Wulfenite is a leadmolybdatemineral with the formula PbMoO4. It often occurs as thin tabular crystals with a bright orange-red to yellow-orange color, sometimes brown, although the color can be highly variable. In its yellow form it is sometimes called "yellow lead ore".
It crystallizes in the tetragonal system, often occurring as stubby, pyramidal or tabular crystals. It also occurs as earthy, granular masses. It is found in many localities, associated with leadores as a secondary mineral associated with the oxidized zone of lead deposits. It is also a secondary ore of molybdenum, and is sought by collectors.
A noted locality for wulfenite is the Red Cloud Mine in Arizona. Crystals are deep red in color and usually very well-formed. Wulfenite was approved as the official state mineral of Arizona in 2017.[5] The Los Lamentos locality in Mexico produced very thick tabular orange crystals.
Another locality is Mount Peca in Slovenia. The crystals are yellow, often with well-developed pyramids and bipyramids. In 1997, the crystal was depicted on a stamp by the Post of Slovenia.[6]
Lesser known localities of wulfenite include: Sherman Tunnel, St. Peter's Dome, Tincup-Tomichi-Moncarch mining districts, Pride of America mine and Bandora mine in Colorado.[7]
Small crystals also occur in Bulwell and Kirkby-in-Ashfield, England. These crystals occur in a galena-wulfenite-uraniferous asphaltite horizon in a magnesian limestone. The wulfenite found in this area is similar in properties (paragenetic sequence, low silver and antimony contents of the galenas and absence of pyromorphite) to the wulfenites of the Alps and may be similar in origin.[8]
Crystallography
Wulfenite crystallizes in the tetragonal system and possesses nearly equal axial ratios; as a result, it is considered to be crystallographically similar to scheelite (CaWO4).[9][10] Wulfenite is classed by a pyramidal-hemihedral (tetragonal dipyramidal) (C4h) crystal symmetry. Therefore, the unit cell is formed by placing points at the vertices and centers of the faces of rhomboids with square bases and the crystallographic axes coincide in directions with the edges of the rhomboids. Two of these lattices interpenetrate such that a point on the first is diagonal to the second and one quarter the distance between the two seconds.
An extensive solid solution exists between the two end members wulfenite and stolzite (PbWO4), such that tungstenian-wulfenite compositions range from 90% wulfenite and 10% stolzite to chillagite (64% wulfenite, 36% stolzite) and so on.[11] Nevertheless, the Commission for New Minerals and Mineral Names of the International Mineralogical Association has deemed that the solid solutions do not require new names. The correct nomenclature of the 90:10 solid state is wulfenite-I41/a and the 64:36 solid state is wulfenite-I4.[11] The structure of the wulfenite-I41/a system can be described as a close packing of tetrahedral MoO42− anions and Pb2+ cations.[11] In the lattice, the MoO42− anions are slightly distorted, though the bond lengths remain equal and the oxygens are linked through Pb-O bonds. Each lead atom has an 8-coordination with oxygen and two slightly different Pb-O bond distances. This structure closely resembles that of pure wulfenite.[11]
The structure of wulfenite-I4 is also very similar to that of wulfenite-I41/a but has an unequal distribution of tungsten and molybdenum which may explain the observed hemihedrism.[11]
It is argued that no miscibility gap exists in the wulfenite-stolzite solid solution at room temperature due to the almost identical size and shape of the MoO42− and WO42− ions, however, arguments have been made for the existence of a miscibility gap at higher temperatures.[11]
The heat capacity, entropy and enthalpy of wulfenite were determined taking into consideration the existence of solid solutions and the inclusion of impurities. The reported values are as follows: Cp°(298.15) = 119.41±0.13 J/molK, S°(298.15) = (168.33±2.06)J/molK, ΔH°= (23095±50) J/mol.[13]
When forced through a tube into a flame, wulfenite disintegrates audibly and fuses readily. With the salt of phosphorus, it yields molybdenum beads. With soda on charcoal it yields a lead globule. When the powdered mineral is evaporated with HCl, molybdic oxide is formed.[12]
Molybdenum can be extracted from wulfenite by crushing the ore to 60–80 mesh, mixing the ore with NaNO3 or NaOH, heating the mixture to about 700°C (decomposing), leaching with water, filtering, collecting the insoluble residues which may include Fe, Al, Zn, Cu, Mn, Pb, Au and Ag, then the NaMoO4 solution is agitated with a solution of MgCl2, filtered, CaCl2 or FeCl2 or any other chlorides is added to the Mo solution and heated and agitated, filtered and the desired product is collected. The full process is patented by the Union Carbide and Carbon Corp.[14]
Synthesis
Wulfenite has been shown to form synthetically through the sintering of molybdite with cerussite as well as that of molybdite with lead oxide. The following will describe both methods of synthesis.
Synthesis from molybdite and cerussite:
Thermal analysis of the 1:1 mix of molybdite and cerussite first displayed the characteristic peaks of cerussite. There is a sharp endothermic peak at 300°C, which occurs during the dehydration of hydrocerussite associated with cerussite. A second peak at 350°C is the first step of cerussite’s dissociation into PbO*PbCO3. Later at 400°C, a medium endothermic peak represents the second step of the dissociation into lead oxide. These transitions involve a decrease in mass, which occurs in steps. First, the dehydration of hydrocerussite is marked by its loss of constitutional OH and later is the freeing of carbon dioxide during the cerussite dissociation. The formation of wulfenite occurs at 520°C, as observed in the exothermic peak. The reaction between lead oxides and molybdenum takes place at 500–600°C, along with the formation of lead molybdate.
The endothermic peaks at 880 and 995°C perhaps denote the vaporization and melting of unreacted lead and molybdenum oxides. A small peak at 1050°C represents the melting of the wulfenite product itself, while an even smaller peak at 680°C may indicate some vaporization of molybdite as the molybdenum oxide volatilizes at 600–650°C.
This reaction occurs as follows:
350°C: 2PbCO3 → PbO*PbCO3+CO2
400°C: PbO*PbCO3 → 2PbO+CO2
500–520°C: MoO 3+PbO → PbMoO4 (wulfenite)
Synthesis from molybdite and lead oxide:
Thermal analysis for molybdite and lead oxide mixes at a 1:1 ratio suggest that the formation of wulfenite occurs at 500°C, as can be seen by an exothermic peak at this temperature. Microscopic investigation of the products show that at 500°C, wulfenite is the main product, while at 950°C, wulfenite is the only constituent of the product, as grains of molybdite and lead oxide melt and undergo volatilization. A small endothermic peak at 640°C may represent the start of vaporization, and a sharp and large endothermic peak at 980°C indicates the melting and volatilization of the unreacted lead and molybdenum oxides.
Characteristics of synthetic wulfenite:
Synthetically-made wulfenite will have the following composition: 61.38% PbO and 38.6% MoO3[further explanation needed]. This synthesis will give you samples of wulfenite that is pale-yellow in thin sections and is optically negative. It crystallizes in the tetragonal system, in the form of square tabular crystals, and with distinct cleavage on {011}. It crystals also display transparency and adamantine luster. The X-ray diffraction data, calculated cell dimensions, constants and optic axial angles of the synthetic wulfenite are consistent with those of the natural mineral.[15]
Coloration
Pure wulfenite is colorless, but most all samples display colors ranging from a creamy yellow to a sharp, intense red. Some samples even display blues, browns, and blacks. The yellow and red coloration of wulfenites is caused by small traces of chromium. Others have suggested that while the lead adds little colors, perhaps the molybdate contributes to wulfenite’s yellow color.[16]
More recent studies suggest that though the source of strong coloration is the presence of extrinsic impurities, the nonstoichiometry in both cationic and anionic sublattices also plays a major role in the coloration of the crystals. Tyagi et al. (2010) found that a reason for coloration in wulfenite is extrinsic impurity, as they were able to grow crystals displaying red, green, and various shades of yellow simply through changing the purity of the starting charges. They also posited that the presence of Pb3+ is not the cause of coloration. Because the crystals they grew in an Ar ambient are light yellow in color, they suggest that the interstitial oxygen concentration may be another cause in the coloration of wulfenite. Tyagi et al. note, however, the Mo is in a lower valence state when in Ar ambient, meaning it is Mo5+ rather than Mo6+. This suggests that the concentration of Mo5+ sites is also a cause of the coloration.[17]
Talla et al. (2013) posits that trace amounts of chromium do in fact play a role in determining the coloration of wulfenite. Here, the CrO42- anion group substitutes for the MoO42- group in the tetrahedral position. They found that as little as 0.002 atoms per formula unit (apfu) of Cr6+ substituting for Mo6+ is adequate to result in an orange-hued specimen. Cr6+apfu values of 0.01 were able to result in a red color. Talla et al. went on to emphasize that the colors result from a change of absorption intensity rather than a change of spectral position.[18]
Gallery
Cluster of translucent, butterscotch-colored wulfenite blades from the Glove Mine, Arizona, US
A plate of very sharp, chocolate-brown crystals of wulfenite to 1.5cm on edge
Wulfenite from Mexico
A rich jumble of tabular, reddish-brown wulfenite crystals fills the vug in the geode-like gossan matrix on this specimen
Wulfenite specimen from the Mina Ojuela, Mapimi, Durango, Mexico
Specimen from the Mina Ojuela, Mapimí, Durango, Mexico
Intensely colored crystals to 1.7cm, from Los Lamentos Mts (Sierra de Los Lamentos), Municipio de Ahumada, Chihuahua, Mexico
A yellow crystal elongated on its sides, with a small attached cerussite in front
A classic cluster of butterscotch-colored wulfenite blades richly dusted with olive-green mimetite botryoids
Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms. The Mo-S bonds are strong, but the interaction between the sulfur atoms at the top and bottom of separate sandwich-like tri-layers is weak, resulting in easy slippage as well as cleavage planes. Molybdenite crystallizes in the hexagonal crystal system as the common polytype 2H and also in the trigonal system as the 3R polytype.
Murdochite is a mineral combining lead and copper oxides with the chemical formula PbCu 6O 8−x(Cl,Br) 2x (x ≤ 0.5).
Mimetite is a lead arsenate chloride mineral (Pb5(AsO4)3Cl) which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name derives from the Greek Μιμητής mimetes, meaning "imitator" and refers to mimetite's resemblance to the mineral pyromorphite. This resemblance is not coincidental, as mimetite forms a mineral series with pyromorphite (Pb5(PO4)3Cl) and with vanadinite (Pb5(VO4)3Cl). Notable occurrences are Mapimi, Durango, Mexico and Tsumeb, Namibia.
Powellite is a calcium molybdate mineral with formula CaMoO4. Powellite crystallizes with tetragonal – dipyramidal crystal structure as transparent adamantine blue, greenish-brown, yellow-to-grey typically anhedral forms. It exhibits distinct cleavage, and has a brittle-to-conchoidal fracture. It has a Mohs hardness of 3.5 to 4 and a specific gravity of 4.25. It forms a solid solution series with scheelite (calcium tungstate, CaWO4). It has refractive index values of nω=1.974 and nε=1.984.
Molybdenum trioxide describes a family of inorganic compounds with the formula MoO3(H2O)n where n = 0, 1, 2. The anhydrous compound is produced on the largest scale of any molybdenum compound since it is the main intermediate produced when molybdenum ores are purified. The anhydrous oxide is a precursor to molybdenum metal, an important alloying agent. It is also an important industrial catalyst. It is a yellow solid, although impure samples can appear blue or green.
Leadhillite is a lead sulfate carbonate hydroxide mineral, often associated with anglesite. It has the formula Pb4SO4(CO3)2(OH)2. Leadhillite crystallises in the monoclinic system, but develops pseudo-hexagonal forms due to crystal twinning. It forms transparent to translucent variably coloured crystals with an adamantine lustre. It is quite soft with a Mohs hardness of 2.5 and a relatively high specific gravity of 6.26 to 6.55.
Massicot is lead (II) oxide mineral with an orthorhombic lattice structure. Lead(II) oxide can occur in one of two lattice formats, orthorhombic and tetragonal. The red tetragonal form is called litharge. PbO can be changed from massicot to litharge by controlled heating and cooling. At room temperature massicot forms soft yellow to reddish-yellow, earthy, scaley masses which are very dense, with a specific gravity of 9.64. Massicot can be found as a natural mineral, though it is only found in minor quantities. In bygone centuries it was mined. Nowadays massicot arises during industrial processing of lead and lead oxides, especially in the glass industry, which is the biggest user of PbO.
Stolzite is a mineral, a lead tungstate; with the formula PbWO4. It is similar to, and often associated with, wulfenite which is the same chemical formula except that the tungsten is replaced by molybdenum. Stolzite crystallizes in the tetragonal crystal system and is dimorphous with the monoclinic form raspite.
Huttonite is a thorium nesosilicate mineral with the chemical formula ThSiO4 and which crystallizes in the monoclinic system. It is dimorphous with tetragonal thorite, and isostructual with monazite. An uncommon mineral, huttonite forms transparent or translucent cream–colored crystals. It was first identified in samples of beach sands from the West Coast region of New Zealand by the mineralogist Colin Osborne Hutton (1910–1971). Owing to its rarity, huttonite is not an industrially useful mineral.
Duftite is a relatively common arsenate mineral with the formula CuPb(AsO4)(OH), related to conichalcite. It is green and often forms botryoidal aggregates. It is a member of the adelite-descloizite Group, Conichalcite-Duftite Series. Duftite and conichalcite specimens from Tsumeb are commonly zoned in color and composition. Microprobe analyses and X-ray powder-diffraction studies indicate extensive substitution of Zn for Cu, and Ca for Pb in the duftite structure. This indicates a solid solution among conichalcite, CaCu(AsO4 )(OH), austinite, CaZn(AsO4)(OH) and duftite PbCu(AsO4)(OH), all of them belonging to the adelite group of arsenates. It was named after Mining Councilor G Duft, Director of the Otavi Mine and Railroad Company, Tsumeb, Namibia. The type locality is the Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia.
Iranite (Persian: ایرانیت) is a triclinic lead copper chromate silicate mineral with formula Pb10Cu(CrO4)6(SiO4)2(F,OH)2. It was first described from an occurrence in Iran. It is the copper analogue of hemihedrite (Pb10Zn(CrO4)6(SiO4)2(F,OH)2).
Plumbogummite is a rare secondary lead phosphate mineral, belonging to the alunite supergroup of minerals, crandallite subgroup. Some other members of this subgroup are:
Plattnerite is an oxide mineral and is the beta crystalline form of lead dioxide (β-PbO2), scrutinyite being the other, alpha form. It was first reported in 1845 and named after German mineralogist Karl Friedrich Plattner. Plattnerite forms bundles of dark needle-like crystals on various minerals; the crystals are hard and brittle and have tetragonal symmetry.
Beudandite is a secondary mineral occurring in the oxidized zones of polymetallic deposits. It is a lead, iron, arsenate, sulfate with endmember formula: PbFe3(OH)6SO4AsO4.
Tsumebite is a rare phosphate mineral named in 1912 after the locality where it was first found, the Tsumeb mine in Namibia, well known to mineral collectors for the wide range of minerals found there. Tsumebite is a compound phosphate and sulfate of lead and copper, with hydroxyl, formula Pb2Cu(PO4)(SO4)(OH). There is a similar mineral called arsentsumebite, where the phosphate group PO4 is replaced by the arsenate group AsO4, giving the formula Pb2Cu(AsO4)(SO4)(OH). Both minerals are members of the brackebuschite group.
Minium is the naturally occurring form of lead tetroxide, Pb2+2Pb4+O4 also known as red lead. Minium is a light-to-vivid red and may have brown-to-yellow tints. It typically occurs in scaly-to-earthy masses. It crystallizes in the tetragonal crystal system.
Tsumcorite is a rare hydrated lead arsenate mineral that was discovered in 1971, and reported by Geier, Kautz and Muller. It was named after the TSUMeb CORporation mine at Tsumeb, in Namibia, in recognition of the Corporation's support for mineralogical investigations of the orebody at its Mineral Research Laboratory.
The Tabataud Quarry is situated in the northwestern French Massif Central. The quarry used to be mined for its granodiorite.
Carminite (PbFe3+2(AsO4)2(OH)2) is an anhydrous arsenate mineral containing hydroxyl. It is a rare secondary mineral that is structurally related to palermoite (Li2SrAl4(PO4)4(OH)4). Sewardite (CaFe3+2(AsO4)2(OH)2) is an analogue of carminite, with calcium in sewardite in place of the lead in carminite. Mawbyite is a dimorph (same formula, different structure) of carminite; mawbyite is monoclinic and carminite is orthorhombic. It has a molar mass of 639.87 g. It was discovered in 1850 and named for the characteristic carmine colour.
Mottramite is an orthorhombic anhydrous vanadate hydroxide mineral, PbCu(VO4)(OH), at the copper end of the descloizite subgroup. It was formerly called cuprodescloizite or psittacinite (this mineral characterized in 1868 by Frederick Augustus Genth). Duhamelite is a calcium- and bismuth-bearing variety of mottramite, typically with acicular habit.
↑ Gašperšič, Primož. "Rudnik svinca in cinka v Mežici"[Lead and Zinc Mine in Mežica]. In Šmid Hribar, Mateja; Torkar, Gregor; Golež, Mateja; etal. (eds.). Enciklopedija naravne in kulturne dediščine na Slovenskem – DEDI (in Slovenian). Retrieved 12 March 2012.
↑ Vesselinov, I. (1971). "Relation between the structure of wulfenite, PbMoO4, as an example of scheelite type structure, and the morphology of its crystals". Journal of Crystal Growth. 10 (1): 45–55. Bibcode:1971JCrGr..10...45V. doi:10.1016/0022-0248(71)90045-5.
1 2 Bayley, William Shirley (1917). Descriptive Mineralogy. United States of America: D. Appleton And Company. pp.257–258.
↑ Bissengaliyeva, Mira R.; Bespyatov, Michael A.; Gogol’, Daniil B. (9 September 2010). "Experimental Measurement and Calculation of Mole Heat Capacity and Thermodynamic Functions of Wulfenite PbMoO". Journal of Chemical & Engineering Data. 55 (9): 2974–2979. doi:10.1021/je901040d.
↑ Talla, D.; Wildner, M.; Beran, A.; Škoda, R.; Losos, Z. (2013-11-01). "On the presence of hydrous defects in differently coloured wulfenites (PbMoO4): an infrared and optical spectroscopic study". Physics and Chemistry of Minerals. 40 (10): 757–769. Bibcode:2013PCM....40..757T. doi:10.1007/s00269-013-0610-8. ISSN0342-1791. S2CID97718142.
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