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

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 yellowsih 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 (upto 3.96), siderite will sink in all common heavy liquids.

Refractometer

Optic nature: birefringent, uniaxial negative.
Refractive index: n_ω = 1.875, n_ε = 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 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').