# Geominerals/Nickels

Native nickel of composition Ni 96.3%, Fe 1.77%, and Co 0.69% was found as tiny flakes up to 0.75 mm across in a heavy mineral concentrate from stream sediments in the Jerry River, South Westland, New Zealand.

## Aerugites

Aerugite is a rare complex nickel arsenate mineral with a variably reported formula: Ni $_9$(AsO $_4$) $_2$AsO $_6$ that forms green to deep blue-green trigonal crystals, has a Mohs hardness of 4 and a specific gravity of 5.85 to 5.95. The IMA symbol is Aru.

## Ahlfeldites

Ahlfeldite ((Ni,Co)SeO $_3$·2H $_2$O) is a mineral of secondary origin with the International Mineralogical Association (IMA) symbol Afe, named after Friedrich Ahlfeld (1892–1982), a German-Bolivian mining engineer and geologist, a type locality of Virgen de Surumi mine, Pakajake Canyon, Chayanta Province, Potosí Department, Bolivia.
In the image on the right is a druse of tiny alfredopetrovite crystals, colorless to greyish tone, coating over blue chalcomenite and pink ahlfeldite, on a kruťait-penroseite matrix. Some areas, with no colored background minerals, let us to see perfectly crystals. You can also observe face reflections of this extremely rare species. Alfredopetrovite is the first aluminium selenite mineral.

## Akaganeites

Akaganeites have the chemical formula Fe$^{3+}$ O(OH,Cl).
Akaganeite (International Mineralogical Association (IMA) symbol: Akg), also written as the deprecated Akaganéite, is a chloride-containing iron(III) oxide-hydroxide mineral, formed by the weathering of pyrrhotite (Fe $_{(1−x)}$S).
Akaganeite is often described as the β phase of anhydrous Iron(III) oxide-hydroxide (ferric oxyhydroxide) FeOOH, but some chloride (or fluoride) ions are normally included in the structure, so a more accurate formula is FeO $_{0.833}$(OH) $_{1.167}$Cl $_{0.167}$. Nickel may substitute for iron, yielding the more general formula (Fe$^{3+}$ ,Ni$^{2+}$ ) $_8$(OH,O) $_{16}$Cl $_{1.25}$
Akaganeite has a metallic luster and a brownish yellow streak, crystal structure is monoclinic and similar to that of hollandite BaMn $_8$O $_{16}$, characterised by the presence of tunnels parallel to the c-axis of the tetragonal lattice. These tunnels are partially occupied by chloride anions that give to the crystal its structural stability.
Occurrence: The mineral was discovered in the Akagane mine in Iwate, Japan, for which it is named. It was described by the Japanese mineralogist Matsuo Nambu in 1968, but named as early as 1961.
Akaganeite has also been found in widely dispersed locations around the world and in rocks from the Moon that were brought back during the Apollo Project. The occurrences in meteorites and the lunar sample are thought to have been produced by interaction with Earth's atmosphere. It has been detected on Mars through orbital imaging spectroscopy.

## Allabogdanites

Allabogdanite is a very rare phosphide mineral with formula (Fe,Ni) $_2$P, found in 1994 in the Onello meteorite. It was described for an occurrence in the Onello meteorite in the Onello River basin, Sakha Republic; Yakutia, Russia; associated with taenite, schreibersite, kamacite, graphite and awaruite. It was named for Russian geologist Alla Bogdanova.
In a June 2021 study, scientists reported the discovery of terrestrial allabogdanite in a sedimentary formation, located in the Negev desert of Israel, just southwest of the Dead Sea.

## Annabergites

Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate, Ni $_3$(AsO $_4$) $_2$·8H $_2$O, crystallizing in the monoclinic system and isomorphous with vivianite and erythrite. Crystals are minute and capillary and rarely met with, the mineral occurring usually as soft earthy masses and encrustations. A fine apple-green color is its characteristic feature. It was long known (since 1758) under the name nickel bloom; the name annabergite was proposed by H. J. Brooke and W H. Miller in 1852, from Annaberg in Saxony, one of the localities of the mineral. It occurs with ores of nickel, of which it is a product of alteration. A variety, from Creetown in Kirkcudbrightshire, in which a portion of the nickel is replaced by calcium, has been called dudgeonite, after P. Dudgeon, who found it.
Closely related is cabrerite wherein some of the nickel is replaced by magnesium. It is named for Sierra Cabrera in Spain where it was originally found.

## Awaruites

Awaruite has the chemical formula Ni $_2$Fe.
Awaruite occurs in river placer deposits derived from serpentinized peridotites and ophiolites, also occurs as a rare component of meteorites, in association with native gold and magnetite in placers; with copper, heazlewoodite, pentlandite, violarite, chromite, and millerite in peridotites; with kamacite, allabogdanite, schreibersite and graphite in meteorites.
It was first described in 1885 for an occurrence along Gorge River, near Awarua Bay, South Island, New Zealand, its type locality.
Awaruite is also known as josephinite in an occurrence in Josephine County, Oregon where it is found as placer nuggets in stream channels and masses in serpentinized portions of the Josephine peridotite. Some nuggets contain andradite garnet.
Awaruite occur as an ore mineral in a large low grade deposit in central British Columbia, some 90 km northwest of Fort St. James, disseminated in the Mount Sidney Williams ultramafic/ophiolite complex.

## Breithauptites

Breithauptite is a nickel antimonide mineral with the simple formula NiSb. Breithauptite is a metallic opaque copper-red mineral crystallizing in the hexagonal - dihexagonal dipyramidal crystal system. It is typically massive to reniform in habit, but is observed as tabular crystals. It has a Mohs hardness of 3.5 to 4 and a specific gravity of 8.23.
It occurs in hydrothermal calcite veins associated with cobalt–nickel–silver ores.

## Carrboydites

Carrboydite has the formula: (Ni $_{(1-x)}$Al $_x$)(SO $_4$) $_{(x/2)}$(OH) $_2$·nH $_2$O, where (x < 0.5, n > 3x/2), is a member of the Glaucocerinite Group > Hydrotalcite Supergroup, in the Hexagonal Crystal System, named for the Carr Boyd nickel mine, Australia, the type locality.
Carrboydite has the Chemical Formula: (Ni,Cu) $_{14}$Al $_9$(SO $_4$,CO $_3$)(OH) $_{43}$•7(H $_2$O). The Empirical Formula is Ni $_{10}$Cu $_4$Al $_9$(SO $_4$) $_4$(CO $_3$) $_2$(OH) $_{43}$•7(H $_2$O).
"As part of the recent re-evaluation of the nomenclature of the hydrotalcite supergroup (Mills et al., 2012), carrboydite was identified as a questionable species which needs further investigation."
Environment: "Surface material at a nickel mine."

## Garnierites

Garnierite is a general name for a green nickel ore which is found in pockets and veins within weathered and serpentinized ultramafic rocks. It forms by lateritic weathering of ultramafic rocks and occurs in many nickel laterite deposits in the world. It is an important nickel ore, having a large weight percent NiO.
Some of the proposed compositions are all hydrous Ni-Mg silicates, a general name for the Ni-Mg hydrosilicates which usually occur as an intimate mixture and commonly includes two or more of the following minerals: serpentine, talc, sepiolite, smectite, or chlorite, and Ni-Mg silicates, with or without alumina, that have x-ray diffraction patterns typical of serpentine, talc, sepiolite, chlorite, vermiculite or some mixture of them all.
The composition of a talc-like garnierite is close to the compositions of stevensite and sepiolite, but with partial replacement of the Mg content by Ni. Chemical analysis of garnierite samples yields non-stoichiometric formulae that can be reduced to formulas like those of talc and serpentine suggesting a talc monohydrate formula of H$_2$O(Mg,Ni)$_3$Si$_4$O$_{10}$(OH)$_2$ for the talc-like garnierite. Mg, Si, Fe, Ni and Al have been found in samples and the compositions of all these garnierite samples lie between the serpentine solid solution series and the sepiolite solid solution series. Using x-ray diffraction, the composition of garnierite samples collected at the Falcondo mine in the Dominican Republic fell into one of three groups: an Ni-talc to willemseite (up to 25 weight percent Ni) group, an Ni-lizardite to nepouite (up to 34 weight percent Ni) group and an Ni-sepiolite to falcondoite (up to 24 weight percent Ni) group. Using Extended X-ray Absorption Fine Structure (EXAFS) analysis to determine the composition of their garnierite samples, they had an almost complete solid solution between Ni-sepiolite and falcondoite, with samples analyzed showing between 3 and 77 percent falcondoite composition. According to X-ray and thermal analysis, the garnierites of the Ural deposits are multiphase formations and consist of a serpentinites (pecoraite 2McI, chrysotile 2McI, chrysotile 2OrcI, lisardite 6T, lisardite 1T, nepuit - nickel lisardite 1T), chlorites (clinochlor IIB, sepiolit, palygorskit), clay minerals (nontronit, saponite, montmorillonite, vermiculite), minerals of the mica supergroup (talc, vilemsite, clintonite, annite, phlogopite) and quartz. Calcite, sauconite, beidellite, halloysite, thomsonite, goethite, maghemite, opal, moganite, nickel hexahydrite, accessory magnesiochromite and rivsit are among the sporadic minerals found in them.
The unit cell parameters, found using transmission electron microscopy (TEM) analysis, are 13.385(4), 26.955(9), 5.271(3) Å and 13.33(1), 27.03(2), 5.250(4) Å, space group Pncn.
Based on the ionic radii and charge alone, Ni$^{2+}$ should easily substitute for Mg$^{2+}$ in octahedral coordination. The fact that Ni readily substitutes for Mg in garnierite explains why as NiO content goes up, MgO content goes down. The nickel in garnierite is not evenly distributed throughout the structure, but is concentrated in small zones of nickel surrounded by magnesium zones.
Garnierite is a layer silicate. The main difference between the serpentine-like and talc-like variants of garnierite is the spacing between layers in the structure, seen in x-ray powder diffraction studies. The serpentine-like variants have 7 Å basal spacings while the talc-like variants have a basal spacing of 10 Å. At 10$^6$X magnifications, the 7 and 10 Å layer spacings (d(001)) are obvious and measureable, with the 7 Å spacings being better defined than the 10 Å spacings. 7 Å, serpentine-like minerals show rod and tube shaped particles, as well as platy particles and fluffy particles that are most likely aggregates while the 10 Å variety shows much less variation in particles, showing only platy and fluffy forms with very few tube or rod shaped particles. Some particles exhibit interstratification of 7 and 10 Å spacings. There is no correlation between NiO content and the shapes of the particles in the mineral. 7 Å type garnierites usually resemble chrysotile or lizardite in their structures, while 10 Å types usually resemble pimelite.
Garnierite is a green mineral, ranging from light yellow-green to dark green. The color comes from the presence of nickel in the mineral structure for magnesium. Noumeaite (later determined to be a member of the garnierite family) varies in hardness, from soft and brittle to hard enough to carve into figurines and the like. Some species of garnierite stick to the tongue and dissolve readily in water or even on the tongue. Garnierite commonly has a colloform texture, typical of minerals that fill open spaces from a solution. In general, darker green garnierites have higher Ni content, higher specific gravity and higher mean index of refraction than lighter green garnierites, which most likely relates to the inclusion of more Ni in the structure. The Specific gravity of garnierite ranges from approximately 2.5 to 3. The mean index of refraction of garnierite ranges from approximately 1.563 to 1.601.
Light colored garnierite is an alteration of olivine-rich rock to a clay-like mineral poor in nickel, light green to bright green garnierite is a result of the leaching of manganese oxide, magnesium, nickel and iron from the original dark green garnierite, rich in nickel, which was deposited by groundwater. This leads to a very common occurrence of garnierite as fracture fillings of millimeter to centimeter thick veins or as a fabric or coatings at the Falcondo mine in the Dominican Republic. X-ray diffraction of samples from that mine show that garnierite veins include sepiolite-falcondoite and quartz (chrysoprase, a green variety of quartz with a nickel content of less than 2 weight %). Breccias found in faults at the Falcondo mine contain garnierite clasts cemented together by a secondary deposition of garnierite, which is evidence of syn-tectonic deposition of garnierite. In the garnierite deposits near Riddle, Oregon, garnierite is found as a weathering product of the underlying peridotite, with the garnierite layer between 50 and 200 ft (15 and 61 m) thick.

## Glaukosphaerites

Glaukosphaerites have the CNMMN/CNMNC approved formula: CuNi(CO $_3$)(OH) $_2$ and the Strunz formula: (Cu,Ni)$_2$[(OH)$_2$|CO$_3$], or (Cu,Ni) $_2$(CO $_3$)(OH) $_2$.
It is a member of the Rosasite Group.
Associated Minerals at Type Locality: Goethite, Quartz, Paratacamite, Gypsum, Magnesite, Malachite, and Clay.

## Heazlewoodites

Heazlewoodite, Ni$_3$S$_2$, is a rare sulfur-poor nickel sulfide mineral found in serpentinitized dunite. It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallize in the trigonal crystal system. It has a hardness of 4, a specific gravity of 5.82. Heazlewoodite was first described in 1896 from Heazlewood, Tasmania, Australia.
Heazlewoodite is formed within terrestrial rocks by metamorphism of peridotite and dunite via a process of nucleation. Heazlewoodite is the least sulfur saturated of nickel sulfide minerals and is only formed via metamorphic exsolution of sulfur from the lattice of metamorphic olivine.
Heazlewoodite forms from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes. Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe$^{2+}$ and Mg$^{2+}$).
During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration.
When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.
In this environment, nickel sulfide mineralogy converts to the lowest-sulfur state available, which is heazlewoodite.
Heazlewoodite is known from few ultramafic intrusions within terrestrial rocks. The Honeymoon Well ultramafic intrusive, Western Australia is known to contain heazlewoodite-millerite sulfide assemblages within serpentinized olivine adcumulate dunite, formed from the metamorphic process.
The mineral is also reported, again in association with millerite, from the ultramafic rocks of New Caledonia.

## Hydrohonessites

In the image on the right, hydrohonessite was riginally described as jamborite, now most green coatings on millerite like this have been shown to be hydrohonessite per David Hospital.
Hydrohonessite has the formula: (Ni $_{1-x}$Fe$^{3+}$ $_x$)(OH) $_2$(SO $_4$) $_{x/2}$ · nH $_2$O (x > 0.5, n > ​$^{3x}$⁄$_2$.
"May convert readily into honessite, depending on humidity and temperature. Appears to be stable between pH 6 and 7."
General Appearance of Type Material: Thin surface encrustation of tiny hexagonal crystals on botryoidal quartz and magnesite in a fracture in supergene Ni-Fe sulphides.
Associated Minerals at Type Locality: Quartz, Magnesite, Violarite, Pyrite, Gaspéite, Goethite, Pecoraite and Gypsum.
"A precipitate corresponding to hydrohonessite was obtained by slowly adding an aqueous 0.1 M ferrous sulphate solution to a 0.1 M nickel sulphate solution. The pH of the solution was maintained between 6.0 and 6.5 by the addition of 0.01 M sodium carbonate. Above pH 7.5, Ni(OH) $_2$ is precipitated, and below pH 6, FeOOH. The oxidation of the iron and precipitation of the hydrohonessite is slow, so care must be taken that the pH does not drop too low during the approximately 24 hours that the precipitation requires. After drying at 25°C, the precipitate gives a diffuse X-ray powder pattern similar to that of hydrohonessite. Chemical analysis of the precipitate confirmed that the composition is close to that of hydrohonessite. The infra-red spectrum of the synthetic hydrohonessite [...] is similar to that of honessite (Bish and Livingstone, 1981), and is characterized by strong absorptions due to H $_2$O and SO $_4$. The synthetic hydrohonessite dehydrates slowly at 25°C, and after ten days it gives a diffuse X-ray diffraction pattern of four lines which correspond to the strongest lines of the honessite diffraction pattern (Bish and Livingstone, 1981), i.e. with a basal spacing in the neighbourhood of 9 Å; the conversion can also be achieved more rapidly by heating the synthetic hydrohonessite at 110°C. The natural hydrohonessite is more stable than its synthetic equivalent, since it retains its integrity at 110°C, and dehydration requires a temperature between 150 and 170°C. The dehydration experiments indicate that hydrohonessite is the hydrated equivalent of honessite."

## Kambaldaites

Kambaldaites have the formula: NaNi $_4$(CO $_3$) $_3$(OH) $_3$ · 3H $_2$O.
General Appearance of Type Material: Cryptocrystalline veins, layers and concretionary growths up to about 2 mm thick, commonly intergrown with gaspeite, also as encrustations of tiny hexagonal prisms.
Geological Setting of Type Material: Nickel sulfide deposit, as a secondary mineral that has been precipitated on fracture surfaces in oxidizing Ni-Fe sulfide ore.
"The primary sulfides, which occur as assemblages of pentlandite-pyrrhotite-pyrite and pentlandite-millerite-pyrite, have been altered to supergene assemblages consisting largely of violarite and pyrite, which have decomposed on further oxidation to a goethitic residue in which the secondary nickel minerals have been deposited."
"The samples in which the kambaldaite was found are from a depth of about 20 meters, and consist largely of goethite with some reevesite and residual pyrite. The kambaldaite, together with gaspeite and some aragonite, occurs on fracture surfaces in the goethite. The kambaldaite occurs in a variety of types: massive, crystalline, nodular and chalky."

## Millerites

Millerite is a nickel sulfide mineral, NiS. Millerite is a common metamorphic mineral replacing pentlandite within serpentinite ultramafics. It is formed in this way by removal of sulfur from pentlandite or other nickeliferous sulfide minerals during metamorphism or metasomatism.
Millerite is also formed from sulfur poor olivine cumulates by nucleation. Millerite is thought to form from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes. Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe$^{2+}$ and Mn$^{2+}$ ).
During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration. When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.
This forms disseminated needle like millerite crystals dispersed throughout the rock mass. Millerite may be associated with heazlewoodite and is considered a transitional stage in the metamorphic production of heazlewoodite via the above process.
"Millerite, NiS, fractured under high vacuum and reacted with air and water has been analyzed by X-ray photoelectron spectroscopy (XPS). The pristine millerite surface gives rise to photoelectron peaks at binding energies of 853.1 eV (Ni 2p3/2) and 161.7 eV (S 2p), thus resolving ambiguities concerning binding energies quoted in the literature. Air-reacted samples show the presence of NiSO $_4$ and Ni(OH) $_2$ species. There is evidence for polysulfide species (S$^{2-􏰄}$ $_n$, where 2 􏰀≤ n 􏰀≤ 8) on air-oxidized surfaces. These may occur in a sub-surface layer or may be intermixed with the Ni(OH) $_2$ in the oxidized layer. The NiSO $_4$ species at the millerite surface occur as discrete crystallites whereas the Ni(OH) $_2$ forms a thin veneer covering the entire millerite surface. The NiSO $_4$ crystallites form on the surface of millerite but not on surfaces of adjacent minerals. Surface diffusion of Ni$^{2+}$ 􏰃 and SO$^{2−}$ $_4$ across the millerite surface [may] be responsible for the transport and subsequent growth of NiSO $_4$ crystallites developed on millerite surfaces. [It] is clear that Ni and SO$^{2−}$ $_4$ does not diffuse onto surfaces of adjacent minerals in sufficient quantity to form crystallites [...]. XPS results for water-reacted surfaces show little difference from the vacuum fractured surfaces with the exception that minor amounts of polysulfide and hydroxy nickel species are present. Similar reaction products to those formed in air [NiSO $_4$ and Ni(OH) $_2$] are believed to be produced, but these are removed from the millerite surface by dissolution, leaving behind a sulfur-enriched surface (polysulfide) and hydroxyl groups chemisorbed to nickel ions at the millerite surface.”
"The presence of NiSO $_4$ can be explained through oxidation of the sulfide ion in millerite to sulfate by molecular oxygen according to the following scheme:
NiS +􏰃 2O $_2$ → NiSO $_4$
In fact, it is most likely that the salt is hydrated. The presence of water in the O 1s spectrum supports the suggestion. The free energy of formation of hydrated NiSO $_4$ species is about 300 to 400 kcal/mol more negative than anhydrous NiSO $_4$, the difference being largest for the greatest degree of hydration. Even without hydration, the oxidation of NiS to NiSO $_4$ by molecular oxygen has a [reaction (rxn)] 􏰉∆G $_{rxn}$ 􏰅= -􏰄162.6 kcal/mol. Therefore, the oxidation of NiS to NiSO $_4$ is thermodynamically favored and should occur provided it is kinetically favored.”
"Coincident with formation of the hydroxy nickel surface complex is the formation of polysulfides. The nickel that reacts with the water and oxygen of ambient air is no longer bonded to sulfide. This sulfide is therefore available to react with other near-surface species, including other sulfide ions, which may lead to the formation of polysulfides (including disulfide) according to the following reaction scheme:”
nNiS 􏰃+ (n-􏰄1)H $_2$O 􏰃+ (n-􏰄1)/2O $_2$ → Ni$^{2􏰃􏰄+}$ $_0$ - S$^{2+}$ $_n$ + S$^{2−}$ $_n$ +􏰃 (n􏰄-1)Ni(OH) $_2$,
"where 2 ≤􏰀 n ≤􏰀 8. The designation Ni$^{2􏰃􏰄+}$ $_0$ - S$^{2+}$ $_n$ is used to denote polysulfide bonded to nickel in the lattice at the millerite surface. The Ni(OH) $_2$ and polysulfide may exist as separate, thin layers on the millerite surface with the Ni(OH) $_2$ presumably forming the overlayer. Alternatively, the polysulfides may be intermixed with the Ni(OH) $_2$ in the oxidized overlayer.”

## Morenosites

The heptahydrate nickel sulfate NiSO $_4$.7H $_2$O, which is relatively unstable in air, occurs as morenosite.
"On standing in the open in dry air, crystals of morenosite generally dehydrate rapidly to the tetragonal hexahydrate, retgersite. This was verified by the writers on several artificial preparations. The product formed, however, varies considerably with circumstances. One artificial preparation, crystallized from a water solution containing a little HCl, proved to be stable under ordinary conditions. Further, the natural morenosite from Minasragra had partially effloresced during the thirty years or so that it had been contained in the collection, but the dehydration product proved to be not the hexahydrate but a mixture of several lower hydrates."
A "complete series extends between the orthorhombic compounds morenosite, NiSO $_4$·7H $_2$O, and epsomite, MgSO $_4$·7H $_2$O, as shown by Dufet (5) and by Hutton (10)."
"Retgersite can be synthesized by crystallization from pure water solution at temperatures between 31.5°, below which orthorhombic NiSO $_4$·7H $_2$O is stable, and 53.3°, above which monoclinic NiSO $_4$·6H $_2$O is stable. A dihydrate forms above about 118°. These transition temperatures are from the data of Steele and Johnson (17); slightly different values have been reported by others (see Seidell (17)), and metastable equilibria commonly occur. Retgersite also can crystallize at temperatures at least as low as 0° from solutions which contain an appropriate excess of free H $_2$SO $_4$, as shown by Rohmer (14) and others. This factor may determine its formation in nature in place of morenosite. Under certain circumstances both retgersite and morenosite have been observed to crystallize simultaneously, one or the other of the two compounds being in metastable equilibrium."

## Népouites

Népouite is a rare nickel silicate mineral which has the apple green colour typical of such compounds. The ideal formula is Ni$_3$(Si$_2$O$_5$)(OH)$_4$, but most specimens contain some magnesium, and (Ni,Mg)$_3$(Si$_2$O$_5$)(OH)$_4$ is more realistic. There is a similar mineral called lizardite (named after the Lizard Complex in Cornwall, England) in which all of the nickel is replaced by magnesium, formula Mg$_3$(Si$_2$O$_5$)(OH)$_4$. These two minerals form a series; intermediate compositions are possible, with varying proportions of nickel to magnesium.

## Niccolites

Niccolite has the chemical formula NiAs.

## Nullaginites

"In nature there exists [the] hydroxy nickel carbonate [mineral] nullaginite Ni $_2$(CO $_3$)(OH) $_2$$^{1-3}$ [...] Nullaginite is monoclinic with point group 2/m and is a member of the rosasite mineral group. Nullaginite is formed in the oxidised zone of nickel rich hydrothermal ore deposits."

## Otwayites

Otwayite has the chemical formula Ni $_2$CO $_3$(OH) $_2$.
Otwayite has the formula: Ni $_2$CO $_3$(OH) $_2$ · H $_2$O.
Geological Setting of Type Material: Narrow veinlets to 1 mm in width, probably late-stage fracture filling, transecting nickeloan serpentine, millerite, polydymite, and apatite.
Otwayite is associated with Widgiemoolthalite.
Occurrence: Otwayite is found in association with nullaginite and hellyerite in the Otway nickel deposit, is found in association with theoprastite, hellyerite, gaspeite and a suite of other nickel carbonate minerals in the Lord Brassey Mine, Tasmania, in association with gaspeite, hellyerite and kambaldaite in the Widgie Townsite nickel gossan, Widgiemooltha, Western Australia, and reported from the Pafuri nickel deposit, South Africa.
It was first described in 1977 from the Otway Nickel Deposit, Nullagine, Pilbara Craton, Western Australia and named for Australian prospector Charles Albert Otway (born 1922).

## Pecoraites

Pecoraite is a nickel phyllosilicate mineral and a member of the Serpentine Subgroup > Kaolinite-Serpentine Group, named after geologist William Thomas Pecora, is monoclinic and has a chemical composition of Ni $_3$(Si $_2$O $_5$)(OH) $_4$.
Common Impurities: Al, Fe, Mg, Ca, H $_2$O.
Geological Setting of Type Material: Weathered meteorite.
Geological setting: Shears in ultramafic rocks, weathering product of millerite in geodes.
Pecoriate is typically a green, lime green, or bluegreen mineral with a waxy, or earthy luster and a mohs hardness of 2.5.
Morphology: curved plates, spirals and tubes, granular and massive.

## Pentlandites

This massive sulfide specimen on the right consists of brassy gray-brown pyrrhotite (Fe $_{(1-x)}$S - imperfect iron monosulfide) with brighter brassy-colored patches of pentlandite ((Ni,Fe) $_9$S $_8$ - nickel iron sulfide), plus a network of grayish to black patches of magnetite (Fe $_3$O $_4$ - iron oxide).
Pentlandite is an iron–nickel sulfide with the chemical formula (Fe,Ni) $_9$S $_8$. Pentlandite has a narrow variation range in Ni:Fe but it is usually described as having a Ni:Fe of 1:1. It also contains minor cobalt, usually at low levels as a fraction of weight.

## Reevesites

Chemical Formula: Ni $_6$Fe$^{3+}$ $_2$(CO $_3$)(OH) $_{16}$•4(H $_2$O).
Environment: Alteration product of a highly weathered iron-nickel meteorite.
Type Locality: Wolf Creek meteorite, found three km west of the Scotia talc mine, Bon Accord area, Barberton, Transvaal, South Africa.
Crystal System: Trigonal.
Member of the Hydrotalcite Group > Hydrotalcite Supergroup.
Geological Setting: Nickel rich ore deposits.
Geological Setting of Type Material: alteration product of a highly weathered iron-nickel meteorite.
Associated Minerals at Type Locality: Goethite, Jarosite, Serpentine Subgroup, Apatite, Lipscombite.

## Retgersites

Aqueous solutions of nickel sulfate react with sodium carbonate to precipitate nickel carbonate, a precursor to nickel-based catalysts and pigments. Addition of ammonium sulfate to concentrated aqueous solutions of nickel sulfate precipitates Ni(NH$_4$)$_2$(SO$_4$)$_2$·6H$_2$O, a blue-coloured solid analogous to Mohr's salt, Fe(NH$_4$)$_2$(SO$_4$)$_2$·6H$_2$O.
Nickel sulfate occurs as the rare mineral retgersite, which is a hexahydrate.
"Oxidization zone of nickel-bearing hydrothermal mineral deposits, formed from H $_2$O solution between 31.5 deg C and 53.5 deg C. Dimorphous with nickelhexahydrite."
The second hexahydrate is known as nickel hexahydrite (Ni,Mg,Fe)SO$_4$·6H$_2$O.
"May occur as a dehydration product of morenosite."
"The tetragonal polymorph of NiSO $_4$•6H $_2$O was first identified [...] as poorly formed, bluish green crystals incrusting a black coke-like mass of patronite from Minasragra, Peru."
Natural "occurrences of retgersite [...] comprised a foot-long mass of niccolite-bearing vein material from a mine in Cottonwood Canyon, Churchill County, Nevada [...]. The geology of the deposit has been described by Ferguson (6). The specimen has been thoroughly oxidized and is crusted over and veined by apple-green, granular masses of annabergite and blue-green fibrous aggregates of retgersite. The retgersiteis an original deposit, in part earlier formed than annabergite, and is not a dehydration product of morenosite."
"The Mg and part at least of the Fe$^{2+}$ is present in substitution for Ni."
"Retgersite is isostructural with the tetragonal polymorphs of the hexahydrated selenates of Ni and Zn."
"The monoclinic polymorph of NiSO $_4$•6H $_2$O has been prepared artificially and its crystallographic and optical properties have been described (8,9). This compound, green in color, is formed from pure water solutions at temperatures over 53.3°C. and below this temperature rapidly inverts to the blue tetragonal polymorph. The occurrence of this unstable and relatively soluble (52.5g. NiSO $_4$ in 100g. water at 54.5°) monoclinic phase in nature seems very unlikely. The monoclinic zinc and magnesium analogues are stable under ordinary conditions, however, and occur in nature as the minerals bianchite and hexahydrite."
The heptahydrate, which is relatively unstable in air, occurs as morenosite.
The monohydrate occurs as very rare mineral dwornikite (Ni,Fe)SO$_4$·H$_2$O.

## Taenites

"Taenite (Fe,Ni) is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with nickel proportions of 20% up to 65%. Taenite is one of four known Fe-Ni meteorite minerals: The others are kamacite, tetrataenite, and antitaenite. It is opaque with a metallic grayish to white color. The structure is isometric-hexoctahedral. Its density is around 8 g/cm³ and hardness is 5 to 5.5 on the Mohs scale. Taenite is magnetic."
The crystal lattice has the c≈a= 3.582 Å ±0.002 Å.
The Strunz classification is I/A.08-20, while the Dana classification is 1.1.11.2. It is a Hexoctahedral (cubic system) in structure."

## Takovites

Takovites have the formula: Ni $_6$Al $_2$(OH) $_{16}$(CO $_3$) · 4H $_2$O.
Member of the Hydrotalcite Group > Hydrotalcite Supergroup.
Morphology: Microcrystalline, platy, to 1 µm; commonly in veinlets and massive.

## Tetrataenites

"Tetrataenite is a native metal found in meteorites with the composition FeNi."
It is one of the mineral phases found in meteoric iron.

## Violarites

Violarite (Fe$^{2+}$Ni$_2$$^{3+}$S$_4$) is a supergene sulfide mineral associated with the weathering and oxidation of primary pentlandite nickel sulfide ore minerals.
Violarite is formed by oxidisation of primary sulfide assemblages in nickel sulfide mineralisation. The process of formation involves oxidation of Ni$^{2+}$ and Fe$^{2+}$ which is contained within the primary pentlandite-pyrrhotite-pyrite assemblage.
Violarite is produced at the expense of both pentlandite and pyrrhotite, via the following basic reaction;
Pentlandite + Pyrrhotite --> Violarite + Acid
    (Fe,Ni)$_9$S$_8$ + Fe$_{(1-x)}$S + O$_2$ → Fe$^{2+}$Ni$_2$$^{3+}$S$_4$ + H$_2$SO$_3$
Violarite is also reported to be produced in low-temperature metamorphism of primary sulfides, though this is an unusual paragenetic indicator for the mineral.
Continued oxidation of violarite leads to replacement by goethite and formation of a gossanous boxwork, with nickel tending to remain as impurities within the goethite or haematite, or rarely as carbonate minerals.
Violarite is reported widely from the oxidised regolith above primary nickel sulfide ore systems worldwide. It is of particular note from the Mount Keith dunite body, Western Australia, where it forms an important ore mineral.
It is also reported from open cast mines around the Kambalda Dome, and Widgiemooltha Dome, in association with polydymite, gaspeite, widgiemoolthalite and hellyerite, among other supergene nickel minerals.

## Willemseites

Falcondoite and willemseite in the image on the right are rare nickel, magnesium silicates found in a serpentinized harzbergite massif or an obducted ophiolite at a plate collision of oceanic crust with continental crust. The locality is in the Dominican Republic, which is the Type Locality for falcondoite. This very showy, bright green thin crust is mostly green falcondoite, with just a bit of lighter, olive willemseite.
Wllemseite has the chemical formula Ni$_3$Si$_4$O$_{10}$(OH)$_2$.