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16.62: Siderite

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

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


    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.


    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.


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


    Transparent to opaque.

    Specific gravity

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


    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.


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


    UV fluorescence: generally inert.


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


    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').

    16.62: Siderite is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

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