RUTILE

Rutile – TiO2 – is a common oxide, an accessory mineral in many igneous rocks. It constitutes an important ore of titanium oxide, which is a high quality white pigment, among other uses.

Rutile is one of the 5 polymorphs of TiO2, the others are anatase (tetragonal), brookite (orthorhombic), akaogiite and TiO2 II. Thermodynamically it is the most stable of the five. It is one of the minerals with the highest known refractive indexes, in addition to having high dispersion. May contain up to 10% Fe, Nb, Ta, V, Sn and Sb. The endings of the crystals, which reach up to 25 cm, can be very complex.

It forms epitaxies with hematite, ilmenite, magnetite, anatase and brookite. The best known epitaxies are those where tabular hematite (specularite) forms pseudo-hexagonal aggregates with epitaxic golden rutile crystals, found in Minas Gerais (Brazil).

There are 11 varieties of rutile, based mainly on additional levels of Nb (“ilmenorutile”), Ta+Fe (“strüverite”), V, Cr, Sn and Sb. Acicular rutile crystals in quartz crystals are called “Venus Hair” or “rutilated quartz”. When in parallel needles, rutile is responsible for asterism in various types of gemstones such as corundum (rubies and sapphires), tourmaline and quartz.

1. Characteristics

Crystal system: Tetragonal, bipiramidal ditetragonal.          

Color: Reddish brown, red, yellow, brown, bluish, violet, rarely green or black (Nb/Ta).

Habit: Prismatic, elongated, equidimensional, massive, granular, bipyramidal.       

Cleavage: {110} good, {100} moderate, {111} in traces. Striations // to [001].

Tenacity: Brittle.        

Twinning: Contact twins are very common, in {011} and {031}. Cyclic and polysynthetic also occur.

Fracture:  Irregular, conchoidal, subconchoidal.

Mohs Hardness: 6 – 6.5

Parting: No.         

Streak: Light red and dark red.         

Lustre: Intense adamantine to submetallic.          

Diaphaneity: Transparent.       

Density (g/cm³):  4.23

 

2. Geology and Deposits

Rutile is a common, high-temperature, high-pressure accessory mineral found as generally very small grains in many types of igneous rocks (anorthosites, pegmatites, etc.). In metamorphic rocks (phyllites, quartzites, mica-schists, gneisses, amphibolites, glaucophane-schists, eclogites) it occurs in the form of minute acicular crystals. It can occur as needles in quartz.

It is also a very common detrital mineral in sediments and sedimentary rocks, it can form placer deposits. Large crystals are found in pegmatites.

 

3. Mineral Associations

It occurs with feldspars (albite, adularia), other TiO2 polymorphs (much rarer: anatase and brookite), carbonates (calcite, siderite), phyllosilicates (biotite, muscovite, chlorite, pyrophyllite), Fe-minerals (lepidocrocite, hematite, ilmenite), kyanite, cordierite, corundum, coulsonite, spinel, apatite, titanite, pyrite, quartz and others. 

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 2.605 – 2.613     nε: 2.899 – 2.901

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Red brown, yellow-brown (depending on Fe content), dark brown, usually with weak pleochroism, which may be barely noticeable. When in small grains, with very dark reddish-brown colors, it can be mistaken for an opaque mineral! Intense lighting or use of compensator is necessary: in CPL and with the compensator inserted, the opaque minerals remain black; the rutile will show color and variation in luminosity. It is also possible to use Oblique Light.   

Relief: Very high, extreme.           

Cleavage: {110} perfect, {100} moderate and {111} in traces. Cleavage is usually difficult to see.           

Habits:  Short prismatic (columnar), acicular parallel to [001], granular, occasionally forms reticular networks (sagenite texture). Occurs as inclusion in biotite, its radioactivity can produce dark halos around the grains. Small, acicular or fibrous crystals may appear black due to their high reflection. In many cases the crystals are extremely small, smaller than the thickness of the thin section (30 micra).           

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: 0.286 to 0.296 birefringence: extreme, very high, 16th order interference colors. It is the 2nd highest birefringence of all minerals. They are cream, pearly colors, masked by the intense color of the mineral. Generally, the color of rutile is the same in PPL and in XPL.           

Extinction: Paralell.

Elongation sign: ES(+), difficult to determine due to high birefringence and intense color.            

Twins:  On (101), in the form of a knee or a heart. In addition, cyclic twins.        

Zoning: No.             

CONVERGENT LIGHT

Character: U(+). Interference figures can exhibit many isochromats. May be anomalous biaxial. The strong colors make it very difficult to obtain the figure.          

2V angle:  No, or small in anomalous biaxial crystals.        

Alterations: it is very stable, it is a detrital mineral, it does not alters.          

May be confused with:  several other minerals, depending on its habit.

When it occurs with a granular habit, it can be very dark and can even be confused with an opaque mineral. But, in XPL, using converging lens and compensator, the rutile is translucent and the opaques are black. Granular rutile resembles perovskite (which always shows twins like leucite), spinel (which is isotropic in XPL), cassiterite (whose pleochroism is much stronger), anatase (which is U(-), can show other habits, but may be difficult to distinguish from rutile), titanite (biaxial) and brookite (biaxial).

When it occurs with an acicular habit, it is usually very small and is easily confused with an aggregate of opaque minerals. It is necessary to observe carefully the small crystals with the objective of maximum magnification. Epidote may be similar, but it is much lighter, greenish in PPL and its relief is slightly lower. Acicular amphiboles are similar, but their relief is smaller and the colors in PPL are different, between greenish and brown. Tourmaline can be very similar, but its pleochroism in the longitudinal sections is stronger.       

In PPL, rutile with xenomorphic granular habit and very dark color in eclogite. Larger grains appear opaque at first glance. Pleochroism is very weak, almost imperceptible. In XPL, the color is almost the same.
In PPL, rutile with xenomorphic granular habit and very dark color in eclogite. Larger grains appear opaque at first glance. Pleochroism is very weak, almost imperceptible. In XPL, the color is almost the same.
Rutile 01: In PPL, rutile in eclogite. The granular habit is characteristic, as well as strong colors and minimal pleochroism.
Rutile 01: The same section as of the previous image, now in XPL. The color is almost the same as in PPL.
Rutile in XPL (center of image) under intense lighting, tending to idiomorphic and with different shades.
Rutile in XPL (center of image) under intense lighting, tending to idiomorphic and with different shades.
In PPL, acicular rutile with a strong red color.
In XPL, acicular rutile.Colors range between red and dark brown and generally do not change between PPL and XPL. Needles can be short to extremely long.
In XPL, acicular rutile. Usually the needles are small and require attention to be detected.
Rutile 02: Acicular rutile in alkaline rock, with plagioclase, muscovite (on the left in the images) and fluorite (on the right in the image), among others. , In PPL, the brown color of rutile is only visible in the larger crystals. Typical is very high relief.
Rutile 02: The same section as of the previous image, now in XPL. In XPL, the finer needles are strongly colored, while the larger ones have the original brown color of rutile.
In PPL, a granular rutile aggregate (left) and an acicular rutile aggregate (right) side by side. In the same rock, anatase, another polymorph of TiO2, occurs too.
In XPL, very long, extreme, needle-like rutile crystals.
Rutile 03: In PPL, acicular rutile in "rutilated quartz". The color is so dark that the mineral appears opaque.
Rutile 03: The same section as of the previous image, now in XPL. The interference colors are difficult to define, as always in deeply-colored minerals.

5. Reflected Light Microscopy

Sample preparation: the rutile polish is of good quality, but requires persistence. It is equivalent to polishing hematite and pyrite, consuming at least twice the time of other minerals.

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray with a blue tint.       

Pleochroism: Weak in shades of gray with blue, perceptible only in intergranular boundaries and in twin lamellae.      

Reflectivity: 19.87 – 23.27%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Strong anisotropy from gray-blue to dark, but heavily masked by internal reflections. In twin lamellae, anisotropy is more evident.

Internal reflections: Colorless, white, light yellow, orange and caramel, can evolve to dark brown red, blood red, brown, green and gray, vary with chemical composition.      

May be confused with:  diagnostic are hardness, twins and frequent and very clear internal reflections despite of the high reflective power.

Stannite and titanite can be quite similar.      

General Characteristics: 

Grain shape: rutile has several characteristic habits, depending on the origin:

a) Crystals tend to be euhedral, prismatic along the z axis as in rutilated quartz.

b) When it occurs as pseudomorphosis, it takes on the shape of the mineral it replaced..

c) Another form of occurrence is in very fine-grained, extremely intergrown aggregates of rutile and hematite. Rutile crystals can form a very fine web with hematite filling the gaps in the lattice; it is a structure derived from the decomposition of an original ilmenite. If this ilmenite contained exsolved discs of hematite, these may still be preserved in the original position, revealing the origin of the aggregate.

d) Likewise, the original lattice of ilmenite unmixed into titanomagnetites can be transformed into rutile in this way, with or without the concomitant removal of the magnetite. Under special conditions, the transformation of ilmenite can generate, instead of rutile, anatase.

e) In some intrusive basic rocks (diabases), a part of the leucoxene is rutile, instead of or alongside sphene and anatase.

f) Rutile can also form by pyritizing the host rocks of hydrothermal veins with metals from minerals with titanium; the iron content forms pyrite, the titanium content forms rutile; there may be rutile networks in pyrites formed from titanomagnetites.

g) Rarely, is rutile substituted and surrounded by ilmenite. Alternatively, the rutile is replaced by a darker brown rutile with lamellar twins.

h) A special case is acicular rutile crystals in “blue quartz” of many granites. These needles have thicknesses of around 1 micron, but lengths of 100 and more microns.

Cleavage parallel to (110) may be visible, even with excellent polish, due to the formation of polishing pits.

Twins in parallel lamellae, usually thin, are almost always present in several directions, very well developed.

Zonation is not observed in polished sections.

Substitutions of ilmenite for rutile can occur. Sometimes it involves ilmenite or hematite grains.

Cataclase is visible with some frequency.        

Unmixings are very rare. Sometimes there are small plates of unmixed ilmenite arranged parallel to (110) of the rutile. Hematite can also form these structures.

Oriented intergrowths of rutile with hematite are well known.

In XPL, large rutile twins chowing anisotropy. Two large crystals forming twins, but without smaller twins and with only few internal reflections. Rotating the stage 90º in either direction, the colors are inverted.
In XPL, large rutile twins showing anisotropy. Two twinned crystals with at least two sets of twins and more frequent internal reflections. Rotating the stage 90º in either direction, the colors are inverted.
Rutile 01: In PPL, rutile with two sets of twins intersecting. It is possible to perceive the discrete pleochroism of rutile.
Rutile 01: The same section as of the previous image, now in XPL+2º and rotated by almost 90º. The twins are very evident.
In PPL, in the horizontal, several parallel alignments of polishing pits (black, tending to triangular), generated in function of the cleavages of the rutile. On the left, diagonally, fracture (larger, black) and several polishing scratches (fine, black, various directions). Diagonally opposite the fracture, very faint, thin and parallel twins, difficult to see in PPL.
Rutile 02: Hand specimen of a cyclic twin in rutile. The next image show the twinning characteristic of such twins.
Rutile 02: In XPL, cyclic twins of rutile. The twins appear as a complex of lamellae and well-defined twin planes. It is also possible to observe the scattered internal reflections in red that characterize the rutile.
Rutile 03: Horizontal alignments of tiny polishing pits (black) generated as a function of the cleavage. On the diagonal, some polishing scratches and, approximately parallel to the grooves, thin parallel twins whose visualization is possible due to the weak pleochroism of rutile.
Rutile 03: The same sections as of the previous image, now in XPL. The twins become more evident and the image shows the anisotropy colors and the orange to caramel internal reflections typical of rutile.
On fractures, imperfections, larger holes, etc. rutile in XPL has intense and luminous internal reflections in various shades of orange and caramel. White reflections occur too.
Granular aggregate of rutile in XPL with typical characteristics: - low quality polish, - anisotropy in gray, - orange and white internal reflections along intergranular contacts.
Rutile 04: In PPL, rutile needles in quartz *("rutilated quartz"). Rutile shows its medium gray reflection color with a blue undertone and quartz is dark gray.
Rutile 04: The same section as of the previous image, now in XPL. Quartz has milky internal reflections and rutile has predominantly yellow to orange, sometimes red reflections.
Rutile 05: In PPL, rutile needle in quartz, form a sample of "rutilated quartz".
Rutile 05: The same section as of the previous image, now in XPL. Quartz with milky internal reflections and rutile with intense red internal reflections.
Rutile 06: In PPL, the color contrast between ilmenite (brown-pink) and rutile (whiter), a frequent association.
Rutile 06: The same section as of the previous image, now in XPL. Ilmenite presents its anisotropy in gray-blue, shows some twins and does not present internal reflections. Rutile shows internal reflections in orange to red brown and thin parallel lamellar twins (diagonally across the image).
In XPL, rutile with an undulating structure probably formed by varying levels of ilmenite.