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16.62: Siderite

  • Page ID
    6490
  • Siderite

    Siderite
    Chemical composition FeCO3
    Crystal system Trigonal
    Habit Rhombohedrons, ganular, oolites
    Cleavage Perfect, rhombohedral {101}
    Fracture  
    Hardness 3.5 - 4.5
    Optic nature Uniaxial -
    Refractive index 1.633 - 1.875
    Birefringence 0.240
    Dispersion  
    Specific gravity 3.83-3.88 (upto 3.96)
    Lustre Vitreous, pearly, silky or dull
    Pleochroism  

    Figure \(\PageIndex{1}\): A rare faceted siderite. Notice the, easily visible, doubling of facet edges.
    Photo by Chaman Golecha, Gem Testing Laboratory, Jaipur

     

    Crystallography

    Trigonal system.
    Habit: rhomb shaped; massive; granular; globular; oolitic

     

    Chemical composition

    FeCO3 - iron carbonate.
    Siderite belongs to the calcite group and forms series with rhodochrosite and magnesite.

     

    Diagnostics

    This stone is too soft to be cut and is hence rarely faceted as a gemstone. As it is a carbonate, it would give effervescence (bubbling) when in contact with acids; strong eye visible doubling due to the high birefringence. Siderite, like most carbonates, is highly birefringent.

     

    Color

    Pistachio brown (gem). Also pale yellowish-brown; pale yellowish; pale green; greenish-gray; grayish brown; reddish-brown; blackish brown; rarely almost colorless. Also massive.

     

    Diaphaneity

    Transparent to opaque.

     

    Specific gravity

    S.G.: 3.83-3.88 (up to 3.96), siderite will sink in all common heavy liquids.

     

    Refractometer

    Optic nature: birefringent, uniaxial negative.
    Refractive index: nω = 1.875, nε = 1.633.
    Maximum birefringence: 0.240.
    On a gemological refractometer, only the extra-ordinary ray may be observed. The shadow edge (of the extra-ordinary ray) will move while there is no constant ordinary reading (above the range of the refractometer). Care should be taken not to mistake the constant shadow of the liquid for the real ordinary ray.

     

    Spectrum

    Not very characteristic; band at 530, 500nm; vague at 440-480; strong 450, 475, 420 giving a cut-off like appearance.

     

    Luminescence

    UV fluorescence: generally inert.

     

    Inclusions

    Strong doubling of inclusions and pavilion facet edges due to very high birefringence.

     

    Occurrence

    Geological occurrence:
    A widespread mineral in sedimentary deposits; hydrothermal ore veins; also in pegmatites; basaltic rocks.

    Geographical locations:
    Pinasquevia, Portugal: brown fine transparent material; Also Colorado; Connecticut; Idaho; Austria; France; Germany; Italy; Brazil (large fine); Quebec, Canada (brown rhombs); Greenland (rich brown, gemmy in cryolite); England, Cornwall (transparent known as 'Chalybite').