Difference between revisions of "Siderite"
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+ | [[Image:Sideritemm2.jpg|thumb|left|300px|A rare faceted siderite. Notice the, easily visible, doubling of facet edges.<br />Photo by Chaman Golecha, Gem Testing Laboratory, Jaipur]] | ||
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==Crystallography== | ==Crystallography== | ||
Trigonal system.<br /> | Trigonal system.<br /> | ||
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==Diagnostics== | ==Diagnostics== | ||
− | This stone is too soft to be cut and | + | 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. |
===Color=== | ===Color=== |
Revision as of 12:45, 15 September 2007
Siderite | |
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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 |
Contents
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.
Color
Pistaschio brown (gem); Also pale yellowsih brown; pale yellowish; pale green; greenish gray; grayish brown; reddish brown; blackish brown; rarely almost colourless; also massive.
Diaphaneity
Transparent to opaque.
Specific gravity
S.G.: 3.83-3.88 (upto 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.
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 pavillion facet edges due to very high birefringence.
Occurence
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 Colarado; Conectiut; Idaho; Austria; France; Germany; Italy; Brazil (large fine); Quebec, Canada (brown rhombs); Greenland (rich brown, gemmy in cryolite); England, Cornwall (transparent known as 'Chalybite').