Titanite (CaTiSiO5) is a common mineral in calc-silicates, metamorphosed igneous rocks, and calc-alkaline plutons. The mineral was first named by Martin Klaproth in 1795 for its high content of the element titanium, which had been discovered only a few years prior, and named by Klaproth for the Titans of Greek mythology. The alternate name sphene was proposed by Rene Haüy in 1801 for the mineral’s characteristic wedge-shape (sphenos in Greek means “wedgeâ€), but in 1982 the IMA recommended that the name titanite be used in technical writing. The name sphene is still used in the gem industry, and retains a loyal following among some mineralogists. Titanite’s unusual crystal structure—including a 7-fold decahedral site—preferentially takes up numerous geochemically interesting elements, especially U, which enhances its geochronologic utility, but also other high field-strength elements like Zr, and the rare-earth elements (REE). It is one of a handful of major Ti-bearing phases that occur in almost every rock either as a silicate (titanite), as a pure Ti-oxide (rutile or anatase) or as a Fe–Ti oxide (ilmenite or magnetite). Although usually present as a minor or accessory mineral, titanite differs from many other accessory minerals in that its main chemical constituents participate in reactions with other major minerals. Significant substitution of Al and OH enhances this reactivity. Thus, although the stability and reactivity of accessory minerals such as monazite, zircon, etc. are also tied to major mineral reactions (Pyle and Spear 1999; Ferry 2000; Wing et al. 2003; Kohn and Malloy 2004; Tomkins and Pattison 2007; Spear 2010; Kohn et al. 2015, etc.), titanite’s connection is much more direct and forms the basis of quantitative thermometry and barometry. More generally, titanite has served a key role in understanding igneous, metamorphic and ore-forming processes (Kerrich and Cassidy 1994; Frost et al. 2000) and is even used to constrain the depositional ages of sedimentary rocks through ...