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Molybdate mineral

Wulfenite is a lead molybdate mineral 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 lead ores 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.

Wulfenite was first described in 1845 for an occurrence in Bad Bleiberg, Carinthia, Austria. 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. 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. 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.

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). 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. 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.

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. 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.

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. 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. 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 of Cr6+ substituting for Mo6+ is adequate to result in an orange hued specimen. Cr6+ 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.