Solubility Of Cassiterite Research Articles

Tin transport and cassiterite precipitation from hydrothermal fluids

Cassiterite (SnO2) is the main ore mineral of tin in magmatic–hydrothermal tin deposits, but tin transport and precipitation mechanisms from hydrothermal fluids remain poorly understood. We critically evaluated aqueous tin speciation in hydrothermal fluids from extensive experimental data and thermodynamic modeling. Sn(II) chloride complexes in hydrothermal fluids exist mainly as SnCl+, SnCl2(aq), and SnCl3−. The revised Helgeson–Kirkham–Flowers model parameters of these three tin species and two tin ions (Sn4+ and Sn2+) were derived from the correlation algorithms among these parameters, and the standard molar properties of cassiterite were optimized to be internally consistent with the available thermodynamic dataset. These thermodynamic parameters, together with the available equilibrium constant equation of Sn(IV) chloride complexes, could reproduce the available solubility data of cassiterite in acidic solutions at 400–700 °C under oxygen fugacity (fO2) levels buffered by hematite–magnetite (HM) or nickel–nickel oxide (NNO). These comparisons allow modeling chemical systems of SnO2–NaCl–HCl–H2O (liquid phase) to examine tin transport and cassiterite precipitation mechanisms under tin-mineralizing conditions: 300–500 °C, 50–150 MPa, 2 molal NaCl, and fO2 levels from QFM (quartz–fayalite–magnetite) to HM. Sn(II) chloride complexes are commonly interpreted to dominate in aqueous tin speciation under fO2 = NNO, but our modeling results indicate that considerable contents of Sn(IV) chloride complexes also exist in those reduced fluids with high HCl contents, consistent with recent in situ high-temperature experiments and molecular dynamic simulations. The Sn(II)/Sn(IV) ratios in fluids depends on fO2, temperature, and HCl contents. A considerable amount of Sn(IV) possibly exist in an early mineralization stage even under fO2 = NNO; if so, redox reactions are unnecessary to precipitate cassiterite from these mineralizing fluids. We find that even if the fO2 levels are constant, simple cooling can alter mineralizing fluids to be more oxidized (e.g., from QFM to HM) and cause cassiterite precipitation, indicating that oxidizing agents are not necessary as previously thought. This explains why cassiterite can precipitate in host rocks (e.g., sandstone or quartzite) that do not provide oxidizing agents. A simple rise in fO2 levels and pH neutralization (e.g., greisenization) also cause cassiterite precipitation. Cassiterite solubility in oxidized acidic hydrothermal fluids (NNO < fO2≤HM) is high enough to account for the tin contents of fluid inclusions from typical tin deposits, but the mineralization potential of oxdized fluids is inferior to reduced fluids (fO2≤ NNO) under the same conditions.

Optimization on the purification of cassiterite from low-grade cassiterite concentrate

Optimization on the purification of cassiterite from low-grade cassiterite concentrate has been successfully done. The concentrate used came from the mine site in the Delas area, Bangka Island with an element content of Sn was 31.60% wt and the impurity phases including zircon, ilmenite, quartz, hematite, and monazite. The purification process consists of two steps: mechanical-magnetical separation and acid leaching. The mechanical-magnetical separation step started by color separation (bright sand removal) and removed the magnetic elements of the concentrate subsequently. The content of the Sn element in acid leaching residue increased significantly was up to 90.28% with cassiterite as the dominant phase. The X-ray diffraction (XRD) revealed that the impurity phase of the filtrate after crystallization in all varying pH consists of cassiterite, ilmenite, and ammonium chloride. The solubility of cassiterite concentrate in the acid leaching process is very low at 4.4%, this indicates that the Sn element is difficult to decompose under these conditions.

Formation of hydrothermal tin deposits: Raman spectroscopic evidence for an important role of aqueous Sn(IV) species

The speciation of tin and the solubility of cassiterite in H2O + HCl were determined at temperatures to 600 °C using in situ Raman spectroscopy. In addition, information on the fluid-melt partition of Sn was obtained at 700 °C and indicated a preference of the fluid only at HCl concentrations that are much higher than in fluids exsolved from natural felsic melts. Dissolution of cassiterite generally resulted in formation of Sn(IV) species unless reduced conditions were generated by hydrogen permeation or carbohydrates in the starting material. The prevalent aqueous Sn(IV) species was [SnCl4(H2O)2]0, with additional [SnCl3(H2O)3]+ and [SnCl5(H2O)]−. The only detectable Sn(II) species was very likely [Sn(II)Cl3]−. Cassiterite solubility increased with HCl concentration and was generally high in H2O+HCl fluids, with no strong dependencies on temperature, pressure, or the oxidation state of tin in the fluid. The Sn(IV) concentrations at 500 and 600 °C determined from the integrated ν1[Sn(IV)Cl] band intensity are in good agreement with literature data on the cassiterite solubility in H2O + HCl at oxygen fugacities along the hematite-magnetite buffer. The combined results from previous experimental studies and this study demonstrate that HCl molality is a crucial parameter for hydrothermal mobilization and transport of tin and for cassiterite precipitation, and that pH, pressure and temperature are less important. Current models on hydrothermal tin deposit formation need to be augmented to include Sn(IV)Cl complexes as significant tin-transporting species. Irrespective of the oxidation state of tin in the fluid, cassiterite precipitates due to reaction of the hydrothermal fluid with the wall rock (greisen or skarn formation), dilution (mixing with meteoric water) or a decrease in the HCl activity in the aqueous liquid by boiling. A redox reaction is only required for tin transported as Sn(II) to be converted to Sn(IV).

Cassiterite dissolution and Sn diffusion in silicate melts of variable water content

Experiments to constrain cassiterite dissolution kinetics in rhyolitic melts with 0.1–5.9wt% H2O were conducted at 1023–1373K and 0.5GPa in a piston-cylinder apparatus. Care was taken to minimize convection in the experimental charge. Tin diffusivity was found to be concentration dependent. The diffusion profiles were fit well by assuming that tin diffusivity depends exponentially on tin concentration, which is roughly linearly related to SiO2 or SiO2+Al2O3 concentration. Tin diffusivity was found to increase with temperature following the Arrhenius relation, with activation energy decreasing from 161kJ/mol in dry rhyolite to 93kJ/mol in hydrous rhyolite containing 5.9wt% H2O. Tin diffusivity increases exponentially with H2O concentration, by about 3.2 orders of magnitude at 1123K from 0 to 6wt% H2O, and decreases exponentially with SiO2 concentration, by about 0.7 orders of magnitude when SiO2 concentration increases by 10wt%. The equation to describe Sn2+ diffusivity in rhyolitic to dacitic melt (64–76wt% SiO2) at about 0.5GPa is:lnDSn=−18.194+0.1776–CSiO2−19418−1389w/T,where DSn is in m2/s, CSiO2 and w are SiO2 and H2O concentrations in wt%, and T is in Kelvin. The solubility of cassiterite (or tin concentration at cassiterite saturation) in rhyolitic melts increases strongly with increasing temperature.

An experimental study of cassiterite solubility in HCl-bearing water vapour at temperatures up to 350 °C. Implications for tin ore formation

The solubility of cassiterite was investigated experimentally in liquid-undersaturated HCl-bearing water vapour at temperatures of 300 to 350 °C and pressures up to 180 bar. Concentrations of tin varied between 0.1 and 32 ppb, depending on experimental conditions, and reached a maximum at 320 °C. These concentrations are at least four orders of magnitude higher than those reported for the water-free system. Positive correlation of the concentrations of tin with f H 2O and f HCl indicate that cassiterite solubility is enhanced by solute–solvent interaction, and that this involves the formation of hydrated gaseous species of tin chloride having a tin–chlorine ratio of 1:2, and statistical hydration numbers of 2.9, 2.7, and 2.5 at 300, 320, and 350 °C, respectively. We therefore propose that the solubility of cassiterite in HCl-bearing water vapour is explained by the reaction: SnO 2 cassiterite + 2 · HCl gas + n · H 2 O vapour = SnOCl 2 · ( H 2 O ) n + 1 vapour Calculations based on this reaction and data for the solubility of tin in aqueous liquids indicate that, under all conditions, tin partitions preferentially into the latter phase. However, in high salinity, two-phase aqueous systems, where vapour pressures can be considerably higher than those in our experiments and chlorinity is not controlled by f HCl, but by NaCl content, tin solubility in the vapour phase may be appreciable.

Solubility of cassiterite in evolved granitic melts: effect of T, fO 2, and additional volatiles

The aim of this experimental study was to determine the solubility of cassiterite in natural topaz- and cassiterite-bearing granite melts at temperatures close to the solidus. Profiles of Sn concentrations at glass–crystal (SnO 2) interface were determined following the method of (Harrison, T.M., Watson, E.B., 1983. Kinetics of zircon dissolution and zirconium diffusion in granitic melts of variable water content. Contributions to Mineralogy and Petrology 84, 66–72). The cassiterite concentration calculated at the SnO 2–glass interface is the SnO 2 solubility. Experiments were performed at 700–850 °C and 2 kbar using a natural F-bearing peraluminous granitic melt with 2.8 wt.% normative corundum. Slightly H 2O-undersaturated to H 2O-saturated melt compositions were chosen in order to minimize the loss of Sn to the noble element capsule walls. At the nickel–nickel oxide assemblage (Ni–NiO) oxygen fugacity buffer, the solubility of cassiterite in melts containing 1.12 wt.% F increases from 0.32 to 1.20 wt.% SnO 2 with an increasing temperature from 700 to 850 °C. At the Ni–NiO buffer and a given corundum content, SnO 2 solubility increases by 10% to 20% relative to an increase of F from 0 to 1.12 wt.%. SnO 2 solubility increases by ∼20% relative to increasing Cl content from 0 to 0.37 wt.% in synthetic granitic melts at 850 °C. We show that Cl is at least as important as F in controlling SnO 2 solubility in evolved peraluminous melts at oxygen fugacities close to the Ni–NiO buffer. In addition to the strong effects of temperature and fO 2 on SnO 2 solubility, an additional controlling parameter is the amount of excess Al (corundum content). At Ni–NiO and 850 °C, SnO 2 solubility increases from 0.47 to 1.10 wt.% SnO 2 as the normative corundum content increases from 0.1 to 2.8 wt.%. At oxidizing conditions (Ni–NiO +2 to +3), Sn is mainly incorporated as Sn 4+ and the effect of excess Al seems to be significantly weaker than at reducing conditions.

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2Liquids

The saturation surface of cassiterite, SnO2, was determined for liquids in the system K2O–Al2O3–SiO2 as a function of bulk composition and temperature. At fixed K2O/Al2O3 cassiterite solubility varies weakly with SiO2 concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log (fO2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K2O/Al2O3 lessthan equal to 1 (peraluminous liquids). As K2O/Al2O3 increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K2O in excess of Al2O3. It is proposed that potassium in excess of aluminum combines with Sn4+ to form quasi-molecular complexes with an effective stoichiometry of K4SnO4.

Origin and evolution of the greisenizing fluid at the East Kemptville tin deposit, Nova Scotia, Canada

The process responsible for greisen-hosted tin mineralization at East Kemptville was investigated using petrographic, chemical, and fluid inclusion analyses of samples from a single deep drill hole (90-1) in the western part of the deposit (Baby zone). Based on their trace element chemistry, the greisens intersected in the hole appear to have all formed from a single leucogranite protolith. Alteration developed as symmetrically zoned halos around central fractures as a result of the interaction of a fluoride-rich orthomagmatic fluid with evolved leucogranite. During alteration, K feldspar was first replaced by albite. With further alteration, albite was replaced by muscovite resulting in the formation of quartz-sericite greisen. Ore minerals are locally present in this zone. Closer to veins, muscovite is replaced by topaz and quartz, producing a quartz-topaz greisen with which the most intense mineralization is associated. This latter greisen is enriched in F, Fe, S, Zn, and Sn, suggesting that these elements were added by the mineralizing fluid. Next to the vein, dissolution of pyrrhotite, sphalerite, and cassiterite characterizes quartz-greisen in which the concentration of ore-forming elements (Sn, Fe, S, F, Zn, Cu) is lower than in the quartz-topaz greisen. Microthermometric measurements of fluid inclusions in quartz from the various alteration zones show that this alteration sequence was formed by only one greisenizing event and that temperature was approximately constant (450 degrees C). The fluid responsible for greisen formation was an NaCl-brine, containing subordinate and variable concentrations of Fe, Mn, and K.Measured eutectic temperatures of fluid inclusions are lowest in quartz-topaz greisen as a result of an increase in the Fe concentration due to pyrrhotite dissolution. Oxygen fugacity, which was calculated from the CO 2 /CH 4 ratio in gases released by crushing fluid inclusion-rich samples, displays a corresponding minimum since pyrrhotite dissolution releases Fe (super 2+) . The distribution of pyrrhotite and the fluid inclusion data indicates that pyrrhotite was precipitated close to the vein during early stages of the alteration and reprecipitated farther out as alteration progressed. This suggests that alteration zones moved away from the vein and widened with time.The salinity of fluid inclusions varies between 27 and 41 wt percent NaCl equiv and increases linearly with increasing distance from the vein, even in the absence of Na-bearing phases. It, therefore, follows that sodium was transported toward the fracture (vein) down a chemical potential gradient. This occurred through compensated infiltration, i.e., a regime in which flow was dominantly parallel to the fracture but individual aliquots of fluid followed complex paths back and forth between the fracture and the rock.Cassiterite precipitated in quartz-topaz greisen in response to a pH increase of the mineralizing fluid due to its interaction with the wall rock. Other components affecting cassiterite solubility were either constant (temperature) or acted against its precipitation (f (sub O 2 ) and a (sub Cl (super -) ) ).

Solubility of Radionuclides in Fresh and Leached Cementitious Systems at 22°C and 50°C

ABSTRACTThe solubility of Ni, Sn, Pb, Eu, and Sr was investigated at 22°C and 50°C in simulated cement porewaters corresponding to fresh cement (‘FPW’, pH 13.2) and cement leached of soluble alkalis, whose porewater chemistry is dominated by equilibrium with portlandite (‘APW’, pH 12.5). Solubility limits were approached from undersaturation, using Ni(OH)2(c), SnO2(c) (cassiterite), PbO(red), and Eu(OH)3(c) as solids. For Sr, no solubility experiments were carried out since the cement contains enough Sr to reach equilibrium with strontianite and celestite. Solubilities generally increase with increasing pH and temperature. The measured solubility of Pb ranges from about 4× 10−3 to 1×1O−2M and is within thermodynamically predicted values. The measured solubility of Ni is &lt;O.5-1.3×10−7M in APW and about 3-4×10−7M in FPW, which is lower than predicted by most thermodynamic data. The measured solubility of Eu ranges from &lt;6.6×10−10M in APW to 1.8-2.7×10−9M in FPW, which agrees well with thermodynamic calculations. The solubility of cassiterite is low, with dissolved Sn concentrations ranging from 3.5× 10−8M in APW to 3.3× 10−6M in FPW. Thermodynamic calculations predict much higher solubilities, presumably for less crystalline phases. However, oversaturation measurements in APW reproduced the solubility limits obtained by undersaturation within a factor of two. To our knowledge, no relevant thermodynamic data are available.

The combined effects of fO2 and melt composition on SnO2 solubility and tin diffusivity in haplogranitic melts

The diffusion profile method has been employed to measure tin diffusion coefficients and SnO2 solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log fO2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn2+ species, whereas at oxidized conditions SnO2 is much less soluble, and tin is present dominantly as a Sn4+ species. There is a strong melt composition control on SnO2 solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log fO2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO2 for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO2 solubility is ∼ 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn02 solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn4+/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from ∼ 0.4 at log fO2 of FMQ + 3.49 to ∼0.01 at FMQ - 0.57. Over the same log foZ range the Sn4+/ΣSn ratio for the A.S.I. 0.6 composition ranges from ∼0.98 to ∼0.4 and for the A.S.I. 1.25 composition, from ∼0.8 to ∼0.02.Tin diffusivity is dependent on both fO2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10−7.5 cm2/sec for the peraluminous compositions and 10−8.2 cm2/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10−8.2 and 10−9.0 cm2/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn2+ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn4+ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn2+ and Sn4+ is the same, but the bond strengths are different. At fixed fO2 the faster diffusivity in the peraluminous compositions reflects the lower Sn4+/Sn2+ ratio. The fact the Sn4+/Sn2+ ratio in melts varies greatly with fO2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.

Cassiterite solubility, tin speciation, and transport in a magmatic aqueous phase

The solubility of SnO 2 in chloride + or - fluoride-bearing fluids in equilibrium with a number of crystalline, silicate acid buffers, has been measured at 2.0 kbars, as a function of temperature (700 degrees -800 degrees C), oxygen fugacity (QFM to QFM + 1.5 log units), and fluid chemistry. Silicate buffers included sanidine-sillimanite-quartz, analbite-sillimanite-quartz, anorthite-sillimanite-quartz, talc-quartz, and (ortho)enstatite-quartz. Hundreds to tens of thousands of parts per million of tin can be dissolved in dilute, chloride-bearing acid solutions. Fluoride was found to have no significant influence on the solubility of SnO 2 at 700 degrees C. In alkali chloride-bearing, acid solutions, across the entire range of oxygen fugacities examined, tin transport is effected by a complex series of stannous chloride-bearing species. These include simple chloride, mixed ligand (hydroxy chloride), and alkali-bearing chloride and hydroxy chloride stannous complexes (SnOHCl, SnCl 2 , KSnOHCl 2 , KSnCl 3 , K 2 SnCl 4 , K 3 SnCl 5 , K 4 SnCl 6 , and NaSnOHCl 2 , NaSnCl 3 , etc). Apparent equilibrium constants for the predicted aqueous tin species have been calculated and species distribution plots derived for different solution compositions. The solubility of SnO 2 is favored by increasing fluid acidity (m (sub HCl, aq ) ), elevated aqueous alkali/acid and K/Na ratios, and reducing conditions, but appears to be only weakly enhanced by temperature increases in the 500 degrees to 750 degrees C range.

Experimental studies of the solubility of cassiterite and the extraction of tin from granitic melts

Experiments indicate that the solubility of cassiterite can be enhanced by increasing either acidity or alkalinity in hydrothermal solutions as a consequence of the duality of tin. The minimum solubility of cassiterite is found in neutral solutions. F- and CL-coordination compounds of Sn can alternate with hydroxy coordination compounds with changing pH in the solutions. In this case, F- and Cl- and OH- can be substituted with each other. The dissolution reaction of cassiterite is of reducing nature. High temperature and acidic reducing environment are favorable for the dissolution of cassiterite and the transport of Sn2+ compounds in fluids or solutions. High-temperature fluoride and chloride fluids can all dissolve, extract and enrich Sn to form F- and /or Cl-coordination compounds, However, F plays a more important role than G. F-coordination compounds are more stable and efficient than Cl-coordination compounds during the transport and enrichment of Sn in melts or solutions. The solubility of cassiterite and the amount of Sn extracted from granitic melt depend not only on T, P, pH and Eh in the fluids or solutions, but also on the amounts of dissociated F- and Cl- in the fluids.

Thermodynamic properties of Sn(ll) complexes in supercritical acidic fluoride solutions. Les propriétés thermodynamiques des complexes Sn(ll) dans les solutions acides fluorurées aux conditions supercritiques

The state of tin in acidic fluoride solutions at 500 °C and 1 kbar in the presence of a Ni-NiO buffer has been studied using the solubility method. The main reaction describing the solubility of cassiterite in the HF concentration range of 0.05 to 0.75 M is : SnO₂(cryst.) + H₂(gas) + 2 HF° = SnF₂ + 2 H₂O Thermodynamic analysis has shown that in the concentration range of 0.01 to 0.07 M HF and 0.005 to 0.05 M NaF (pH 4.8 to 5.65), SnF₂(OH)- is the major tin complex, and cassiterite dissolution proceeds according to the following reaction : SnO₂(cryst.) + H₂(gas) + 2 HF° = SnF₂(OH)⁻+ H⁺ + H₂O Within the range of 0. 1 to 0.75 M HF and 0.01 to 0. 1 M NaF (pH 4.0 to 5.05), two tin complexes predominate : SnF₂(OH)⁻ and SNF₃⁻, the latter according to the reaction : SnO₂(cryst.) + H₂(gas) + 3 HF° = SnF₃⁻ + H+ + 2 H₂O The equilibrium constants for reaction (1) through (3) and the values of the standard free energy of formation of these tin complexes at 500 °C and 1 kbar have been calculated. The origin of tin deposits associated with Li-F granites is discussed.

Chemical and thermodynamic constraints on the hydrothermal transport and deposition of tin: I. Calculation of the solubility of cassiterite at high pressures and temperatures

Estimation of equation of state parameters for Sn ++ and calculation of the thermodynamic properties of other aqueous species and dissociation constants for various stannous and stannic complexes as a function of temperature permit prediction of the high temperature solution chemistry of tin and calculation of the solubility of cassiterite in hydrothermal solutions. The results of these calculations indicate that in the absence of appreciable chloride and fluoride concentrations, Sn(OH) 2 0 and Sn(OH) 4 0 are the predominant tin species in H 2O up to 350°C at ~2 $ ̌ pH $ ̌ 7.5. The calculations also indicate that chloride complexes of Sn ++ predominate by several orders of magnitude over their fluoride and hydroxide counterparts in 1–3 molal (m) NaCl solutions, except in the presence of geologically unrealistic concentrations of fluoride or a pH greater than ~3.5 at 250°C or ~5.0 at 350°C. At higher pH values, most of the tin in solution is present as hydroxide complexes, even at concentrations of NaCl as high as 3 m. Calculated values of the solubility of cassiterite at high temperatures compare favorably with experimental data reported in the literature. Depending on the fugacity of oxygen and solution composition, the solubility of cassiterite in hydrothermal solutions may exceed 100 ppm under geologically realistic conditions.

Some aspects of the hydrothermal reactions of tin during skarn formation

Abstract The reactions of tin under hydrothermal conditions in skarns (calcsilicate replacements of carbonate rock) may result in the precipitation of discrete tin minerals such as malayaite (CaSnSiO5) or cassiterite, or tin‐rich calcsilicate minerals by cation substitution. Thermodynamic calculations show that cassiterite has low solubilities (< 118 ppm, 10‐3m) between 250 and 350°C and near neutral pH. These conditions, which are indicated by the occurrence of calcite with pyrrhotite or pyrite, are the most favourable for the precipitation of cassiterite. At higher temperatures and pH, magnetite displaces the iron sulphides as the stable iron mineral, and the solubility of cassiterite as aqueous hydroxy complexes is increased by several orders of magnitude. Hydrothermal solutions that are ore‐forming at 250 to 350°C and neutral pH are undersaturated with respect to cassiterite at higher temperatures and pH. Synthesis experiments and the mineral associations of natural malayaite, the most common tin silicate, show that its occurrence is favoured by hydrothermal reactions at low iron and magnesium activities. Malayaite has been synthesised at temperatures between 240 and 830°C, but geological occurrences suggest that it crystallises after garnet and clinopyroxene. At high activities of iron and magnesium, calcsilicate minerals form in preference to malayaite. High tin concentrations in calcsilicate minerals have been reported for andradite (up to 5.8%; McIver & Mihalik, 1975), epidote (2%) and actinolite (0.3%; Mulligan & Jambor, 1968). Electron microprobe determinations of tin in minerals from the ‘number two anomaly’ calcsilicate horizon, Mt Lindsay, Tasmania, indicate high tin contents in ferrohastingsite (up to 0.4%) and grossularite‐andradite (up to 0.1%) and significantly lower tin contents in tremolite (up to 0.06%), biotite (<0.03%, diopside‐hedenbergite (<0.03%) and vesuvianite (<0.03%). These data, together with Mössbauer measurements reported for tin‐rich andradites, are consistent with the coupled substitution: 2Fe3+ [??] Fe2+ + Sn4 + , being an important factor in the occurrence of high tin concentrations in some calcsilicate minerals. The occurrence of lattice‐bound tin is favoured by hydrothermal reactions at high oxygen fugacities between the haematite‐magnetite and nickel‐nickel oxide oxygen buffers. The formation of cassiterite‐bearing skarn requires cooling under conditions of low oxygen fugacity that leads into the cassiterite stability field without prior precipitation of lattice‐bound tin. Remobilisation of lattice‐bound tin at temperatures below about 350°C may also precipitate cassiterite.