16.62: Siderite
- Page ID
- 6490
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| 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').