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14.10: Tungstates, Molybdates, and Chromates

  • Page ID
    18683
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    wolframite series
    huebnerite MnWO4
    ferberite FeWO4

    others
    scheelite CaWO4
    wulfenite PbMoO4
    crocoite PbCrO4

    Tungstates, molybdates, and chromates are chemically and structurally related to the anhydrous sulfates.

    Generally rare, these minerals may be locally concentrated in ore deposits. Wolframite is the most common tungsten ore mineral. Scheelite, too, is an important tungsten ore mineral. Wulfenite is a minor molybdenum ore mineral. Crocoite is a minor secondary mineral sometimes found with other lead ore minerals.

    Wolframite (Fe,Mn)WO4

    Origin of Name
    The name’s origin is unclear; perhaps it comes from the German words for “wolf” and rahm, meaning “soot,” in reference to its color. Wolframite is not a single mineral species, but is a solid solution series between end members ferberite and huebnerite.

    14.422.png
    Figure 14.422: Wolframite with quartz and calcite from Hunan Province, China, 3.1 cm across

    Hand Specimen Identification
    Very high specific gravity, dark color, and one good cleavage help identify wolframite. It is sometimes confused with hornblende but is much heavier. It is occasionally confused with scheelite or other heavy dark-colored minerals. Figure 14.422 shows a typical example of black wolframite. It is perched on top of clear quartz and orangish calcite.

    Physical Properties

    hardness 4 to 4.5
    specific gravity 7.25 to 7.60
    cleavage/fracture one perfect (010)/uneven
    luster/transparency submetallic/opaque unless very thin color black, red-brown
    color brownish black or iron black
    streak black, red-brown

    Properties in Thin Section
    Wolframite is generally opaque. Biaxial (+), α = 2.17 to 2.31 , β = 2.22 to 2.40, γ = 2.30 to 2.46, δ = 0.13 to 0.15, 2V = 73° to 79°.

    Crystallography
    Wolframite is monoclinic, a = 4.71 to 4.85, = 5.70 to 5.77, c = 4.94 to 4.98, b = 90° to 91°, Z = 2; space group \(P\dfrac{2}{c}\); point group \(\dfrac{2}{m}\).

    Habit
    Wolframite crystals are short to long prisms or tabs, often showing vertical striations. Wolframite may form bladed, subparallel crystal groups.

    Structure and Composition
    A complete solid solution exists between ferberite (FeWO4) and huebnerite (MnWO4), the two principal wolframite end members. The basic structure consists of layers of distorted (WO4)2- tetrahedra joined by octahedral Fe or Mn.

    Occurrence and Associations
    Wolframite is a rare mineral. Usually found in high-temperature quartz veins associated with granitic igneous rocks, it is, however, the most important tungsten ore mineral. It is also found in sulfide-rich veins, associated with scheelite, cassiterite, pyrite, or galena. It may contain minor amounts of Ca, Mg, or rare earth elements.

    Related Minerals
    Similar minerals include the rare tungstates sanmartinite, (Zn,Fe)WO4, and raspite, PbWO4.

    Scheelite CaWO4

    Origin of Name
    Named after K. W. Scheele (1742–1786), a Swedish chemist.

    14.423.png
    Figure 14.423: Scheelite, quartz, and muscovite from China, 6.2 cm across

    Hand Specimen Identification
    High specific gravity, crystal habit, distinct cleavage, and fluorescence under ultraviolet light are characteristic for scheelite. It may be confused with quartz or feldspar when uncolored.

    Figure 14.423 shows scheelite crystals, with sheelite‘s classic orange diagnostic color, on quartz and muscovite. Scheelite, however, may have other hues.

    Physical Properties

    hardness 4.5 to 5
    specific gravity 6.11
    cleavage/fracture good pyramidal {101}, poor {112}/uneven
    luster/transparency subadamantine/transparent to translucent
    color tan, orange, brown, yellow, white, or colorless
    streak white

    Properties in Thin Section
    Scheelite is uniaxial (+), ω = 1.920, ε = 1.934, δ = 0.014.

    Crystallography
    Scheelite is tetragonal, a = 5.25, c = 11.40, Z = 4; space group \(I\dfrac{4_1}{a}\); point group \(\dfrac{4}{m}\).

    Habit
    Scheelite is found in massive, columnar, or granular aggregates and as individual crystals, usually dipyramids.

    Structure and Composition
    Scheelite is usually close to end-member composition, but a limited solid solution exists with powellite, CaMoO4. It may also incorporate small amounts of Cu or Mn. In the structure, isolated (WO4)2- tetrahedra are linked by 8-coordinated Ca.

    Occurrence and Associations
    Scheelite is a high-temperature mineral found in metamorphic aureoles, in granites and pegmatites, and in some hydrothermal veins. It may be found with cassiterite, SnO2; topaz, Al2SiO4(F,OH)2; fluorite, CaF2; apatite, Ca5(PO4)3(OH,F,Cl); and with other tungstates or molybdates.

    Related Minerals
    Scheelite is isostructural with wulfenite, PbMoO4.

    Wulfenite PbMoO4

    Origin of Name
    Named after Austrian mineralogist F. X. von Wulfen (1728–1805).

    14.424.png
    Figure 14.424: Wulfenite specimen from the Mammoth Mine, Arizona
    14.425.png
    Figure 14.425: Wulfenite display in the New York Museum of Natural History

    Hand Specimen Identification
    Crystal habit and orange-yellow color are distinctive. Wulfenite is occasionally confused with native sulfur but has much greater density.

    Figure 14.424 shows a specimen of wulfenite from southern Arizona. Figure 14.425 is a photo of a display case with a dozen samples from different places. The orange to yellow-orange color, although a bit variable, is a key to identifying this mineral.

    Physical Properties

    hardness 3
    specific gravity 6.7 to 7.0
    cleavage/fracture good but rarely seen pyramidal {011}, poor (001)/ subconchoidal
    luster/transparency adamantine, silvery/ transparent to translucent
    color yellow, orange, red, brown, green
    streak white

    Optical Properties
    Wulfenite is uniaxial (-), ω = 2.404, ε = 2.283, δ = 0.121.

    Crystallography
    Wulfenite is tetragonal, a = 5.42, c = 12.10, Z = 4; space group \(I\dfrac{4_1}{a}\); point group \(\dfrac{4}{m}\).

    Habit
    Wulfenite crystals are most often square tablets, often thin or pyramidal. They may also be prismatic.

    Occurrence and Associations
    Wulfenite is a rare secondary mineral found in the oxidized portions of Pb deposits. It may be associated with galena, PbS; cerussite, PbCO3; vanadinite, Pb5(VO4)3Cl; or pyromorphite, Pb5Cl(PO4)3. Wulfenite is isostructural with scheelite, CaWO4, with which it forms limited solid solutions. It also forms limited solid solutions with powellite, CaMoO4, and raspite, PbWO4.

    Crocoite PbCrO4

    Origin of Name
    From the Greek word krokos, meaning “saffron,” referring to its color.

    14.426.png
    Figure 14.426: Orange crocoite on black goethite, from Tasmania, Australia
    14.427.png
    Figure 14.427: Crocoite from the Ural Mountains, 26 cm across

    Hand Specimen Identification
    Distinctive reddish-orange color and adamantine luster, its high specific gravity and habit, and association with other Pb-minerals usually make crocoite easy to identify. It is occasionally confused with wulfenite, PbMoO4, or with vanadinite, Pb5(VO4)3Cl, both of which may have strong red or orange colors.

    Figure 14.426 shows crocoite on goethite from Australia. Figure 14.427 shows crocoite from the Ural Mountains, Russia. See also the photo of crocoite with pyromorphite and galena in Figure 14.433.

    Physical Properties

    hardness 2.5 to 3
    specific gravity 6.0
    cleavage/fracture perfect {110}, poor (001)/subconchoidal
    luster/transparency adamantine/translucent
    color reddish orange
    streak orange

    Optical Properties
    Crocoite is biaxial (+), α = 2.31 , β = 2.37, γ = 2.66, δ = 0.35, 2V = 54°.

    Crystallography
    Crocoite is monoclinic, a = 7.11, b = 7.41, c = 6.81, β = 102.55°, Z = 4; space group \(P\dfrac{2_1}{n}\); point group \(\dfrac{2}{m}\).

    Habit
    Crocoite crystals are generally acicular or columnar. The thin prismatic crystals often have striations parallel to prism faces. More rarely, crocoite forms granular aggregates or patches.

    Structure and Composition
    Crocoite is isostructural with monazite, (Ce,La,Th,Y)PO4. It consists of distorted (CrO4)2- tetrahedra alternating with Pb in 9-fold coordination.

    Occurrence and Associations
    Crocoite is a rare secondary Pb mineral associated with veined lead deposits. It may be found with cerussite, PbCO3; pyromorphite, Pb(PO4)3Cl; or wulfenite, PbMoO4.

    Related Minerals
    Crocoite is isostructural with monazite, (Ce,La,Th,Y)PO4, and with several other rare earth silicates and phosphates. It is closely related to xenotime, YPO4, and pucherite, BiVO4.


    This page titled 14.10: Tungstates, Molybdates, and Chromates is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.