The industrial mineralogy of magnesite and huntite from eastern Iran
Vein and hydrothermal - sedimentary types of magnesite from eastern Iran probably originated from ascending hydrothermal solutions. The host rocks are part of an ophiolite unit. The cryptocrystalline magnesites are very pure although they have a variable CaO/SiO₂ ratio. The magnesite is compact with a very fine and uniform texture. The mineralogy of samples of dead burned magnesites calcined at different temperatures and times are variable and can be predicted from phase equilibria studies. The textural relationships studied using scanning electron microscopy show that matrix is concentrated at periclase crystal boundaries particularly at the triple points. The Periclase crystals are larger at increased calcination temperatures and times. The amount of periclase - periclase grain contact reduces by increasing the amount of impurities. Electron microprobe results confirm that CaO and FeO in periclase increase steadily with the increase in the CaO/SiO₂ ratio and FeO content of the bulk chemistry respectively. The mineral chemistry of forsterite, monticellite, merwinite, dicalcium silicate and tricalcium silicate in dead burned magnesite indicates some solid solution between some phases, although sometimes it is difficult to analyse single phases. The observations indicate that high quality dead burned magnesia bricks can be produced from Iranian natural magnesite with low CaO and SiO₂ impurities and by maintaining the CaO/SiO₂ ratio of around 2:1. The effects of time and temperature on the transformation of the Iranian natural magnesite into caustic calcined magnesia have been used to establish the optimum industrial properties for the chemical and allied industries. High quality magnesia can be produced by calcination at 600°C for 2 hours. A medium quality product can be made rapidly (within 30 min) between 800 - 1000°C, but at lower temperatures (600 - 800°C), the formation of medium quality magnesia is largely independent of time in the range of 2-8 hours. Many occurrences of huntite are present near to the magnesite deposits. Three different modes of occurrence are recognised: 1) in veins as a weathering products of magnesite, 2) As sporadic nodules near fault zones between ultrabasic rocks and 3) in low temperature and near surface environments as a result of alteration of Mg-rich solutions with pre-existing weathered carbonate fragments in alluvial sediments. The mineralogy, chemistry, microstructure and industrial properties of huntite suggest that it may make an excellent white filler in paint.