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14.11: Phosphates, Arsenates, and Vanadates

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    The phosphate group contains many minerals, but most are extremely rare. Apatite is the only common example. Vanadates and arsenates, which are closely related to the phosphates in chemistry and structure, are also rare.

    Phosphate Group Minerals
    monazite (Ce,La,Th,Y)PO4
    triphylite Li(Fe,Mn)PO4
    apatite Ca5(PO4)3(OH,F,Cl)
    pyromorphite Pb5(PO4)3Cl
    amblygonite LiAl(PO4)F
    lazulite (Mg,Fe)Al2(PO4)2(OH)2
    wavellite Al3(PO4)2(OH)3•5H2O
    turquoise CuAl6(PO4)4(OH)8•4H2O
    autunite Ca(UO2)2(PO4)2•10H2O

    Vanadate Group Minerals
    vanadinite Pb5(VO4)3Cl
    carnotite K2(UO2)2(VO4)2•3H2O

    Arsenate Group Minerals
    erythrite CO3(AsO4)2•8H2O

    Monazite (Ce,La,Th,Y)PO4

    Origin of Name
    From the Greek word monazein, meaning “to live alone,” referring to its rare occurrences in the outcrops where it was first found.

    14.428.jpg
    Figure 14.428: Single crystal of monazite, 5 cm across
    14.429.png
    Figure 14.429: Monazite with quartz, Mubashir, Morocco, about 4 cm across

    Hand Specimen Identification
    Radioactivity, color, crystal habit, and associations help identify monazite. It may be confused with zircon, but is not as hard and has different forms. It can be distinguished from titanite by its crystal shape and high density.

    Physical Properties

    hardness 5 to 5.5
    specific gravity 4.9 to 5.2
    cleavage/fracture perfect (001), good (100)/subconchoidal
    luster/transparency variable, subresinous/translucent
    color red, brown, yellowish
    streak white

    Optical Properties
    Monazite is colorless, gray, or yellow-brown in thin section. It has high positive relief and displays up to third or fourth order interference colors. Biaxial (+), α = 1.785 to 1.800 , β = 1.786 to 1.801, γ = 1.838 to 1.850, δ = 0.005, 2V = 10° to 20°.

    Crystallography
    Monazite is monoclinic, a = 6.79, b = 7.01, c = 6.46, β = 104.4°, Z = 4; space group \(P\dfrac{2_1}{n}\); point group \(\dfrac{2}{m}\).

    Habit
    Monazite crystals are usually small, tabular, or prismatic, often forming granular masses or individual grains in sand.

    Structure and Composition
    Monazite is isostructural with crocoite. In its structure, distorted (PO4)3- polyhedra are bonded to rare earth elements in 9-fold coordination. All the rare earths may be present, but Ce, La, and Th are usually the dominant large cations. Small amounts of Si may substitute for P in the tetrahedral sites.

    Occurrence and Associations
    Monazite is a rare secondary mineral is silicic igneous rocks. It is also found in unconsolidated beach or stream sediments, where it is associated with other heavy minerals such as magnetite and ilmenite.

    Varieties
    Rare earth content varies, so the names monazite-(Ce), monazite-(La), and so on are sometimes used to designate the dominant rare earth.

    Related Minerals
    Monazite is isostructural with crocoite, PbCrO4, and forms solid solutions with huttonite, ThSiO4. It is chemically related to xenotime, YPO4, with which it forms a minor solid solution.

    Triphylite Li(Fe,Mn)PO4

    Origin of Name
    From the Greek words for “three” and “family,” in reference to its three cations.

    14.430.png
    Figure 14.430: Triphylite from Chandler’s Mill, New Hampshire; the largest crystal is 1 cm tall

    Hand Specimen Identification
    Brown-green to gray or bluish color, association with other pegmatite minerals, 90° cleavage angle, and resinous luster help identify triphylite.

    Physical Properties

    hardness 5 to 5.5
    specific gravity 3.5 to 5.5
    cleavage/fracture perfect (001), good (010)/subconchoidal
    luster/transparency vitreous, resinous/ transparent to translucent
    color variable brown-green to gray or bluish
    streak white, gray

    Optical Properties
    Triphylite is biaxial (-), α = 1.68 , β = 1.68, γ = 1.69, δ = 0.01, 2V = 0° to 56°.

    Crystallography
    Triphylite is orthorhombic, a = 6.01, b = 4.68, c = 10.36, Z = 4; space group \(P\dfrac{2_1}{m}\dfrac{2_1}{c}\dfrac{2_1}{n}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).

    Habit
    Euhedral crystals are rare; triphylite is typically fine grained and massive.

    Structure and Composition
    In triphylite, the cations occupy octahedra forming zigzag chains between (PO4)3- tetrahedra. A complete solid solution exists between the Fe and Mn end members. Compositions near the Mn end member are given the name lithiophilite.

    Occurrence and Associations
    Triphylite, typically a pegmatite mineral, is found with other phosphates, quartz, feldspar, spodumene, and beryl.

    Apatite Ca5(PO4)3(OH,F,Cl)

    Origin of Name
    From the Greek word apate, meaning “deceit,” because it is often difficult to distinguish it from other minerals.

    14.431.png
    Figure 14.431: Typical olive-green apatite crystals, 0.75 to 1 cm long
    14.432.png
    Figure 14.432: Apatite crystals up to 6 cm long from Bahia, Brazil

    Hand Specimen Identification
    Apatite may be any of a number of different colors. The olive-green color seen in Figure 14.431 is most common. Other shades of green and blue are quite common, too, such as the blue color of the crystals in Figure 14.432. Other hues are relatively rare.

    When euhedral, apatite crystals are easy to discern hexagonal prisms, often with terminating pyramidal faces. Cleavage traces run perpendicular to prism faces and long dimension. (Cleavage is quite obvious in Figure 14.432) Also aiding identification: apatite has moderate density, is softer than quartz and feldspar, but harder the calcite and fluorite.

    Physical Properties

    hardness 5
    specific gravity 3.2
    cleavage/fracture good (001), poor (100)/conchoidal
    luster/transparency subresinous/transparent to translucent
    color green, yellow, variable
    streak white

    Optical Properties
    Apatite appears similar to quartz in thin section. It is colorless, does not develop good cleavage, and has very low birefringence. Quartz, however, has lower relief and is uniaxial (+). Apatite is uniaxial (-), ω = 1.633, ε = 1.630, δ = 0.003.

    Crystallography
    Apatite is hexagonal, a = 9.38, c = 6.86, Z = 2; space group \(P\dfrac{6_3}{m}\); point group \(\dfrac{6}{m}\).

    Habit
    Apatite typically forms prismatic crystals, but may be colloform, massive, or granular.

    Structure and Composition
    Complete solid solution exists between hydroxyapatite (OH end member), fluorapatite (F end member), and chlorapatite (Cl end member). In addition, transition metals or Sr2+ may replace Ca2+; and (CO3)2-, OH, or (SO4)2- may replace some (PO4)3-. In the apatite structure, Ca-PO4 chains run parallel to the c-axis. Ca2+ is located around channels occupied by (F,Cl,OH).

    Occurrence and Associations
    Apatite is a common accessory mineral but only rarely a major rock former. It is common in all igneous rocks, including pegmatites and hydrothermal veins, in metamorphic rocks, and in marine sediments.

    Varieties
    Collophane is a massive cryptocrystalline form of apatite that comprises some phosphate rocks and bones.

    Related Minerals
    A large number of phosphates, sulfates, arsenates, vanadates, and silicates are isostructural with apatite, but none are common.

    Pyromorphite Pb5(PO4)3Cl

    Origin of Name
    From the Greek words meaning “fire” and “form,” because it typically develops large faces when crystallizing from a magma.

    14.433.jpg
    Figure 14.433: Pyromorphite with crocoite on galena, ant for scale
    14.434.png
    Figure 14.434: Pyromorphite with cerussite, 2.5 cm across

    Hand Specimen Identification
    Occurrence with other Pb-minerals, bright green, green-yellow, or brown color, habit, high density, and sometimes resinous luster identify pyromorphite. It is sometimes confused with apatite, mostly because of its green color, but is softer than apatite.

    Figure 14.433 shows green pyromorphite on gray galena (Pb-sulfide). Minor orange crocoite (Pb-chromate) can also be seen. The specimen in Figure 14.434 contains green pyromorphite and white/clear cerrusite (Pb-carbonate). The brown material around the edges may be crocoite.

    Physical Properties

    hardness 3.5 to 4
    specific gravity 7.0
    cleavage/fracture poor {100}, poor {101}
    luster/transparency resinous/transparent to translucent
    color bright green, yellow, variable
    streak white, yellow

    Optical Properties
    Pyromorphite is uniaxial (-), ω = 2.058, ε = 2.048, δ = 0.010.

    Crystallography
    Pyromorphite is hexagonal, a = 9.97, c = 7.32, Z = 2; space group \(P\dfrac{6_3}{m}\); point group \(\dfrac{6}{m}\).

    Habit
    Pyromorphite is typically prismatic, having a barrel shape; it is less commonly granular, fibrous, cavernous (hollow prisms), globular, or reniform.

    Structure and Composition
    Pyromorphite is isostructural with apatite (see apatite structure). Some Ca may substitute for Pb. P may be replaced by As.

    Occurrence and Associations
    Pyromorphite is a secondary mineral, found with other oxidized Pb or Zn minerals, in oxidized zones associated with Pb deposits. It is isostructural with apatite and with mimetite, Pb(AsO4)3Cl, with which it forms a complete solid solution.

    Amblygonite LiAl(PO4)F

    Origin of Name
    From the Greek word amblygonios, referring to its cleavage angle.

    14.435.png
    Figure 14.435: Amblygonite from Minas Gerais, Brazil, 2.8 cm across

    Hand Specimen Identification
    Amblygonite is most often a nondescript white to creamy translucent mineral that is difficult to distinguish from other light-colored minerals with moderate density and hardness. Its single perfect cleavage and conchoidal fracture help identification and distinguish it from feldspars, feldspathoids, and zeolites. Its occurrence in pegmatites, especially if it contrasts with any feldspar present, also aids identification. Figure 14.435 shows typical nondescript amblygonite. Figure 10.64 (from an earlier chapter) shows an unusual yellow specimen.

    Physical Properties

    hardness 6
    specific gravity 3.0
    cleavage/fracture perfect (100), good (110), poor (011)/ subconchoidal
    luster/transparency vitreous, greasy/transparent to translucent
    color white, beige, green, sometimes other colors
    streak white

    Optical Properties
    Amblygonite is biaxial (-), α = 1.59 , β = 1.60, γ = 1.62, δ = 0.03, 2V = 52° to 90°.

    Crystallography
    Amblygonite is triclinic, a = 6.644, b = 7.744, c = 6.91, α = 90.35°, β = 117.33°, γ = 91.01°, Z = 2; space group \(P\overline{1}\); point group \(\overline{1}\).

    Habit
    Rare crystals may be equant or columnar. Amblygonite is more commonly found as rough masses or in irregular aggregates.

    Structure and Composition
    Amblygonite is composed of alternating (PO4) tetrahedra and AlO5F octahedra linked by Li+ ions. F may be replaced by OH. Minor Na may be present.

    Occurrence and Associations
    Amblygonite is a rare mineral found in pegmatites rich in Li and P. Typical associated minerals include lepidolite, spodumene, apatite, and tourmaline.

    Related Minerals
    Several other phosphate minerals, some which can be gems, are similar to amblygonite and are found in pegmatites: herderite, CaBe(PO4)(F,OH); beryllonite, NaBePO4; and brazilianite, NaAl3(PO4)2(OH)4. Complete solid solution exists between amblygonite and a hydroxy end member, montebrasite, LiAl(PO4)(OH).

    Lazulite (Mg,Fe)Al2(PO4)2(OH)2

    Origin of Name
    From an Arabic word meaning “heaven,” referring to its sky-blue color.

    14.436.png
    Figure 14.436: Lazulite from Salzburg, Austria, 8.9 cm across
    14.437.png
    Figure 14.437: Lazulite from the Yukon Territory, Canada, 6.2 cm tall

    Hand Specimen Identification
    Lazulite’s azure-blue color is distinctive. Few minerals ever have a strong blue color like lazulite‘s. When crystals are visible, their pyramidal form distinguishes lazulite from the other blue minerals. When massive, identification is problematic.

    Figure 14.436 shows typical lazulite crystals, up to several centimeters long, from Austria. The light blue color is typical. Some lazulite has a darker color; the photo of the specimen from the Yukon in Figure 14.437 shows an example.

    Physical Properties

    hardness 5 to 5.5
    specific gravity 3.0
    cleavage/fracture poor (011)/uneven
    luster/transparency vitreous/translucent
    color azure-blue
    streak white

    Optical Properties
    Lazulite is biaxial (-), α = 1.612 , β = 1.634, γ = 1.643, δ = 0.031, 2V = 70°.

    Crystallography
    Lazulite is monoclinic, a = 7.16, b = 7.26, c = 7.24, β = 120.67°, Z = 2; space group \(P\dfrac{2_1}{c}\); point group \(\dfrac{2}{m}\).

    Habit
    Lazulite is typically massive, granular, or compact; rare crystals are prismatic or pyramidal with steep faces.

    Structure and Composition
    Octahedral Mg and Fe are linked to octahedral Al by sharing of O2- and OH. The (Mg,Fe,Al)6 octahedra bond to (PO4)3- tetrahedra. Complete solid solution exists between the Mg and Fe end members. Scorzalite is the name of Fe-rich members of the series.

    Occurrence and Associations
    Lazulite and related minerals are rare, found only in some pegmatites and high-grade metamorphic rocks. They may be associated with rutile, kyanite, corundum, and sillimanite.

    Wavellite Al3(PO4)2(OH)3•5H2O

    Origin of Name
    Named after W. Wavel (d. 1829), who discovered it.

    14.438.png
    Figure 14.438: Wavellite from the Czech Republic; typical radiating structures like the ones seen here are 1-2 cm across
    14.439.png
    Figure 14.439: Wavellite cotton balls from Snyder County, Pennsylvania; the specimen is 21 cm across

    Hand Specimen Identification
    Wavellite is a secondary mineral that most commonly forms planar radiating clusters of crystals along fracture surfaces, creating “starburst” structures like those seen in Figure 14.438. Wavellite also forms globular radiating aggregates of acicular crystals that develop in voids. This produces spherical structures with a “cotton-ball” appearance, like the wavellite balls seen in Figure 14.439. The planar or cotton-ball radiating textures, and a light green to yellow color, are diagnostic for this mineral. Other occurrences are known, but not as easily identified as wavellite.

    Physical Properties

    hardness 3.5 to 4
    specific gravity 2.36
    cleavage/fracture perfect prismatic {101}, perfect (010)/subconchoidal
    luster/transparency vitreous/translucent
    color white, greenish yellow, gray, brown
    streak white

    Optical Properties
    Wavellite is biaxial (+), α = 1.525 , β = 1.534, γ = 1.552, δ = 0.027, 2V = 72°.

    Crystallography
    Wavellite is orthorhombic, a = 9.62, b = 17.36, c = 6.99, Z = 4; space group \(P\dfrac{2_1}{c}\dfrac{2_1}{m}\dfrac{2_1}{n}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).

    Habit
    2-dimensional or 3-dimensional radiating acicular crystal aggregates typify wavellite. Less commonly wavellite forms dense opal-like or stalactitic masses. Large visible individual crystals are very rare.
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    Structure and Composition
    Wavellite’s structure is incompletely known. It may contain small amounts of Fe and Mg.

    Occurrence and Associations
    Wavellite is a secondary mineral found in rock cavities or on joint surfaces in low-grade aluminous metamorphic rocks. It is also found in phosphorite deposits. Common associated minerals include other phosphate minerals and limonite.

    Turquoise CuAl6(PO4)4(OH)8•4H2O

    Origin of Name
    From the French word turquoise, meaning “Turkish,” in reference to the source of the original stones imported into Europe.

    14.440.jpg
    Figure 14.440: Turquoise on rhyolite from Kingman, Arizona, 7.8 cm across
    14.441.png
    Figure 14.441: 7 cm tall sample of turquoise from Kingman, Arizona

    Hand Specimen Identification
    Turquoise forms in veins or as void fillings and is generally cryptocrystalline (exceptionally fine- grained), sometimes appearing amorphous. It typically has a distinctive turquoise-blue color seen in these two photos, but different shades of blue and green are well known. Its color and cryptocrystalline character generally serve for identification.

    Turquoise is not often confused with other minerals. Chrysocolla, a blue-green hydrated Cu-silicate may have similar colors but is much softer. (Chrysocolla has hardness of 2-3 compared with turquoise‘s 6.) Malachite (green) and azurite (blue), both hydrated copper carbonates, have colors distinct from turquoise and hardnesses of 3.5 to 4.

    Physical Properties

    hardness 6
    specific gravity 2.7
    cleavage/fracture perfect but rarely seen (001), good (010)/subconchoidal, brittle
    luster/transparency resinous, waxy/translucent
    color turquoise and related blue-green shades, other hues are exceptionally rare
    streak white, green

    Optical Properties
    Turquoise is biaxial (+), α = 1.61, β = 1.62, γ = 1.65, δ = 0.04, 2V = 40°.

    Crystallography
    Turquoise is triclinic, a = 7.48, b = 9.95, c = 7.69, α = 111.65°, β = 115.38°, γ = 69.43°, Z = 1; space group P1; space group P1; point group 1.

    Habit
    Rare small crystals of turquoise are known, but reniform, massive, or granular varieties are more typical.

    Structure and Composition
    The structure of turquoise consists of a framework of (PO4)3- tetrahedra and Al octahedra. Holes in the structure contain Cu, which bonds to the polyhedra, OH and to H2O. Fe3+ may substitute for Al3+.

    Occurrence and Associations
    Turquoise occurs as a secondary mineral associated with Al-rich volcanic rocks. It forms in small seams, veins, stringers, and crusts. Associated minerals typically include kaolinite, Al2Si2O5(OH)4; limonite, Fe2O3nH2O; or chalcedony, SiO2.

    Varieties
    Zn-rich blue-green varieties of turquoise are called faustite.

    Related Minerals
    Complete solid solution exists between turquoise and chalcosiderite, CuFe6(PO4)4(OH)8•4H2O.

    Autunite Ca(UO2)2(PO4)2•10H2O

    Origin of Name
    Named for Autun, France, where it is found.

    14.442.png
    Figure 14.442: Autinite from Burgundy, France
    14.443.png
    Figure 14.443: Autinite cluster from Aldeia Nova, Portugal, 3.2 cm across

    Hand Specimen Identification
    A strong yellow or yellow-green color, radioactivity, fluorescence under ultraviolet light, and typically tetragonal platy crystals identify autunite.

    Figure 14.442 shows autunite from France. Figure 14.443 show a reticulated cluster of autunite crystals from Portugal.

    Physical Properties

    hardness 2 to 2.5
    specific gravity 3.15
    cleavage/fracture perfect basal (001), good prismatic (100), (010) and {110 }/uneven
    luster/transparency pearly, adamantine/transparent to translucent
    color typically lemon yellow to greenish yellow to pale green
    streak yellow

    Optical Properties
    Autunite is uniaxial (-), ω = 1.577, ε = 1.553, δ = 0.024.

    Crystallography
    Autunite is tetragonal, a = 7.00, c = 20.67, Z = 4; space group \(I\dfrac{4}{m}\dfrac{2}{m}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\dfrac{2}{m}\dfrac{2}{m}\).

    Habit
    Thin tabular or flaky crystals, often square, are typical for autinite. It commonly forms reticulated masses of crusts.

    Structure and Composition
    In autunite, (PO4)3- tetrahedra link to U octahedra to form uneven layers. The layers are connected by weakly bonded H2O molecules. Other alkaline earths may substitute for Ca in small amounts; the amount of water in the structure is somewhat variable.

    Occurrence and Associations
    Autunite is a secondary uranium mineral, often forming after uraninite, UO2. Torbernite, Cu(UO2)2(PO4)2nH2O, and uraninite are common associated minerals.

    Related Minerals
    Torbernite is isostructural with autunite and has similar properties.

    Vanadinite Pb5(VO4)3Cl

    Origin of Name
    Named in reference to its vanadium content.

    14.444.png
    Figure 14.444: Vanadinite with calcite from Chihuahua, Mexico
    14.445.png
    Figure 14.445: Vanadinite from Mibladene, Morocco, 9 cm across

    Hand Specimen Identification
    Vanadinite generally has a distinctive bright red to orange-red color and forms hexagonal crystals. Color is the key to identification and distinguishes this mineral from apatite, pyromorphite, or mimetite. It may be transparent, translucent, or opaque and has an adamantine to resinous luster.

    Figure 14.444 shows a typical occurrence of vanadinite with calcite from Mexico. Figure 14.445 is a photo of a spectacular museum specimen that originally came from Morocco. Enlarge this photo and the hexagonal nature of the crystals is easily seen.

    Physical Properties

    hardness 3
    specific gravity 6.9
    cleavage/fracture none/subconchoidal
    luster/transparency resinous/translucent
    color ruby red, orange-red, brown, yellow
    streak white, yellow

    Optical Properties
    Vanadinite is uniaxial (-), ω = 2.416, ε = 2.350, δ = 0.066.

    Crystallography
    Vanadinite is hexagonal, a = 10.33, c = 7.35, Z = 2; space group \(P\dfrac{6_3}{m}\); point group \(\dfrac{6}{m}\).

    Habit
    Vanadinite often forms hexagonal prisms, sometimes hollow, with or without pyramidal faces. It may also form rounded or globular masses.

    Structure and Composition
    Vanadinite has the same structure as apatite (see apatite structure). P or As may substitute for V in small amounts. Minor amounts of Ca, Zn, and Cu may also be present.

    Occurrence and Associations
    Vanadinite is a rare mineral found in the oxidized portions of Pb deposits where it is often associated with galena, cerussite, or limonite.

    Related Minerals
    Vanadinite is isostructural with apatite, Ca5(PO4)3(OH,F,Cl), and with a number of other arsenates, vanadates, and phosphates. It forms solid solutions with mimetite, Pb5(AsO4)3Cl, and intermediate compositions are called endlichite.

    Carnotite K2(UO2)2(VO4)2•3H2O

    Origin of Name
    Named after M. A. Carnot (1839–1920), a French mining engineer and Inspector General of Mines.

    14.446.png
    Figure 14.446: Carnotite on sandstone, 13 cm across
    14.447.jpg
    Figure 14.447: Small crystals of carnotite from Yavapai County, Arizona; FOV is 6 mm across

    Hand Specimen Identification
    Radioactivity and yellow color, often with a greenish stain, characterize carnotite. It is sometimes confused with other secondary uranium minerals.

    Figure 14.446 shows typical fine-grained carnotite deposited in a sandstone. Individual crystals are exceptionally small. Figure 14.447 is a much magnified photo of carnotite crystals smaller than millimeter across.

    Physical Properties

    hardness 1
    specific gravity 4.5
    cleavage/fracture perfect but rarely seen (001)/uneven
    luster/transparency dull, earthy/translucent
    color yellow, yellow-green
    streak yellow

    Optical Properties
    Carnotite is biaxial (-), α = 1.75 , β = 1.92, γ = 1.95, δ = 0.20, 2V = 38° to 44°.

    Crystallography
    Carnotite is monoclinic, a = 10.47, b = 8.41, c = 6.91, β = 103.67°, Z = 1; space group \(P\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\).

    Habit
    Fine powder or crumbly aggregates characterize carnotite. It may also be disseminated.

    Structure and Composition
    Carnotite‘s structure contains layers of edge-sharing uranium and vanadium polyhedra. The layers are joined by weak bonds to interlayer K and H2O.

    Occurrence and Associations
    Carnotite is a secondary uranium mineral typically found as crusts or flakes in sandstones or conglomerates that have been altered by circulation of meteoric waters.

    Related Minerals
    A number of other hydrated uranium oxides are known, including tyuyamunite, Ca(UO2)2(VO4)2nH2O; torbernite, Cu(UO2)2(PO4)2nH2O; and autunite, Ca(UO2)2(PO4)2 •10H2O.

    Erythrite Co(AsO)2•8H2O

    Origin of Name
    From the Greek word erythros, meaning “red.”

    14.448.png
    Figure 14.448: Erythrite from Australia
    14.449.png
    Figure 14.449: Erythrite from Bou Azzer, Morocco; FOV is about 3 cm wide

    Hand Specimen Identification
    Erythrite is a secondary cobalt-arsenic mineral. It is one of only a few minerals that have its kind of red-pink-purple color; the two photos here show examples. Color, association with other cobalt minerals, and often crusty or drusy appearance generally identify this mineral.

    Figure 14.448 is a photo of typical crusty erythrite. The specimen comes from Australia. Figure 14.449 shows a much enlarged view of a similar specimen from Morocco that contains both needles and cottony balls of erythrite. The largest needles and balls are several millimeters in longest dimension.

    Physical Properties

    hardness 1.5 to 2.5
    specific gravity 3.06
    cleavage/fracture perfect basal (010)/sectile
    luster/transparency adamantine/transparent to translucent
    color crimson, pink, purple-red
    streak pale purple

    Optical Properties
    Erythrite is biaxial (-), α = 1.626, β = 1.661, γ = 1.699, δ = 0.073, 2V = 90°.

    Crystallography
    Erythrite is monoclinic, a = 10.26 , β = 13.37, c = 4.74, β = 105.1°, Z = 2; space group \(C\dfrac{2}{m}\); point group \(\dfrac{2}{m}\).

    Habit
    Large, easily seen crystals are rare. Erythrite is typically in prismatic, acicular, reniform, or globular groups/clusters. Erythrite typically forms as drusy coatings and crusts and may be earthy or powdery.

    Structure and Composition
    The atomic structure in erythrite is layered, with vertex sharing by As tetrahedra and Co octahedra. Ni may substitute for Co.

    Occurrence and Associations
    Erythrite may form as a pink powdery coating, called cobalt bloom, on other cobalt minerals such as cobaltite, (Co,Fe)AsS, or skutterudite, (Co,Ni)As3.

    Related Minerals
    Annabergite, Ni3(AsO4)2•8H2O, also called nickel bloom, is isostructural with erythrite, but has an apple-green color. A complete solid solution exists between erythrite and annabergite.


    This page titled 14.11: Phosphates, Arsenates, and Vanadates 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.

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