Geochemistry to produce Widgiemoolthalite
"Widgiemoolthalite is a rare hydrated nickel(II) carbonate mineral with the chemical formula (Ni,Mg)5(CO3)4(OH)2·5H2O. Usually bluish-green in color, it is a brittle mineral formed during the weathering of nickel sulfide. Present on gaspéite surfaces".
One consequence of the 1966 discovery of nickel deposits in Western Australia and subsequent nickel mining boom was the discovery of novel secondary mineral species in mined regions starting in the mid-1970s.
"Widgiemoolthalite was first found at 132 North, a nickel deposit near Widgiemooltha, Western Australia, controlled by the Western Mining Corporation. Blair J. Gartrell collected the holotype widgiemoolthalite specimen from a stockpile of secondary minerals at the site."
The mineral was discovered in 1992 and was first reported in American Mineralogist in 1993 by Ernest H. Nickel, Bruce W. Robinson, and William G. Mumme, when it received its name for its type locality. Widgiemoolthalite's existence was confirmed and name was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association the same year."
The holotype specimen was stored in Perth's Western Australian Museum (specimen M.1.1993).
"Widgiemoolthalite occurs as a secondary mineral."
It is found overlaying nickel sulfide that has undergone weathering, often in hollow spaces on gaspéite surfaces [...], and often exhibiting fibrous and rarely massive crystal habits. Other minerals associated with widgiemoolthalite include annabergite, carrboydite, dolomite, glaukosphaerite, hydrohonessite, kambaldaite, magnesite, nepouite, nullaginite, olivenite, otwayite, paratacamite, pecoraite, reevesite, retgersite, and takovite. Two additional unnamed minerals were also reported as associated secondary minerals from the 132 North site, the only locality at which widgiemoolthalite had been found as of 2016. The 132 North waste pile from which widgiemoolthalite was first recovered is no longer in existence, making it a rare mineral. In support of the designation of an Anthropocene epoch, the existence and provenance of widgiemoolthalite, along with 207 other mineral species, were cited in 2017 by Robert M. Hazen et al. as evidence of uniquely human action upon global stratigraphy.
Def. "leached and then deposited by descending waters" is called a supergene process.
"In ore deposit geology sugergene processes or enrichment occurs relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomittant oxidation and chemical weathering. The descending meteoric waters oxidize the primary (hypogene) sulfide ore minerals and redistribute the metallic ore elements. Supergene enrichment occurs at the base of the oxidized portion of an ore deposit at which point the metals are redeposited on hypogene sulfides creating a zone of increased ore content."
This is particularly noted in copper ore deposits where the copper sulfide minerals chalcocite Cu2S, covellite CuS, digenite Cu1.8S, and djurleite Cu31S16 are deposited by the descending surface waters.
All such processes take place at essentially atmospheric conditions, 25 °C and atmospheric pressure.
From the surface down they are different zones: a gossan cap, a leached zone, an oxidized zone, the water table, an enriched zone (supergene enriched zone) and the primary zone (hypogene zone).
Pyrite FeS2 is generally abundant, and near the surface it oxidises to insoluble compounds such as goethite FeO(OH) and limonite, forming a porous covering to the oxidized zone known as gossan or iron hat.
The groundwater contains dissolved oxygen and carbon dioxide, and as it travels downwards it leaches out the minerals in the rocks to form sulfuric acid, and other solutions that continue moving downwards.
Above the water table the environment is oxidizing, and below it is reducing. Solutions traveling downward from the leached zone react with other primary minerals in the oxidised zone to form secondary minerals such as sulfates and carbonates, and limonite, which is a characteristic product in all oxidised zones.
In the formation of secondary carbonates, primary sulfide minerals generally are first converted to sulfates, which in turn react with primary carbonates such as calcite CaCO3, dolomite CaMg(CO3)2 or aragonite (also CaCO3, polymorphic with calcite) to produce secondary carbonates. Soluble salts continue on down, but insoluble salts are left behind in the oxidised zone where they form. An example is the lead mineral anglesite PbSO4. Copper may be precipitated as malachite Cu2(CO3)(OH)2 or azurite Cu3(CO3)2(OH)2. Malachite, azurite, cuprite Cu2O, pyromorphite Pb5(PO4)3Cl and smithsonite ZnCO3 are stable in oxidising conditions and they are characteristic of the oxidation zone.
At the water table the environment changes from an oxidizing environment to a reducing one.
Copper ions that move down into this reducing environment form a zone of supergene sulfide enrichment. Covellite CuS, chalcocite Cu2S and native copper Cu are stable in these conditions and they are characteristic of the enriched zone.
The net effect of these supergene processes is to move metal ions from the leached zone to the enriched zone, increasing the concentration there to levels higher than in the unmodified primary zone, possibly producing a deposit worth mining.
The primary zone contains unaltered primary minerals.
Chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite Cu5FeS4, covellite CuS and brochantite Cu4SO4(OH)6.
Pyrite FeS2 (primary) alters to secondary melanterite FeSO4.7H2O.
If the original deposits contain arsenic and phosphorus bearing minerals secondary arsenates and phosphates will be formed.
"Gaspéite's formula is (Ni,Fe,Mg)CO3 and it is a bright green mineral. It forms massive to reniform pappillary aggregates in fractures, bottryoidal concretions in laterite or fracture infill. It is also present as stains and patinas on iron oxide boxworks of gossanous material."
"Gaspéite is formed in the regolith as a supergene alteration mineral of nickel sulphide minerals, generally in arid or semi-arid environments which produce conditions amenable to concentration of calcareous or carbonate minerals in the weathering profile."
"Gaspéite from Widgiemooltha is associated with talc carbonated komatiite-associated nickel sulphide gossans and is probably formed by substitution of nickel into carbonates such as magnesite which are formed by oxidation of the talc-carbonate lithology, and of primary and supergene nickel sulphide minerals."
"Gaspéite is formed from a similar process to the weathering of other sulphide minerals to form carbonae minerals. The sulphide minerals which are weathered to produce gaspeite are pentlandite, violarite, millerite and rarely niccolite."
Def. formed "underground, often by ascending solutions" is called a hypogene process.
In ore deposit geology, hypogene processes occur deep below the earth's surface, and tend to form deposits of primary minerals, as opposed to supergene processes that occur at or near the surface, and tend to form secondary minerals.
At great depth the pressure is high, and water can remain liquid at temperatures well above 100 °C. Hot aqueous solutions originating in the magma contain metal and other ions derived from the magma itself, and also from leaching of surrounding rocks. Hypogene deposition processes include crystallization from the hot aqueous solutions rising through the earth's crust, driven by heat provided by the magma.
Major dissolved components are chlorine, sodium, calcium, magnesium and potassium, and other important components include iron, manganese, copper, zinc, lead, sulfur (as SO42− or S2− or both) carbon (as HCO3− and CO2) and nitrogen (as NH4). Most ore fluids contain chloride as the dominant anion.
As the solutions rise the temperature and pressure fall. Eventually a point is reached where the minerals start to crystallise out. Minerals formed in this way are called primary, or hypogene, minerals. Sulfur is a common component of the fluids, and most of the common ore metals, lead, zinc, copper, silver, molybdenum and mercury, occur chiefly as sulfide and sulfosalt minerals such as pyrite (FeS2), galena (PbS), sphalerite (ZnS), and chalcopyrite (CuFeS2).
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