, dolomite, magnesite and serpentine, and as a diagenetic product in recent sediments (Dana). It is typically precipitated by evaporative concentration of meteoric solutions weathering magnesite- or dolomite- bearing rocks, or serpentinite. Associated minerals include magnesite, dolomite, aragonite, calcite and hydromagnesite (HOM).

Localities

At the Tea Tree Gully, South Mt Lofty Ranges, Mt Lofty Ranges, South Australia, huntite occurs at depths of up to 30 feet below the surface, as nodules associated with a dolomite horizon. Weathering of the dolomite has resulted in calcite being formed; the magnesium in part has formed nodules of magnesite and in part percolated down into fissures and precipitated as huntite. No evidence was found of the huntite replacing dolomite or magnesite (AM 52.1332-1340).

. At the Grotte de la Clamouse, Saint-Jean-de-Fos, Lodève, Hérault, Occitanie, France, huntite has been found as a precipitated carbonate in caves in dolostone. It was precipitated from waters which had become relatively rich in magnesium after traversing the dolostone (AM 52.1332-1340).

In the Emirate of Abu Dhabi, United Arab Emirates, along the Trucial Coast, the partial infilling of marine lagoons by recent carbonate sediments has formed a coastal plain which lies just above the level of normal high tides. The carbonate sediments are predominantly of aragonite. Huntite has been found in the unlithifred sediments of the coastal plain. It occurs in the upper 16 inches of the sediments, as irregular wisps and blebs to 0.3 inches, associated with a sandy mud of detrital quartz and carbonates, together with abundant corroded crystals of anhydrite, celestine, gypsum and traces of dolomite (AM 52.1332-1340).

At the Clear Creek claim, Goat Mountain, New Idria Mining District, San Benito county, California, USA, huntite has been found on magnesite associated with gypsum (Minrec 36.4.354).

At the Crestmore quarries, Crestmore, Riverside county, California, USA, the mineral assemblage magnesium calcite-aragonite-huntite has been found as an encrustation on brecciated calcite-monticellite rock. The magnesium-calcite-aragonite-huntite assemblage was considered to represent a metastable early mineral assemblage. In some places replacement by an equilibrium dolomite-calcite assemblage was found (AM 52.1332-1340).

At the Sierra Magnesite Mine, Gabbs, Gabbs Mining District, Nye county, Nevada, USA, early dolomite has been mineralised and in part converted to magnesite; subsequent igneous activity has resulted in brucite being developed. The huntite occurs as veinlets in the fractured and weathered mantle of the brucite and is associated with hydromagnesite. Surface waters percolating downwards are considered to have precipitated the huntite (AM 52.1332-1340).

At the type locality, the Ala-Mar deposit, Currant Mining District, White Pine county, Nevada, USA, huntite was deposited by cool waters in cavities and vugs in rocks composed of magnesite, dolomite and deweylite.
The magnesite deposits are directly overlain by dacite; in parts the dacite is overlain by volcanic breccia, which in turn is overlain by basaltic andesite. The host rock of the magnesite and associated minerals is a bedded tuff formation, which overlies the lower volcanics. The tuff is overlain by latite.
The minerals of the tuff include hypersthene, augite, hornblende, biotite, plagioclase, sanidine, quartz, apatite, magnetite and calcite. Subsequent to deposition some of the tuff beds were altered by cool meteoric waters. The volcanic glass was in part changed to montmorillonite and there was some replacement by calcite. The magnesite deposits were formed by hydrothermal solutions rich in magnesium and carbon dioxide which ascended along channelways provided by the fault zones. Where calcite was present, the replacing solutions produced dolomite until no more calcite was in actual contact with the solution. Thereafter, the hydrothermal solutions replaced the tuff with magnesite and deweylite.
In the magnesite deposit the early stage represents replacement of the tuff by calcite and montmorillonite.
Then magnesium-rich solutions react with the calcite of the tuff to form dolomite and the solutions became impoverished in calcium.
Then magnesite and later deweylite formed.
At the close of this stage calcite and silica minerals (quartz, chalcedony and opal) were deposited along with some dolomite and deweylite. Finally the silica minerals and calcite were the sole phases forming. The solutions gradually cooled to the temperature of the ground waters and calcite continued to form. During the late stage the vugs in the replaced rocks were lined with botryoidal calcite. The last mineral to fill these cavities was huntite, which occurs as a white powdery mass. Huntite is believed to have been precipitated as a very fine powder from cool ground waters which gathered their magnesium locally in traversing the magnesite deposits (AM 38.4-24).

At the Carlsbad Caverns, Carlsbad Caverns National Park, Eddy county, New Mexico, USA, huntite has been reported associated with aragonite, calcite, hydromagnesite and dolomite. Aragonite, calcite and hydromagnesite are precipitated directly from solution whereas dolomite and huntite apparently are derived from the alteration of hydromagnesite. It was concluded that huntite was a metastable mineral under the cave conditions (AM 52.1332-1340).

At the Kurgashinkan mine, Okhangaron District, Tashkent, Uzbekistan, huntite was formed as fracture fillings in the weathered mantle of lightly serpentinised dolostone and was associated also with hydromagnesite and opal. The opal often penetrated the huntite in a network of line fractures, while hydromagnesite occurred as very small crystals in cavities in the huntite. The huntite was considered to result from supergene conditions connected with circulation of surface waters rich in magnesium from traversing the local dolostone (AM 52.1332-1340).

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