Cassiterite Crystals Research Articles

Interstitial Sn2+ in synthetic and natural cassiterite crystals

Evidence for the presence of Sn2+ in an octahedral interstitial site in synthetic and natural cassiterite (SnO2) is presented. The absorption and relative absorption spectral peaks measured are similar to ones found in Sn2+-doped KI by absorption spectrophotometry and Sn2+-doped soda-silica glass by reflection spectroscopy. The estimated quantity of interstitial Sn2+ present is found by calculating that needed to balance the uncompensated M3+ impurity in substitutional Sn4+ sites where M3+ is mainly Al3+ and Fe3+. Estimates of the oscillator strengths of three Sn2+ bands detected by absorption spectrophotometry in the synthetic crystal are given. The interstitial Sn2+ content in synthetic and natural cassiterite is not affected by heating,60Co gamma irradiation, or UV light treatment with a high pressure xenon-mercury lamp.

Geochemistry of tin-bearing granitic rocks

The use of Rb/Ba and Rb/Sr ratios as guides to tin mineralization has only local interest, because each of the correlations log Rb/Ba-log Sn and log Rb/Sr-log Sn of granitic series has a different slope in every area. The ternary relation between Rb, Ba and Cs is useful to distinguish between Sn-bearing granitic rocks and non-mineralized granitic rocks. The Sn-bearing granitic rocks are late differentiates of anatectic melts (S-type), with primary albite and muscovite, and greisenized granites with metasomatic albite and muscovite. They have high Sn, Rb contents, Rb/Ba and Rb/Sr ratios, and low Ba and Sr contents. The Portuguese Sn-bearing muscovite granites, aplites, pegmatites and hypothermal quartz veins have muscovites richer in Sn than those of greisenized granites. F and Sn of muscovites from Portuguese Sn-bearing granitic rocks are positively correlated. Cassiterite crystallization seems to be independent of f H 2 O f in HF and favoured by a decrease in f O 2.

Fractional crystallization and the origin of tin deposits in granitoids

A comparison between tin-bearing granitoids in an anorogenic setting (Bushveld Complex) and an orogenic setting (Blue Tier Batholith, Tasmania) reveals a number of genetically important similarities. These include: in situ fractional crystallization characterised by marked decrease in Ba and Sr and increase in Rb; the accumulation of late melt in a sheet-like form near the roof zone; the association of barren pegmatites overlying the ore; and of aplites; and the occurrence of conformable tin-bearing sheets, often exhibiting greisenization. These features allow the formulation of the following genetic model. A crustally-derived granitoid magma is emplaced and undergoes fractional crystallization from the margins inwards, with bottom crystallization dominating. Disruption of earlier formed solids by rest liquid commonly occurs. Continued fractional crystallization causes enrichment in volatiles and incompatible elements in the late rest melts, which have a sheet-like habit. The efficiency of enrichment of incompatible elements is critically dependant on the degree of separation of melt from solids throughout crystallization. An early, tin-poor vapour may separate after initial water-saturation of the magma is achieved, and this collects under the roof, commonly forming an impermeable barrier to later tin-bearing fluids. Continued fractional crystallization on the floor further enriches incompatible elements, and at a very late stage a Sn-rich vapour separates within the intercumulus phase and becomes concentrated by progressive crystallization of the intercumulus melt. At a late stage of solidification, this vapour loses equilibrium with the earlier formed feldspars and greisenization ensues, accompanied by the crystallization of cassiterite and other ore minerals. The nature of the mineralization changes if through-going fractures tap the late fluids. This model predicts systematic changes in trace element geochemistry with crystallization which provide useful tools for assessing the tin potential of a granitoid, and for indicating the direction of crystallization of the magma, and hence the location of possible ore.

Caract�ristiques de la phase fluide associ�e � la gen�se des gisements d'�tain d'Abbaretz et de La Villeder (Bretagne Meridionale)

The fluid inclusions occurring in quartz of cassiterite-bearing quartz veins from two localities of Southern Brittany have been studied (microthermometry and chemical analysis). In both localities, two sorts of fluids have been recognized: 1. Early fluids, related to the precipitation of cassiterite. Those fluids are closed to a chloride-bearing aqueous solution, with very little CO2 and hydrocarbons. The salinity is rather low (6 to 9 wt % eq. NaCl). The inclusions homogenize between 150 a 300 °C. The K/Na atomic ratio is about 0,1. From these data and the mineralogical associations (muscovite + kaolinite), the physical and chemical properties of the solution at the time of cassiterite crystallization have been calculated: temperature ∼350°C, pressure ∼800 bars; molalities of NaCl, KCl and HCl are, respectively about 1, 0.1 and 0.01 (pH at 25 °C, 1 bar ∼2, lower than 3 in any case). 2. Late fluids, related to an important kaolinization. They are generally colder, and have a either lower or higher salinity, than the early fluids.

Origin of weak ferromagnetism and remanence in natural Cassiterite crystals

The weak ferromagnetism of cassiterite (SnO2) can be attributed to its 1% iron impurity content. Previous workers have postulated two hypothetical iron-tin compounds to explain the magnetism, namely, Fe4/3;Sn2/3O3 and single-phase magnetic Sn1−xFexO2. Our saturation magnetization measurements on large single crystals as a function of temperature show super-paramagnetic Fe2O3 or Fe2−xSnxO3 impurities to be responsible for the weak ferromagnetism. Thermomagnetic curves confirm this interpretation. Contrary to the results of previous workers, no correlation was found between magnetism and the Fe2+/Fe3+ ratio, and the observed variability of Curie points is attributed by us to a range of blocking temperatures. More importantly, we suggest that the NRM in cassiterite may be CRM, and hence these crystals are liable to yield incorrect paleointensity values of the geomagnetic field.

Magnetism in cassiterite

Large cassiterite crystals from the Pelepah Kanan tin mine in central Johore differ from those at Aberfoyle and Renison Bell, Tasmania, in showing strong colour zoning and possessing a small but conspicuous magnetic fraction. By separating crystals from these localities into fractions of varying magnetic susceptibility, there is found to be a distinct relationship between colouration and the magnetic properties of cassiterite. Furthermore, Mossbauer spectroscopy reveals that ferroan inclusions in the dark brown magnetic fraction contain a higher ferrous to ferric ion ratio than in the colour-less fraction and that only undetectable amounts (less than about 10%) of the iron atoms in these inclusions can have a magnetically ordered environment. Although no significant differences in unit cell dimensions between the two fractions are apparent, electron diffraction d-spacings not assignable to cassiterite are found to compare with paramagnetic FeSn(OH)6 in the dark magnetic fraction and probably with lepidocrocite in the colourless non-magnetic fraction. When the magnetic fraction is heated in air to 600°C, however, the finely dispersed particles of hydrated ferrous stannate seem to coagulate and form a rhombohedral phase which closely resembles ilmenite. It is suggested that this phase is anhydrous ferrous stannate and that it forms a series of compounds with haematite which are analogous to Fe2−xTixO3. For x near 2/3, the latter series becomes appreciably ferrimagnetic and it is concluded that the magnetisation in these cassiterite samples is primarily due to the presence of a similar tin phase which may occur in small dehydrated regions of the primary ferrous stannate hydrate. These conclusions are further substantiated by thermogravimetric and infrared studies for both the Aberfoyle and Pelepah Kanan samples and by thermal demagnetisation analyses for the dark magnetic cassiterite. The thermal demagnetisation data indicate that it is possible to have in cassiterite a series of magnetic compounds with Curie points in the range of 50–250°C which corresponds closely to the magnetic region of the Fe2−xTixO3 series. — It is shown that many features associated with cassiterite paragenesis are readily explicable in terms of colloidal systems following condensation of the primary gaseous ore fluids. Based on this hypothesis and confirmatory experimental evidence, a mechanism for the origin of zoning in cassiterite is outlined

Crystal forms and lattice constants of cassiterites from Miyoshi, Otani and Kaneuchi mines

Miyoshi mine, Okayama Prefecture, Otani and Kaneuchi mine, Kyoto Prefecture are pneumatolitic tungsten deposits, in where cassiterite are found in quartz vein associated with wolframite, scheelite and arsenopyrite. Single crystals of casisterite from Miyoshi mine consist of s(111), e(101), a(100) and m(110), and (101) and ninelings twinning are also found. Prismatic crystals of cassiterite from Otani and Kaneuchi mine are s(111), z(231), sometimes e(101) and narrow prismatic faces of h(120) to h(210), and (101) twinning are found. Unit cell constant of cassiterite from Miyoshi mine is a0 4.740A, c 0 3.185, Otani mine is a0 4.739A, c0 3.186 and Kaneuchi mine is a0 4.739 A, c0 3.186.

Crystals of artificial cassiterite

The crystals of artificial cassiterite, described below, were produced by volatilization of South African low-grade tin ores. In the sinter-pot method employed, air was blown through a charge of alternate layers of coke and of a 12 % metallic tin concentrate mixed with fine coke. This charge was placed above a basal layer of previously ignited coal. Occasionally sodium chloride was mixed with the concentrate to remove tin as chloride as well as oxide fume. The air-flow was carefully controlled to prevent slagging and was maintained until the evolution of copious fumes of tin oxide, and at times of chloride, ceased.Optical pyrometer readings on the charges during the experiments gave temperatures of 1050°C. approximately, but it is certain that this value is a grave under-estimate of the actual temperatures in the interior of the sinter-pot.

Recent Acquisitions at the Natural History Museum

AMONG recent additions to the zoological collections are specimens of Ungulate mammals from the Sxidm presented by Major P. H. G. Powell-Cotton and Miss Diana Powell-Cotton. A fine specimen of adult beaver from Norway has been purchased, and an interesting series of Crustacea from the Bering Sea and the Sea of Okhotsk, regions previously very poorly represented in the Museum collection, have been received by exchange. A specimen of a rare mollusc, Halicardia flexuosa, from the Atlantic Ocean off Cape Point, South Africa, has been presented by the director of the South African Museum, Cape Town. The Department of Geology has recently acquired, through the generosity of the Committee of the Torquay Natural History Society, more than 160 Devonian invertebrates from Devon, all of which are type or figured specimens; and has received from Mr. M. H. Donald a large collection of fossil invertebrates, formed by the late Mrs. J. Longstaff, chiefly from the Carboniferous of Great Britain, and including more than 20 figured specimens. The Mineral Department has received by gift from Prof. E. D. Mountain a specimen of boksputite, a species new to the collection; from Dr. E. S. Simpson distorted crystals of cassiterite from Pilbara goldfield; and from Mr. R. Murray-Hughes a series of rocks collected by him on the Aberdare Range, Kenya Colony, in 1933. A piece, weighing 99 gm., of the meteoric stone which fell on May 26, 1932, at Kuznetzovo, Tatarsk district, Siberia, has been acquired by exchange.

Notes on cassiterite crystals from the New England district, New South Wales, and Stanthorpe, Queensland

Notes on cassiterite crystals from the New England district, New South Wales, and Stanthorpe, Queensland

Additional notes on Wood-tin

In my previous papers I described and illustrated several examples of the concentrically banded and radiated varieties of cassiterite known as ‘wood-tin’, and stated that one very constant character of this variety was its rather remarkable opacity as compared with ordinary cassiterite, which is indeed somewhat exceptionally transparent considering its (often) deep coloration. I also noted the frequent occurrence of specimens of wood-tin encrusted with more or less distinct crystals of cassiterite, or traversed by fissures filled with such crystals, as indicating successive formation.