Formula: Fe(CO3) carbonate
Specific gravity: 3.7 to 3.9
Hardness: 4 to 4½
Colour: Yellowish white, yellowish brown to dark brown
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
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,
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).
At the Kabwe mine, Central Province, Zambia, siderite has been found associated with smithsonite (R&M 94.2.134).
olivine and CO2 to enstatite- ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3
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)
Common impurities: Mn,Mg,Ca,Zn,Co
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