Siderite

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Sphaerosiderite is a microcrystalline, botryoidal to spheroidal variety of siderite that occurs in voids in volcanic rocks or hydrothermal veins; also as concretions. It is a A low-temperature formation (Mindat).

Formula: Fe(CO₃)
Carbonate, calcite group
Crystal System: Trigonal
Specific gravity: 3.96 measured 3.932 calculated
Hardness: 4 to 4½
Streak: White
Colour: Yellowish white, yellowish brown to dark brown
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Common impurities: Mn,Mg,Ca,Zn,Co
Environments:

Pegmatites
Sedimentary environments
Metamorphic environments
Hydrothermal environments

Pelosiderite is a fine grained, concretionary variety of siderite.

Siderite is frequently found mixed with clay minerals, in concretions with concentric layers. As black-band ore it is found, contaminated by carbonaceous material, in extensive stratified formations in shale and commonly associated with coal measures. It is also formed by the action of iron-rich solutions on limestone. Siderite is a common vein mineral associated with silver minerals, pyrite, chalcopyrite, tetrahedrite and galena in the oxidation zone of hypothermal (high temperature) hydrothermal veins.
Siderite may be found in limestone, and in the alkaline syenite pegmatites of Mont Saint-Hilaire, Canada (R&M 93.2.147).

Localities

At Mount Moliagul, Moliagul, Central Goldfields Shire, Victoria, Australia, aggregates to 3 cm of siderite crystals are found with calcite in cavities in quartz (AJM 21.1.44).

At Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Quebec, Canada, siderite pseudomorphs after serandite have been found (KL p159).
Siderite from Mont Saint-Hilaire - Image

The Bairendaba Ag-polymetallic deposit, Hexigten Banner, Chifeng City, Inner Mongolia, China, is a mesothermal magmatic-hydrothermal vein-type silver - lead - zinc deposit, hosted in Hercynian (about 419 to 299 million years ago) quartz diorite.
It is suggested that, with decreasing temperature, mineral compositions changed progressively from tungstate and oxide, to diatomic sulphide, to simple sulphide, to an antimony sulphosalt mineral, and finally to an antimonide.
Siderite has been found at Bairendaba as platy, yellow-brown, lenticular crystals to 1 or 2 cm, associated with green cubic fluorite and corroded purple cubic fluorite, and also as large, platy crystals to 5 cm with fluorapatite and cubic pyrite (Minrec 53.347-359).

In the Erongo Mountains, Namibia, exact location not specified, in 2017 “umbrella siderite” was found featuring sharp, dull brown, lenticular siderite crystals to 5 cm across resting lightly at miscellaneous angles, like tilting umbrellas, on crystals of other minerals, including prisms of smoky quartz, parallel groups of lustrous black, short-prismatic crystals of schorl, and a group of translucent green fluorite crystals (Minrec 55.1.114).
Siderite from Erongo - Image

At the Panasquiera mine, Portugal, siderite pseudomorphs after apatite have been found (KL p157).

At the Turt mine, Muramures, Romania, siderite pseudomorphs after calcite have been found (KL p158).
Siderite from the Turt Mine - Image

At the Aggenys mine, South Africa, siderite pseudomorphs after sphalerite have been found (KL p160).

At the Rampgill Mine, Nenthead, Alston Moor, Eden, Cumbria, England, UK, an attractive specimen has been found that is a thin plate of sugary quartz, probably pseudomorphous after fluorite, covered by small, almost black crystals of sphalerite on the back side. On the front there is a liberal dusting of tiny golden brown siderite crystals, as well as some more sphalerite (AESS).
Siderite from Rampgill - Image

At Teller county, Colorado, USA, a goethite pseudomorph after siderite has been found with microcline (KL p145).

At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the pegmatites are beryl-type rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most complex zonation and diverse mineralogy of any of the Keyes pegmatites. Six zones are distinguished, as follows, proceeding inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to 1.2 meters in size that were altered to vuggy albite-muscovite with fluorapatite crystals. This unit presumably was the source of the albite, muscovite, fluorapatite, quartz and other crystallised minerals found in pieces of vuggy albite rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including tourmaline, graftonite, triphylite, vivianite, pyrite, pyrrhotite, and beryl crystals to 30.5 cm long and 12.7 cm across.
Siderite occurs as blocky to disc-shaped orange crystals in vuggy albite at the Keyes No. 1 mine. Associated minerals include quartz, pyrite and muscovite (R&M 97.4.325).
Siderite from the Keyes Quarries - Image

At the Kabwe mine, Central Province, Zambia, siderite has been found associated with smithsonite (R&M 94.2.134).

Alteration

chalcopyrite, arsenopyrite, CO₂ and O₂ to Fe-tennantite, siderite and sulphur
10CuFeS₂ + 4FeAsS + 4CO₂ + 8O₂ → Cu₁₀Fe₂As₄S₁₃ + 4Fe(CO₃) + 11/2S₂
(CM 28.725-738)

olivine and CO₂ to enstatite- ferrosilite and magnesite-siderite
(Mg,Fe)₂SiO₄ + CO₂ → (Mg,Fe²⁺)SiO₃ + (Mg,Fe)CO₃
(DHZ 2A p139)

siderite, oxygen and H₂O to hematite and silicic acid
2Fe₂CO₃ + O₂ + 4H₂O → 2Fe₂O₃ + 2H₂CO₃
On prolonged exposure to the air Fe²⁺ compounds are oxidised to Fe³⁺ compounds according to reactions such as the one above (KB p334).

siderite and quartz to fayalite and CO₂
2Fe(CO₃) + SiO₂ = Fe²⁺₂(SiO₄) + 2CO₂
(DHZ 1A p265)

Mg-rich siderite and quartz to olivine, orthopyroxene and CO₂
3(Fe,Mg)(CO₃)→ (Fe,Mg)₂SiO₄ + 2SiO₂ → (Fe,Mg)₂SiO₄ + 3CO₂
(DHZ 1A p266) Fe-tetrahedrite, siderite and sulphur to chalcopyrite, stibnite, CO₂ and O₂
Cu₁₀Fe₂Sb₄S₁₃ + 8Fe(CO₃) + 13/2S₂ → 10CuFeS₂ + 2Sb₂S₃ + 8CO₂ + 4O₂
(CM 28.725-738)

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