Titanite: The Wedge-Shaped Timepiece

By Ronald L. Parker, Senior Geologist, Borehole Image Specialists

Titanite, CaTiO(SiO4) - also commonly reported as CaTiSiO5 - is a common accessory mineral in many types of igneous and metamorphic rocks. Also known as sphene, a name that has fallen out of favor, this calcium titanium nesosilicate is an important reservoir in the titanium geochemical cycle. Titanite has a distinctive crystal form that it flaunts, commonly appearing  as recognizable wedgeshaped crystals. Titanite has a dispersion value that gives it a high luster, lending some crystals an intense brilliance and fire. Because it is comparatively soft, it is not well-suited to jewelry, but can be a prized mineral specimen for collectors. On top of it all, the calcium coordination site is sometimes occupied by other cations, most famously, uranium. This, combined with a high temperature of crystallization and a low diffusivity for lead (Pb), make titanite one of the most important minerals for U/Pb geochronology. Because titanite is a central contributor to quantitative thermometry and barometry, it has played a critical role in deciphering igneous, metamorphic and ore-forming processes. Titanite is a titan among minerals!

The name of the mineral titanite is an obvious reference to the element titanium contained therein. Titanium was discovered in 1791 from ilmenite (FeTiO3) ore in 1791 by William Gregor and 1st named by Martin Klaproth, also in 1791, for the Titans of Greek mythology (Chaline, 2012). Martin Klaproth was also the one to apply the name titanite, in 1795. [The Titans were the 2nd generation of divine beings descended from the primordial deities Uranus (Father Sky) and Gaia (Mother Earth, for whom the science of geology is named). The Titans were twelve: six daughters and six sons, with a few names that are familiar to all geologists. The female Titans were Mnemosyne, Tethys, Theia, Phoebe, Rhea and Themis. The male Titans include Kronos, Oceanus, Iapetus, Coeus, Hyperion and Crius. The descendant (2nd and 3rd) generations of Titans also sport names that permeate geology: Helios, Eos, Atlas, Prometheus, Zeus, Hades and Poseidon].


Cluster of many intersecting translucent brown titanite crystals fully crystallized on all surfaces with no matrix from Bear Lake Diggings, Tory Hill, Ontario, Canada. Several of the titanite crystals have rough areas on the edges where they intersected other nearby crystals. Ex. Bill Plavac collection #2743. Used with permission from John Betts Fine Minerals.Titanite was previously known as sphene and that name is ensconced in the older literature. The name “sphene” was coined by Rene Hauy, in 1801. Sphene derives from the Greek, “sphenos”, which  translates to “wedge” (Bonewitz, 2005). In 1982, the International Mineralogical Association (IMA) recommended that official use of titanite replace the name sphene. In spite of being discredited, sphene is still an informal label amongst geoscientists and it is frequently encountered in the gem and mineral trade (Kohn, 2017).

Titanite has a hardness of 5 to 5½, a specific gravity of 3.4 to 3.6 and distinct cleavage on {110}. Titanite is monoclinic (2/m) and euhedral crystals commonly display a wedge-shaped exterior by intersection of the common forms {100}, {001}, {110} and {111} (Klein, 2002). Titanite crystals are often flattened parallel to the c-crystallographic axis {001} and twinning is most often observed on {100}, yielding contact or penetration twins (Johnsen, 2002). Wenk and Bulakh (2004) show a photo of titanite penetration twins that superficially resemble staurolite iron crosses. The axial lengths of titanite are a=6.56Å, b=8.72Å and c=7.44Å for an axial ratio (a:b:c) of 0.752 : 1 : 0.853. The β angle (between the a and c crystallographic axes) is 119.43°, giving titanite a decidedly “inclined” character. This factor is central to the wedge-like appearance (Klein, 2002).

Titanite is usually brown, but can be yellow, green or gray. Titanite has a resinous, brightly vitreous or adamantine luster, and this is one of its diagnostic properties (Johnson, 2002). In fact, titanite is among the very few minerals that enjoy a larger color dispersion than that of diamond (Bonewitz, 2005). Faceted titanite crystals are noted for a fiery brilliance, a character that makes titanite a valued addition to a mineral collection. Unfortunately, titanite is usually too brittle and too soft for use in jewelry (Bonewitz, 2013). Titanite can be discriminated from look-alike minerals by hardness: titanite is harder than sphalerite and softer than staurolite (Klein, 2002).

Titanite is a nesosilicate, in which isolated silica tetrahedra (SiO4 4-) are bonded to cations or anionic groups, not to other silica tetrahedra. In the case of titanite, the isolated silica tetrahedral are bonded to a kinked chain of corner-sharing TiO6 octahedra that parallel the a-crystallographic axis (Klein and Philpotts, 2013). Calcium ions occupy relatively large cavities with (unusual) 7-fold coordination (Johnson, 2002; Kohn, 2017). Substitution of the cationic site includes Na+ and the middle rare-earth elements (MREEs). Substitution for titanium may include Al, Fe, Mn, Mg, Nb and Zr (Klein and Philpotts, 2013, Johnsen, 2002). Although zirconium (Zr4+) substitution clearly occurs at the Ti4+ octahedral sites, some of the other substitutions, niobium (Nb5+) and tantalum (Ta5+), for instance, are more complicated (Liferovich and Mitchell, 2005).


Titanite, Tory Hill, Haliburton County, Ontario, Canada, 3 cm x 2.9 cm x 1.4 cm, Jamison Brizendine Collection #944.jpg. Photo by Jamison Brizendine.Titanite embodies a unique suite of optical properties. Titanite enjoys extremely high birefringence (2.0) and refractive index (1.3) values (Wenk and Bulakh, 2004). When observed in thin-section under plane-polarized light, titanite is easily seen due to its very high positive relief. The high birefringence makes titanite crystals appear the same brown color in plane-polarized and cross-polarized light (Klein and Philpotts, 2002). Euhedral or subhedral titanites, under most orientations in thin-section, are distinctive as diamond or wedge-shaped crystals. These characteristics make titanite a favorite of optical mineralogy and petrography students. As Wenk and Bulakh (2004) state, titanite “…cannot be mistaken in thin-section.” (p. 437).

Titanite is a common accessory mineral in felsic and intermediate plutonic igneous rocks including granites, granodiorites, diorites, syenites and nepheline syenites (Klein, 2002; Johnsen, 2002). It is also found in pegmatites and in metamorphic rocks enriched in Ti and Ca, including gneisses, metabasalts, chlorite schists, marbles, skarns and pyroxenites (Klein and Philpotts, 2013). Titanite appears in evolved gabbroic rocks and “…is, therefore, widespread in oceanic crust.” Titanite sheds light on processes of crystallization and metasomatism under different conditions of pressure, temperature, oxidation state and hydrothermal fluid chemistry (Colwell et. al., 2011). Titanite is also sometimes observed in the accessory mineral suite of sandstones (Nesse, 2004). Other minerals commonly associated with titanite include albite, chlorite, epidote, apatite, allanite magnetite, ilmenite, biotite, diopside, oxides, pyroxene, amphiboles, scapolite, and quartz (Klein, 2002).

Titanite has commercial value as a source of titanium, but deposits with enough titanite are rare (Klein and Philpotts, 2013). Titanium is used primarily as a metal and as a pigment. Titanium metal is well known as a material with a superior strength to weight ratio and corrosion resistance. These characteristics have made titanium the most important metallic component in aeronautical and aerospace applications. Sputnik 1 (1957), Vostok 1 with the 1st human in space (Yuri Gagarin, 1961) and all subsequent space flight vehicles, have depended upon titanium structural elements and engine parts. Today, titanium is used for more mundane applications like high-tech bicycle frames and, because it is biologically inert, is finding widespread application in medicine as artificial joints, heart valves, pacemakers and dental implants (Chaline, 2012). Titanium dioxide (TiO2) is used widely as a bright white pigment in paints and coatings, papermaking and plastics. Other industrial uses for titanium include enamels, glassmaking, ceramics, catalysts, welding and electronics (Chang, 2002).

One of the most interesting characteristics of titanite is its strength as a thermobarometer. It is especially useful because it preserves information at the high-end of the temperature range (~600°C to 1000°C). In 2006, Hayden, Watson and Wark established that the relative concentration of Zr4+ in synthetic and natural titanite is systematically related to pressure and temperature conditions. Since this work, the “Zrin- Titanite” thermobarometer has been demonstrated to be a robust tool with a large temperature and pressure range that finds application in a wide variety of rocks and tectonic settings (Kohn, 2017).


Titanite, Capelinha, Minas Gerais, Brazil, 13 cm x 5 cm x 3 cm, Anne Cook Collection #12099. Photo by Jamison Brizendine.In addition to thermobarometry, titanite is also a reliable geochronometer. Although zircon is the rock star of the U-Pb dating world, titanite – along with monazite and baddeleyite - form a strong second line (Dickin, 2005). Titanite is useful for geochronology because it is widespread, it incorporates uranium into its crystal lattice (substituting for Ca2+), and it has a high closure temperature for Pb and other cations (Frost et. al, 2001; Engi et. al., 2017). Development of titanite as a geochronometer was first accomplished by Tilton and Grunenfelder in 1968. Since, titanite has been used to date many igneous and metamorphic rocks in diverse orogenic settings (Kohn, 2017; Dickin, 2005). Initial work on dating titanites concluded that Pb diffusivity is high above ~600°C, suggesting that titanite dates reflect cooling, not crystallization events. More recent work shows that Pb diffusivity in titanite is 2-4 orders of magnitude slower than previous experimental estimates, expanding the versatility of titanite as an archive of processes in the middle crust (Kohn and Penniston-Dorland, 2017). Linked to thermobarometry, titanite geochronology provides information about the P-T-t evolution of rocks in orogenic belts, a power enjoyed by very few mineral species (Engi et. al., 2017).

Important localities for titanite include the Kola Peninsula of Russia, where it is mined for titanium from a nepheline syenite. Other important localities include Austria, Madagascar, Canada, Mexico, Brazil, Sweden, Germany, Russia, Pakistan, Switzerland, Italy and Norway. In the U.S. titanite is found in abundance in the Adirondacks and Adirondack lowlands of Upstate New York, in New Jersey and in Riverside, California (Klein, 2002; Johnsen, 202; and Bonewitz, 2005).

WEBLINKS:

  • http://www.minerals.net/mineral/titanite.aspx
  • https://en.wikipedia.org/wiki/Titanite
  • https://www.mindat.org/min-3977.html
  • http://www.handbookofmineralogy.org/pdfs/titanite.pdf
  • http://webmineral.com/data/Titanite.shtml#.Ws0hBYgbOHs
  • https://geology.com/minerals/titanite.shtml

REFERENCES:

Bonewitz, Ronald Louis, 2005, Gem and Mineral: The Definitive Guide to Rocks, Minerals, Gems and Fossils, New York, New York: Dorling-Kindersley Limited, 360 pp.

Chaline, Eric, 2012, Titanium, in Fifty Minerals that Changed the Course of History, Buffalo, New York, Firefly Books, Inc., pp. 198-201.

Chang, Luke L.Y., 2002 Industrial Mineralogy: Materials, Processes and Uses, Prentice Hall: Upper Saddle River, New Jersey, 472 pp.

Colwell, L. E., B. E. John, M. J. Cheadle and J. L> Wooden, 2001, Chemistry of Titanite (Sphene) in Ocean Crust: A Tool for Understanding Late-Stage Igneous and Metasomatic Processes at Mid-Ocean Ridges, Abstract V11A-2493 presented at 2011 Fall Meeting, AGU, San Francisco, Ca.

Dickin, Alan P. 2005, /radiogenic Isotope Geology, Cambridge, U.K.: Cambridge University Press, 492 pp.

Engi, Martin. Pierre Lanari and Matthew J. Kohn, 2017, Significant Ages – An Introduction to Petrochronology, Chapter 1 in Petrochronology: Methods and Applications, Reviews in Mineralogy and Geochemistry, Volume 83, ed. by. Matthew J. Kohn, Martin Engi and Pierre Lanari, Mineralogical Society of America pp. 1-12.

Frost, B., Ronald, Kevin R. Chamberlain and John C. Schumacher, 2001, Sphene (Titanite): Phase Relations and Role as a Geochronometer, Chemical Geology 172(1-2):131-148.

Hayden, Leslie A., E. Bruce Watson and David A. Wark, 2008, A Thermobarometer for Sphene (Titanite), Contributions to Mineralogy and Petrology, 155(4): 529-540.

Johnsen, Ole, 2002, Minerals of the World: Princeton University Press, Princeton, N.J. 439 pp.

Klein, Cornelis, 2002, The 22nd Edition of the Manual of Mineral Science: New York, John Wiley & Sons, Inc., 641 pp.

Klein, Cornelis, and Anthony Philpotts, 2013, Earth Materials: Introduction to Mineralogy and Petrology, Cambridge University Press, 536 pp.

Kohn, Matthew J., 2017, Titanite Petrochronology, Chapter 13 in Petrochronology: Methods and Applications, Reviews in Mineralogy and Geochemistry, Volume 83, ed. by. Matthew J. Kohn, Martin Engi and Pierre Lanari, Mineralogical Society of America pp. 419-442.

Kohn, Matthew J. and Sarah C. Penniston-Dorland, 2017, Diffusion: Obstacles and Opportunities in Petrochronolgy, Chapter 4 in Petrochronology: Methods and Applications, Reviews in Mineralogy and Geochemistry, Volume 83, ed. by. Matthew J. Kohn, Martin Engi and Pierre Lanari, Mineralogical Society of America pp. 103-152.

Liferovich, Ruslan P. and Roger H. Mitchell, 2005, Composition and Paragenesis of Na-, Nb- and Zr-Bearing Titanite from Khibina, Russia, and Crystal Structure Data for Synthetic Analogues, The Canadian Mineralogist, 43: 795-812.

Nesse, William D., 2004, Introduction to Optical Mineralogy, 3rd Edition: New York: Oxford University Press, 348 pp.

Tilton, G. R. and M. H. Grunenfelder, 1968, Sphene: Uranium-Lead Ages: Science, 159: 1458-61.

Wenk, Hans-Rudolf and Bulakh, Andrei, 2004, Minerals – Their Constitution and Origin: New York: Cambridge University Press, 646 pp.

 

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