TITANITE

Titanite – CaTiSiO5 – is a very common nesosilicate, an accessory mineral to many types of igneous rocks. A traditional name for titanite, still widely used, is “sphene”. It is a TiO2 ore, an important white pigment. Furthermore, it can be used as a gemstone and for dating by the U-Pb method.

It may contain Al, Fe and F. Furthermore, it contains Nb, Ta, Fe, Mg, Mn, Na, Y, Ce, Sr, V, Zr and Sn as impurities. Forms a series with malayaite, an extremely rare monoclinic nesosilicate that is the Sn analogue of titanite. Titanite has 9 varieties (with Cr, with Ce, with Mn, with Y, with Sn, a triclinic one, etc.). Depending on its content of radioactive elements, it can be metamict.

1. Characteristics

Crystal system:  Monoclinic prismatic.         

Color: Colorless, brown, black, green, yellow, pink-red.    

Habit: Granular, tabular, wedge-shaped, prismatic, compact, massive.       

Cleavage: {110} good.      

Tenacity: Brittle.        

Twinning: On {100}, penetration twins or contact twins, may be lamellar.       

Fracture: Subconchoidal.       

Mohs Hardness: 5 – 5.5

Parting: Due to twinning, on {221}.         

Streak:  Reddish white.        

Lustre: Adamantine, resinous.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.48 – 3.60

      

2. Geology and Deposits

Titanite is very common as an accessory mineral in many types of igneous rocks. In plutonic igneous rocks it is more common in diorites, syenites, monzonites, tonalites and granodiorites, usually associated with a greenish amphibole. Large crystals are found in pegmatites. It also occurs in Alpine Type tectonic fissures.

In volcanic rocks it is more common in phonolites; it does not occur in gabbros and basalts because the Ti in these rocks goes to titanoaugite and ilmenite, respectively. It is a common mineral in ijolites.

In metamorphic rocks, it is more frequent in rocks such as mica-schists, amphibolites, glaucophane-schists, gneisses and marbles. It can occur in metasomatic rocks such as skarns and phenites.

It is often found in sedimentary rocks such as sandstones (graywackes) as a heavy mineral.

 

3. Mineral Associations

In plutonic igneous rocks it is associated with rock-forming minerals such as quartz, plagioclase (albite), alkali feldspars (orthoclase, microcline), micas (biotite, muscovite, chlorite, sericite) and amphiboles (green and brown hornblende).

Furthermore, it is associated with accessory minerals such as epidote, zircon, apatite, allanite, monazite, magnetite, ilmenite, pyrite, chalcopyrite and pyrrhotite.

When in marbles, it is associated with diopside, calcite and clinochlore (chlorite).

In low-silica rocks occurs with nepheline and sanidine.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.843 – 1.950     nβ: 1.870 – 2.034    nγ: 1.943 – 2.110

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Brown, gray-brown to yellow-brown, with absent or weak pleochroism:

X = almost colorless,          

Y = pale yellow,            

Z = brownish.

It is necessary to pay close attention during the rotating of the stage to verify the pleochroism of the tianite.    

Relief: Very high to extreme.           

Cleavage: {110} good. It is a prismatic cleavage with two directions that, however, is generally not visible or poorly defined. Titanite normally has many fractures. It has a partition paralell to the twinning that can be more prominent than the cleavage.           

Habits: Usually anhedral, completely xenomorphic. It only occasionally shows basal sections with rhombic outlines or diamond-shaped (wedge).

In metamorphic rocks, titanite tends to form clusters of small round grains.

When it occurs as inclusions in biotite, chlorite or hornblende and contain higher levels of Th, it generates black halos around it. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Birefringence from 0.1 to 0.192, very high, resulting in a high order white color, a very typical cream color. But the original strong brown color of the mineral masks this high interference color.

Generally the color of titanite does not change much from PPL to CPL. Some grains are very colored in CPL. Low birefringence grains often do not extinguish properly and show anomalous dark orange to dark blue colors.          

Extinction: Oblique from 36 to 45º; symmetrical in basal (rhombic) sections.

Due to the high dispersion, some grains do not completely extinguish.           

Elongation sign: Does not apply as titanite may be elongated in several directions.            

Twins: Contact and penetration twins by {110} are common. Lamellar twins according to {221} occur more rarely.         

Zoning: It can be zoned, in this case the margins have a higher refractive index.             

CONVERGENT LIGHT

Character: B(+), hard to get.          

2V angle:  17 a 56º          

Alterations: under hydrothermal conditions it decomposes to leucoxene, a very fine-grained white to pale yellow mixture composed of rutile, quartz, calcite, apatite, ilmenite and minerals with Rare Earth Elements (allanite, monazite and bastnäsite). Leucoxene can be easily recognized using the Oblique Light technique.

Completely altered titanite grains become opaque due to the presence of ilmenite.          

May be confused with: several other minerals.

Epidote has a lower relief, only one cleavage, and a much lighter color.

Cassiterite, zircon, rutile and xenotime are uniaxial.

Anatase and carbonates are U(-).

In very fine grains, titanite can be confused with epidote and allanite.         

 

Titanite 01: In PPL, losangular basal section of titanite showing the first ot its two colors of pleochroism. In the next image is the second one.
Titanite 01: The same crystal as of the previous image, now rotated 90 degrees to show the second color of pleochroism, a little bit darker. Poorly defined cleavage, many fractures, high relief and few inclusions are typical.
Titanite 01: The same crystal as of the two previous images, now in XPL. The strong own color of titante masks the high interference colors. The color in XPL is almost the same as in PPL. In gray/white, quartz. In lilac, chlorite. In red/yellow, a biotite rest.
In PPL, anhedral titanite grain in a plutonic rock. These anhedral sections are much more frequent than basal rhombic sections, which are comparatively rare. Pleochroism is often barely noticeable, but, since the color in PPL is almost very similar to the color in CPL, titanite is relatively easy to recognize.
In PPL, anhedral titanite grain in plutonic rock. Some rocks never show titanites with the rhombic form.
In PPL, titanite in an anorthosite, a plutonic rock composed mainly of plagioclase.
In most cases the color of titanite in PPL is almost the same as in XPL. But sometimes the titanites show in XPL very colorful colors, as here.
In most cases the color of titanite in PPL is almost the same as in XPL. But sometimes the titanites show in XPL very colorful colors, as here.
In XPL, twinned titante.
In XPL, twinned titante.
In XPL, titante with lamellar twin, in this image with one set of twins.
In XPL, titante crystal in the extinction position, showing two sets of lamellar twins.
Titanite 02: Marble with titanite altered to leucoxene. In PPL, the titanite still shows high relief and a texture reminiscent of the original grain.
Titanite 02: The same section as of the previous image, now in XPL. The titanite is completely black, looks isotropic. In this situation, Oblique Light is useful (next image).
Titanite 02: The same section as of the previous two images, now in Oblique Light. Turning off the microscope light and illuminating from above with a strong LED lamp, this titanite presents an intense white luminosity, slightly yellowish, typical of the alteration products of titaniferous minerals, generically called leucoxene. This observation confirms the diagnosis of “altered titanite”.
In XPL, titanite grain in a sandstone.
Titanite 03: In granitic rock, grouping of idiomorphic titanite crystals with shapes different from the usual ones. On the left, garnet. At right, quartz and biotite. Among the titanite crystals, calcite. PPL.
Titanite 03: The same section as of the previous image, now in XPL. Titanite shows strong interference colors.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of titanite. However, identification of titanite is quite simple in this technique. In addition, it is important to make a polished thin section or section to identify the opaque minerals that occur associated with titanite, such as ilmenite, magnetite, pyrite, chalcopyrite, pyrrhotite and others.

Sample preparation: titanite is very easily polished. Its polish is generally of better quality than other silicates that accompany it, such as quartz, feldspar, etc.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, much lighter than other silicates such as quartz, feldspars in general, amphiboles and pyroxenes.       

Pleochroism: No.      

Reflectivity: 9.43 – 11%, much lighter than other common gangue silicates!

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: In basal sections there is no anisotropy or is masked by internal reflections, but in other sections the anisotropy is very strong, from light gray to dark gray.        

Internal reflections: Generalized, abundant, yellowish to orange in color, can evolve to caramel and red. It resembles the internal reflections of large grain sphalerites and rutile.

When the color of titanite in hand specimen is greenish, the internal reflections also are green, a much rarer situation.      

May be confused with: sphalerite and rutile.       

General Characteristics: 

Twins sometimes can be observed.

Pseudomorphoses of titanite on ilmenite or of titanite on the network of unmixed ilmenites of titanomagnetites can have an excellent development.

Substitutions of titanite for anatase (lighter in PPL) can occur, especially near deposits of metallic elements.

Extinction is symmetric in the basal rhombic sections.

Titanite 01: In PPL, titanite rhombic section surrounded by silicates. In PPL, it is important to note the much lighter color of titanite compared to the surrounding quartz. It reminds ilmenite at first sight.
Titanite 01: The same section as of the previous image, now in XPL. The orange to caramel internal reflections are very typical.
Titanite 02: Titanite grain surrounded by amphibole. The reflection color of titanite is much lighter.
Titanite 02: The same grain as of the previous image, now in XPL. Here the anisotropy is strong, with this image showing the lighter color. The next image will show the darker one.
Titanite 02: The same grain as in the two previous images, also in XPL, this time rotated 90 degrees. It shows golden internal reflections. The dark reflections of the amphibole mask the situation somewhat.
Titanite 03: In PPL, ilmenite grains (very light gray, a little pink) surrounded by titanite (bluish gray, a little darker than ilmenite). Around it, feldspar and amphibole in dark gray and holes (black). These titanite coronas around ilmenite, with or without associated titaniferous magnetite, are a very characteristic feature.
Titanite 03: The same section as of the previous images, now in XPL. Ilmenite (dark in this position), titanite with very strong anisotropy in grey-white, other silicates with dark reflections.
In PPL, typical colors: chalcopyrite (golden), pyrrhotite (brown-pink, poorly polished (black holes)), ilmenite (pink gray, included in titanite, good polishing), titanite (lighter gray, excellent polish), feldspar (darker gray), amphibole (the darkest gray).
In XPL, titanite with lamellar twins. Some twins that appear as thin, lighter, parallel lines on the diagonal of the image.
In XPL, twinned titanite. Here, the lamellar twins are almost horizontal.
Titanites in oblique light. Turn off the light of the microscope and illuminate the polished section from above with a strong LED light. Titanite always shows these white, very luminous reflections. This technique is extremely helpful for the identification of titante.
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