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14.3.2: Octahedral Sulfide Group Minerals

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    Octahedral Sulfide Group Minerals
    galena PbS
    pyrrhotite Fe1-xS
    niccolite NiAs

    Galena and pyrrhotite are the two most important octahedral sulfides; niccolite, although not containing sulfur, is included in this group because of structural similarities. In the octahedral sulfide structure, S and As are closest packed and metal atoms occupy only octahedral sites.

    Fore more general information about sulfide minerals, see the discussion in Chapter 9.

    Galena PbS

    Origin of Name
    From the Latin word galene, a name originally given to lead ore.

    14.260.jpg
    Figure 14.260: Galena cube on dolomite with minor chalcopyrite, from Joplin, Missouri; 6.3 cm across
    14.261.jpg
    Figure 14.261: Galena with anglesite from Morocco; 7.5 cm tall
    14.262.jpg
    Figure 14.262: 3.5-cm wide sample of gray galena with golden pyrite

    Hand Specimen Identification
    Galena is recognized by its density, silver-gray metallic appearance, blocky (often cubic) habit, good cleavage, and softness. Figure 14.260 shows a typical galena cube.

    Galena commonly alters to secondary minerals. Figure 14.261 is a photo of subhedral galena accompanied by clear tan and yellow anglesite (PbSO4). Galena does not always for cubes, the crystals in Figure 14.262 are not cubes but, instead, are pseudododecahedral. See also the photo of galena with pyrrhotite in Figure 14.264, below.

    Physical Properties

    hardness 2.5
    specific gravity 7.6
    cleavage/fracture perfect {100}/subconchoidal
    luster/transparency metallic/opaque
    color lead-gray
    streak lead-gray

    Crystallography
    Galena is a cubic mineral, a = 5.94, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

    Habit
    Galena crystals are typically cubes or cubes modified by octahedra; less commonly they contain other cubic forms. Penetration and contact twins are common. Lamellar twins are less common. Aggregates of galena crystals are massive, finely granular, or plumose.

    Structure and Composition
    The structure of galena is similar to that of halite. Alternating Pb and S are arranged in a face-centered cubic pattern. Galena often contains small amounts of Fe, As, or Sb, and even smaller amounts of Zn, Cd, Bi, and Se. Other impurities may be present in trace amounts.

    Occurrence and Associations
    Galena is common in many types of sulfide deposits. Associated minerals include sphalerite, chalcopyrite, pyrite, fluorite, barite, marcasite, cerussite, anglesite, calcite, dolomite, and quartz. Galena is often associated with silver minerals such as silver, acanthite, or pyrargyrite.

    Related Minerals
    Besides halite (NaCl), other minerals isostructural with galena include periclase (MgO), wüstite (FeO), alabandite (MnS), altaite (PbTe), and clausthalite (PbSe).

    Pyrrhotite Fe1-xS

    Origin of Name
    From the Greek word pyrrhos, meaning “flame colored.”

    14.263.jpg
    Figure 14.263: 8.2 cm tall pyrrhotite crystal with quartz; from the Nikolaevskyi Mine, Russia
    14.264.jpg
    Figure 14.264: Tarnished bronze-brown pyrrhotite with gray galena; from the Potosi Mine near Chihuahua, Mexico

    Hand Specimen Identification
    Pyrrhotite is recognized by its metallic luster, and its (usual) gold or bronze color that tarnishes to a reddish-brown hue. Tarnishing sometimes produces a faint blue to red iridescence. Pyrrhotite is also weakly magnetic. It may be easily mistaken for pyrite, pentlandite, or bornite – other typically gold metallic minerals – unless it is tarnished.

    Figure 14.263 shows a golden “barrel” of untarnished pyrrhotite from Russia. It appears quite similar to pyrite. Figure 14.264 shows tarnished pyrrhotite from Mexico. The tarnishing identifies this mineral. The specimen also includes gray galena.

    Physical Properties

    hardness 4
    specific gravity 4.6
    cleavage/fracture poor (001)/uneven
    luster/transparency metallic/opaque
    color golden yellow to reddish bronze
    streak gray to black

    Crystallography
    Pyrrhotite is a monoclinic mineral but has a hexagonal polytype. a = 11.88, b = 6.87, c = 22.79, β = 90.47 Z = 26; space group \(A\dfrac{2}{a}\); point group \( \dfrac{2}{m}\).

    Habit
    Pyrrhotite may be massive or disseminated. Rare crystals are hexagonal plates or tabs, often twinned.

    Structure and Composition
    Pyrrhotite‘s complex structure is similar to the structure of niccolite (NiAs). Fe occupies sites between hexagonally closest packed S. The amount and distribution of Fe are complex functions of composition and crystallization history, so composition is variable. Most pyrrhotite has less Fe than S. Ni, Co, Mn, and Cu are often present in small amounts.

    Occurrence and Associations
    Pyrrhotite is typically found in mafic igneous rocks. Associated minerals include pyrite, pentlandite, galena, magnetite, and chalcopyrite. Other pyrrhotite occurrences are in pegmatites, contact aureoles, and vein deposits.

    Related Minerals
    Pyrrhotite has a hexagonal polymorph stable at high temperature. Pyrrhotite is generally slightly deficient in Fe, which is why its formula is written as Fe1-xS. Troilite is end-member FeS. Minerals that are isotypical with pyrrhotite include troilite (FeS), niccolite (NiAs), and breithauptite (NiSb).

    Niccolite NiAs

    Origin of Name
    The name refers to this mineral’s nickel content.

    14.265.jpg
    Figure 14.265: Reddish niccolite with quartz, from Saxony, Germany; 4 cm across
    14.266.jpg
    Figure 14.266: Metallic niccolite on top of white barite, from Eisleben, Germany
    14.267.jpg
    Figure 14.267: Tarnished niccolite that is altering to green annabergite

    Hand Specimen Identification
    Niccolite is often easily recognized by its pale metallic copper-red color. Figure 14.265 shows a good example of reddish niccolite surrounded by quartz. The niccolite in Figure 14.266, on top of barite, is not quite as red-colored.

    Alteration to a green hydrated nickel arsenate called nickel bloom, or annabergite, is diagnostic for this mineral. Figure 14.267 is a photo of dark-colored tarnished niccolite with green annabergite. Some annabergite has a much brighter emerald-green color than the green seen in this photo.

    Physical Properties

    hardness 5 to 5.5
    specific gravity 4.6
    cleavage/fracture poor (001)/uneven
    luster/transparency metallic/opaque
    color copper-red
    streak brownish black

    Crystallography
    Niccolite is hexagonal, a = 3.58, c = 5.11, Z = 2; space group \(P\dfrac{6_{2}}{m}\dfrac{2}{m}\dfrac{2}{c}\); point group \(\dfrac{6}{m}\dfrac{2}{m}\dfrac{2}{m}\).

    Habit
    Rare crystals are tabular with pyramidal faces and sometimes cyclic twins. Niccolite is usually massive and sometimes colloform or columnar.

    Structure and Composition
    Niccolite‘s structure involves hexagonal closest packed As with Ni between. Sb usually replaces some of the As; Fe, Co, and S are also present in small amounts.

    Occurrence and Associations
    Niccolite is found in veins with Co and Ag minerals and in sulfide deposits hosted by mafic igneous rocks. Associated minerals include pyrrhotite, chalcopyrite, skutterudite, silver, and a variety of other sulfosalts.

    Related Minerals
    Breithauptite, NiSb; freboldite, CoSe; and kotulskite, Pd(Te,Bi) are isostructural with niccolite. Other related minerals include millerite, NiS; pentlandite, (Ni,Fe)9S8; and langisite, (Co,Ni)As. Annabergite, Ni3As2O8•8H2O, also called nickel bloom, is a common green alteration product of niccolite.

    This page titled 14.3.2: Octahedral Sulfide Group Minerals 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.