Brookite
Brookite, TiO2, is the orthorhombic, biaxial titanium dioxide: hardness 5.5 to 6, density 4.08 to 4.18. Its indices work out to a 2V near 12°, inside 0° to 28°.
Brookite is titanium dioxide, TiO2 on the IMA list (G, 1825, United Kingdom): orthorhombic, hardness 5.5 to 6, density 4.08 to 4.18, brown to iron-black with a white to yellowish white streak. It is the biaxial one of the three TiO2 sheets: its indices work out to a 2V near 12°, inside the printed 0° to 28°.
In short
- The species. The Handbook of Mineralogy sheet gives TiO2, orthorhombic, in crystals to 12 cm, typically tabular on {010} and elongated and striated parallel to [001], or pyramidal {111}; cleavage only indistinct on {120} and in traces on {001}; hardness 5.5 to 6; density 4.08 to 4.18; brown to iron-black, streak white to yellowish white. The September 2026 IMA-CNMNC Master List lists it as TiO2, status G, 1825, United Kingdom.
- The 2V check. The sheet prints α 2.5831, β 2.5843, γ 2.7004. β sits only 0.0012 above α, 1.0% of the full birefringence of 0.1173, so the two optic axes nearly close up: worked from the three indices, 2V comes out at 12.0°, inside the measured 0° to 28°. Of the three TiO2 sheets, only brookite’s optical class is biaxial: anatase is uniaxial (−), anomalously biaxial only in deeply colored crystals, and rutile uniaxial (+).
- Documented failure: one number for a moving angle. The sheet says 2V is variable with wavelength, with very strong, crossed axial dispersion. A single set of indices yields a single angle, so the 12.0° is a check that the printed numbers agree, not a figure to measure against. Separately, brookite’s cell gives a density of 4.118 against the sheet’s 4.133, a gap the anatase page documents and this page leaves open.
- The shared-locality ledger. Of the 16 places the sheet names for brookite, 3 also appear on the anatase sheet (Fron Oleu, Bourg d’Oisans, Hardangervidda) and 2 on the rutile sheet (Magnet Cove, and Bahia, where the rutile sheet names Conquista and Ibitiara); none on both. Diamantina lies in Minas Gerais, a state the anatase sheet also names.
- On a balance and a magnet. A 20.00 g piece reads 15.10 to 15.22 g in water, between anatase’s 14.72 to 14.96 g and rutile’s 15.27 g. USGS pulled part of its 2 brookite samples from 0.70 amp but left most behind at 1.70: see is brookite magnetic.
| Name | System (sheet) | Optical class | Indices | Birefringence, worked | 2V | Streak | 20.00 g piece in water | Places shared with brookite |
|---|---|---|---|---|---|---|---|---|
| Brookite | Orthorhombic, Pcab | Biaxial (+) | α 2.5831, β 2.5843, γ 2.7004 | 0.1173 | 0°–28° measured, variable with wavelength; 12.0° worked | White to grayish white or yellowish white | 15.10–15.22 g | — (16 named) |
| Anatase | Tetragonal, I41/amd | Uniaxial (−); anomalously biaxial in deeply colored crystals | ω 2.5612, ε 2.4880 | 0.0732 | None (uniaxial) | White to pale yellow | 14.72–14.96 g | 3: Fron Oleu, Bourg d’Oisans, Hardangervidda |
| Rutile | Tetragonal, P42/mnm | Uniaxial (+) | ω 2.605–2.613, ε 2.899–2.901 | 0.286–0.296 | None (uniaxial) | Pale brown, yellowish brown | 15.27 g | 2: Magnet Cove; Bahia (as a state) |
What is brookite? The orthorhombic titanium dioxide
Brookite is TiO2 in an orthorhombic structure, the third of the three TiO2 minerals on the Handbook sheets. The Handbook of Mineralogy brookite sheet gives point group 2/m 2/m 2/m and space group Pcab, with a 5.4558, b 9.1819, c 5.1429 Å and Z 8. Crystals reach 12 cm and are typically tabular on {010} and elongated and striated parallel to [001]; rarely tabular on {001}; commonly pyramidal {111}, or pseudohexagonal with {120} and {111}. Twinning on {120} is marked uncertain. Cleavage is indistinct on {120} and in traces on {001}; the fracture is subconchoidal to irregular and the mineral brittle. Hardness 5.5 to 6; measured density 4.08 to 4.18, calculated 4.133.
Colour and composition. Brown, yellowish brown, reddish brown, dark brown to iron-black; yellowish brown to dark brown in transmitted light; opaque to translucent, transparent in thin fragments. Streak white to grayish white or yellowish white; lustre metallic adamantine to submetallic. The sheet’s two analyses, from Snowdon, Wales, and Magnet Cove, Arkansas, give 98.59 and 98.77 wt% TiO2 with 1.41 and 1.48 wt% Fe2O3, written (Ti0.99Fe0.01)O2. Worked on standard atomic weights, TiO2 is 59.93% titanium; the two analyses carry 59.09% and 59.20% titanium and 0.99% and 1.04% iron.
What the IMA list records. The September 2026 list gives brookite TiO2, status G, 1825, United Kingdom, first reference Annals of Philosophy 9 (1825), 140. A script finds 5 rows written TiO2: akaogiite, anatase, brookite, rutile and srilankite, as on rutile. The sheet calls brookite trimorphous with anatase and rutile and names it for Henry James Brooke (1771–1857), English crystallographer and mineralogist.
Is brookite biaxial? The 2V check
Yes, and nearly uniaxial: two of its three refractive indices differ by 0.0012. The sheet prints Biaxial (+), α 2.5831, β 2.5843, γ 2.7004, with Z parallel to b. The full birefringence, γ − α, is 0.1173; β − α is 0.0012 of it, 1.0%. When β sits that close to α, the optic axes close up around the acute bisectrix Z.
Worked. For a biaxial crystal, cos²Vz = α²(γ² − β²) / (β²(γ² − α²)). With the sheet’s indices that gives Vz = 6.0°, so 2V = 12.0°; the simpler approximation tan²Vz = (β − α)/(γ − β) gives 11.6°. Both sit inside the measured 0° to 28°. On the other two TiO2 sheets the question does not arise: anatase is uniaxial (−), anomalously biaxial only in deeply colored crystals, and rutile uniaxial (+).
Documented failure: the angle moves with the light. The sheet says 2V is variable with wavelength, and gives the dispersion as r > v, very strong, with crossed axial dispersion. A single set of indices, printed without a wavelength, gives a single angle, so the 12.0° shows only that the sheet’s numbers are consistent with its measured range. The separation of anatase from rutile is on anatase vs rutile; this page does not repeat it.
How heavy is brookite? Between anatase and rutile
Brookite’s measured density, 4.08 to 4.18, lies between anatase’s 3.79 to 3.97 and rutile’s 4.23. At those densities a 20.00 g solid piece reads 15.10 to 15.22 g in water as brookite, 14.72 to 14.96 g as anatase and 15.27 g as rutile, the same figures as the TiO2 ladder on rutile. Brookite is 0.14 g from anatase’s top and 0.05 g from rutile at the nearest. The method is on how to measure specific gravity at home.
Documented failure, shared with the anatase page. Worked from the sheet’s cell, brookite’s density comes out at 4.118, while the sheet prints a calculated 4.133. anatase sets that out with its cell check and leaves it unresolved; so does this page. The measured range, 4.08 to 4.18, contains both figures.
Streak and cleavage. The streak, white to grayish white or yellowish white, is closer to anatase’s white to pale yellow than to rutile’s pale brown. Brookite’s cleavage is indistinct and in traces, where anatase’s is perfect on {001} and {011} and rutile’s good on {110}. How to read a break is on cleavage or fracture.
Where is brookite found? The shared-locality ledger
Where it is found. The sheet calls brookite a widespread mineral, but says only a few localities afford fine crystals: Fron Oleu, near Tremadog, Wales; Bourg d’Oisans, Isère, France; the Maderanthal and Griessertal, Uri, and Salvan, Valais, Switzerland; near Prägraten and Virgen, Tirol, Austria; Hardangervidda, Norway; Atliansk, near Miass, Ilmen Mountains, and large crystals in the Dodo mine, Subpolar Urals, Russia; Monte Bregaceto, near Genoa, Italy; the diamond placers of Bahia and Diamantina, Minas Gerais, Brazil; and Magnet Cove, Arkansas, Ellensville, New York, and near Powderhorn, Colorado.
The ledger. Method: the 16 named places were matched by script against the Distribution fields of the anatase and rutile sheets. 3 appear on the anatase sheet, Fron Oleu, Bourg d’Oisans and Hardangervidda; 2 on the rutile sheet, Magnet Cove and Bahia, the second only as a state, since the rutile sheet names Conquista and Ibitiara there and brookite’s names diamond placers; none on both. Diamantina is in Minas Gerais, which the anatase sheet names for carbonatites. A shared name says the sheets list both minerals there, not that they grow on one specimen.
How it forms. An accessory mineral in alpine veins in gneiss and schist, in contact metamorphic zones and hydrothermal veinlets, and a common detrital mineral. The Association line names anatase, rutile, titanite, orthoclase, quartz, hematite, calcite, chlorite and muscovite. The sheet describes no inclusions, and this page makes no claim about brookite in quartz beyond that association. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.