CASSITERITE

Cassiterite – SnO2 – is a relatively rare oxide that constitutes the main ore of Sn.

It is classified in the Rutile Group and may contain Fe, Nb, Ta, Zn, W, Mn, Sc, Ge, In and Ga. It rarely forms botryoidal masses, called “wood tin”, which is one of the four varieties recognized for cassiterite. It can be epitaxial on quartz and on nordenskiöldina.

Twins are very common in cassiterite, by {011}. They can be single contact twins or interpenetration twins. Lamellar twins and cyclic twins with five individuals (“fivelings”) also occur. Typical are the knee twins, of two crystals with an angle of approximately 60º to each other.

Alluvial grains of cassiterite can be recognized by grinding with silicon carbide on a glass plate: the color of the abrasive becomes very light, whitish, slightly yellowish, as the cassiterite trace is very clear. Other minerals leave the abrasive with other colors: magnetite (=black), hematite (=red), wolframite, columbite and tantalite (=reddish brown), Mn minerals (=deep black), etc.

1. Characteristics

Crystal system: Tetragonal, bipiramidal ditetragonal.         

Color: Black, black-brown, reddish-brown, red, gray, yellow, rarely colorless. Often zoned.     

Habit: Prismatic, pyramidal, stalagtite, botryoidal, concretionary, fine to coarse granular, massive. Crystals up to 10 cm.       

Cleavage: {110} indistinct, {100} imperfect.      

Tenacity: Very brittle.         

Twinning: See above.       

Fracture: Subconchoidal/irregular       

Mohs Hardness: 6 – 7

Parting: Distinct in {111} ou {011}         

Streak: Light gray, white, light brown.         

Lustre:  Metallic to adamantine, greasy on fractures.         

Diaphaneity: Transparent.           

Density (g/cm³):  6.8 – 7.1

 

2. Geology and Deposits

The primary deposits of cassiterite are almost exclusively occurrences associated with granitic intrusions: hydrothermal veins and pegmatitic-pneumatolitic deposits such as veins, dykes, pipes, stockworks and greisen impregnation deposits; always in medium to high temperature deposits. The secondary deposits are alluvial placers, from which almost all of the mined cassiterite comes from.

It occurs in the form of “wood tin” filling fine fractures in rhyolites. It can precipitate from volcanic gases at temperatures between 240 and 550ºC. It also occurs in contact rocks (cornubianites) with carbonate rocks, associated with sulfides.

 

3. Mineral Associations

It occurs with the typical minerals of high temperature hydrothermal veins and greisen associated with granites: quartz, feldspars (orthoclase, albite), micas (muscovite, phlogopite, lepidolite), tourmaline, topaz, fluorite, apatite, wolframite, columbite and others.

It is associated with many sulfides such as pyrite, pyrrhotite, marcasite, arsenopyrite, molybdenite, sphalerite, tetrahedrite-tennantite, galena, jamesonite, bournonite, stibnite, stannite and bismuthinite.

Also Ag sulfosalts, Pb-Sb sulfosalts, scheelite, siderite, wurtzite, nordenskiöldine, andorite, nickel and cobaltite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1.990 – 2.010    nε: 2.093 – 2.100

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually yellow, reddish to brownish, may be orange or green. Rarely colorless to grey.

Strong to absent pleochroism, but usually weak pleochroism in yellow, green, red to brown, with uneven color distribution, in patches.   

Relief: Very high.           

Cleavage: {110} indistinct and {100} imperfect, usually not visible under a microscope, but may be present as very discrete features.

Habits: Prismatic and granular crystals are common, very rarely it is acicular or botryoidal (“wood tin”). Cataclase is very frequent. It can generate pleochroic halos in neighboring minerals.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.103: very high interference colors, always masked if the mineral has a strong own color.

Extinction: Tends to parallel.

Elongation sign: ES(+). The strong own color of cassiterite turns it difficult to determine de elongation sign.

Twins: Single twins on (101) are common, easy to recognize. It rarely presents lamellar (polysynthetic) pressure twins.        

Zoning:  Frequent, recognized by the distribution of colors, pores or unmixing bodies.   

CONVERGENT LIGHT

Character:  U(+), may be anomalous biaxial. Due to the strong colors of cassiterite, the determination of the optical signal is very difficult. 

2V angle:  if anomalous biaxial, 2V from 0 – 38º 

Alterations:  does not alters. It is very resistant to weathering processes.         

May be confused with: few other minerals.

Rutile has a lighter color, higher relief and birefringence, and stronger pleochroism.

Allanite is B(+) or B(-) and may be metamictic.        

Cassiterite 01: In PPL, muscovite (colorless) and cassiterite (brownish). Zoning is very evident, with lighter and darker spots. Pleochroism is not strong, but distinct enough to be easily noticed in the brightly colored portions.
Cassiterite 01: The same section as of the previous image, now in XPL. Muscovite show strong colors, but the interference colors of cassiterite are overshadowed by the mineral´s own color.
Cassiterite 02: In PPL, two cassiterite grains. Color zoning is always present, but sometimes very faint, as here.
Cassiterite 02: The same grains of the previous image, rotated 90 degrees to show its pleochroism.
Cassiterite 02: The same grains as of the two previous images, now in XPL. Cassiterite's intense birefringence colors are masked by the mineral's own strong color.
In the center of the image, in XPL, twinned cassiterite grain. Twin plane is very evident. Around it, muscovite (colorful) and quartz (gray/white).
In XPL, idiomorphic cassiterite with muscovite around it. Well-formed grains are common in cassiterite.
In XPL, cassiterite on the diagonal of the image, almost in the extinction position, showing two lamellar twins (reddish parallel bars).

5. Reflected Light Microscopy

Sample preparation: cassiterite is one of the worst minerals for polishing due to its high hardness associated with great fragility, as it is very brittle. Grinding should only be done with fine abrasive; good quality polished sections require much more time than usual. Impregnation may be necessary. There will always be a high and uncomfortable relief in relation to other neighboring minerals. Sections with different orientations have very contrasting hardness to polish.

Cassiterite grains leave the abrasive whitish during grinding, because the trace (the color of the powder) of the cassiterite is very light, between white and yellowish. 

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray, much lighter than quartz and feldspars.

Pleochroism: Very weak, only visible in intergranular contacts and twins.

Reflectivity:  10.86 – 12%       

Bireflectance: Weak to strong.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Strong grayscale anisotropy, which can be observed even with typical internal reflections. 

Internal reflections: White, yellow and yellow-brown, very common. Even in the almost black cassiterites, reflections are abundant.

May be confused with: Typical for cassiterite are poor polishing, strong anisotropy, yellow to brown internal reflections, and large numbers of single twins.

Sphalerite can look very similar, especially if the polish is of good quality.

Rutile is lighter and usually has lamellar twins, which in cassiterite are rarer.

Titanite and zircon may be similar, but they occur in other paragenesis.       

General Characteristics: 

Grain shape: cassiterite has a strong tendency to idiomorphy. Shapes often reflect their formation environment – pegmatites, high- or low-temperature pneumatolytic ores, or deposits formed at very low temperatures. The union between adjacent grains is very weak.

Grain sizes are very varied, roughly corresponding to the formation temperature: high temperatures generate large crystals; low temperatures generate small crystals. The alteration of cassiterite and teallite – (Pb,Sn)SnS2 – forms massive aggregates

The variety “wood tin” (botrioidal stalagtite cassiterite) shows banding with successive layers formed by cryptocrystalline acicular crystals oriented perpendicularly to the bands; very reminiscent of the structure of chalcedony.           

Twins are very common and easy to recognize in most cases. They are only absent in the “wood tin” variety and in some occurrences of acicular tin. The twins are usually single growth twins. Relatively rare are pressure twins, which are very thin lamellar.

Cleavage is observed only rarely.

Zonation, often very well developed, can be recognized in many cases through the distribution of pores or unmixing bodies. Also the internal reflections allow in many cases to visualize the zones.

Unmixing bodies of tapiolite (Fe, Mn)(Ta,Nb)2O6 occur in some deep-colored cassiterites or in some areas thereof. They can form extremely fine needles, oriented or not. In granites, cassiterite can unmix columbite or tantalite in the form of small rounded grains. 

Rhythmic precipitation occurred in the “wood tin”, forming very well developed structures. In high temperature hydrothermal occurrences, cassiterite may show textures with gradational grain sizes or banded structures, alternating bands with cassiterite, pyrite or quartz.

Cataclasis is very frequent, as cassiterite is one of the first minerals to form, suffering all the tectonic stresses that the ore later went through. Cataclase is especially common in ores where cassiterite is associated with sulphide ores. Even in some pegmatites the cassiterite is so cataclased that it causes processing problems.

Radial structures, dark red with rhythmic colors, may occur in some deposits.

Cassiterite myrmekites are rare, but can occur with quartz, calcite and pyrite. Stannite alteration produces myrmekites between cassiterite and chalcopyrite.         

Exsolution intergrowths can occur with columbite and tantalite. Intergrowths that resemble the graphic texture can be formed with galena, boulangerite and franckeite.

Inclusions 1: Cassiterite inclusions can occur in sphalerite, arsenopyrite and galena.

Inclusions 2: Inclusions in cassiterite can be of magnetite, hematite, ilmenite, columbite, tantalite, rutile, tapiolite, wolframite, native bismuth, oxystannomicrolite and wodginite.

Magnetism in cassiterite occurs as a function of intergrowth or inclusions of magnetite or the presence of exsolutions of columbite and tapiolite, which may be dispersed or agglutinated.          

Substitutions 1: Cassiterite is replaced by sulfides in some cases, such as pyrite, arsenopyrite, pyrrhotite, stannite, sphalerite and chalcopyrite.

Substitutions 2: Cassiterite replaces various minerals such as quartz, feldspar, topaz, tourmaline, stannite, pyrite, arsenopyrite, sphalerite, frankeite, teallite, cylindrite and bismuthinite

Alluvial cassiterite grains mounted on epoxy, in PPL, showing twins, which are a very common feature in cassiterite.
Alluvial cassiterite grains mounted on epoxy, in PPL, showing twins, which are a very common feature in cassiterite.
Cassiterite 01: Cassiterite in PPL has a non-diagnostic gray color and low reflectivity. Holes (black dots) are very common due to the difficulty of polishing.
Cassiterite 01: The same section as of the previous image, now in XPL. Cassiterite exhibits yellowish-brown internal reflections (the same color as under Transmitted Light in PPL) and twins. Anisotropy is visible to varying degrees.
Cassiterite 02: In PPL, cassiterite grains forming pyramidal crystals (lighter gray), gangue (dark gray) and pyrite (yellow, far right).
Cassiterite 02: The same section as of the previous image, now in XPL. The gray anisotropy of the cassiterite as well as the yellow to golden reflections are evident.
Cassiterite 03: In PPL, pyrite (yellowish, bottom left), gangue (very dark gray) and cassiterite (lighter gray with black holes) in prismatic and bipyramidal grains.
Cassiterite 03: The same section as of the previous image, now in XPL+2 degrees. Pyrite (isotropic, poorly polished), gangue (milky internal reflections) and cassiterite showing its anisotropy in gray and its internal reflections in yellow to golden.
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