Rutile

Rutile was first described in 1803 by the German geologist Abraham Gottlob Werner, who named the mineral after its frequent reddish color, derived from the Latin word rutilus, meaning “red” or “reddish.”

Until 1795, when its chemical composition was determined, rutile was mistakenly believed to belong to the tourmaline group.

In the now outdated 8th edition (1982) of the Strunz Mineral Classification, rutile belonged to the mineral class of “Oxides and Hydroxides” and specifically to the division of oxides with the general formula “MO₂ and related compounds.” Within this system, it served as the namesake of the Rutile series with the system number IV/D.02, which also included cassiterite, plattnerite, and varlamoffite as members.

In the most recent Lapis Mineral Directory (updated in 2018) by Stefan Weiß, which continues to follow the traditional Strunz classification for the sake of private and institutional collections, rutile received the system and mineral number IV/D.02-10. In the Lapis system, this corresponds to the division “Oxides with a metal-to-oxygen ratio of 1:2 (MO₂ and related compounds).”

The 9th edition of the Strunz Mineral Classification, valid since 2001 and updated by the International Mineralogical Association (IMA) until 2009, also places rutile in the division “Oxides with a metal-to-oxygen ratio of 1:2 and related compounds.” However, this edition further subdivides minerals according to the size of the involved cations and the crystal structure. Based on its composition and crystal structure, rutile is assigned to the subgroup “With medium-sized cations; chains of edge-sharing octahedra.” Within this group, rutile again serves as the namesake of the Rutile group with the system number 4.DB.05, which includes the minerals argutite, cassiterite, plattnerite, pyrolusite, tripuhyite, tugarinovite, and varlamoffite.

In the Dana classification system, rutile is likewise assigned to the class “Oxides and Hydroxides,” though it is first placed within the division of “Oxides.” Here, it is found together with ilmenorutile, struverite, pyrolusite, cassiterite, plattnerite, argutite, squawcreekite, and stishovite in the Rutile group (tetragonal: P4₂/mnm) with the system number 04.04.01, within the subdivision “Simple oxides with a cation charge of 4+ (AO₂).”

The rutile structure is a common structural type for AB₂ compounds and, in contrast to the fluorite structure, is not based on a close-packed arrangement. The oxide anions are arranged in distorted and wavy “hexagonal” layers, with half of the octahedral interstices between them occupied by titanium cations. However, due to the tetragonal symmetry, these wavy layers do not form a true close packing.

The crystal structure is therefore better described as a tetragonal rod packing of chains of edge-sharing [TiO₆] octahedra (according to Niggli notation: [TiO₄/₂O₂/₁]) running parallel to the crystallographic c-axis. These chains are further connected via shared corners to form a three-dimensional [TiO₆/₃] network, which simplifies to the overall chemical formula TiO₂.

Each titanium cation is octahedrally coordinated by six oxygen atoms (coordination number 6), while each oxide anion is surrounded by three titanium atoms in a slightly distorted trigonal planar arrangement (coordination number 3).

A number of other inorganic compounds also crystallize in the rutile structure, including the oxides NbO₂, TaO₂, MnO₂, and SnO₂, as well as the fluorides CrF₂, MnF₂, FeF₂, CoF₂, NiF₂, CuF₂, and ZnF₂.

Morphology

Rutile commonly forms prismatic crystals with a thick-columnar to fine-needle-like habit, often dominated by the crystal forms {110} and {010}. The crystal faces are typically elongated and striated parallel to the prism axis. Among many other forms, ditetragonal prisms may also occur.

In fine needle-like or fibrous inclusions, rutile is responsible for the optical phenomenon of asterism (the “star effect”) observed in gemstones such as sapphires and rubies. In microscopic inclusions, together with hematite and other minerals, rutile can accentuate “phantom crystals” often seen in quartz.

Twinning is common in rutile and can occur according to two distinct laws. Twins, triplets, and polysynthetic multiples, often lamellar or cyclic, are most frequently formed on (101). The individuals meet at an angle of 65°35′, producing characteristic knee-shaped, visor-shaped, or V-shaped twin forms and even sixlings that form closed rings. Less common are heart-shaped twins on (301), where the vertical axes meet at an angle of 54°44′. Both twinning laws can occur simultaneously, forming lattice-like or net-like aggregates known as sagenite.

Color

Rutile can occur in various colors, but is most commonly found in reddish-brown to deep red and black. As inclusions in other minerals, such as quartz, rutile also shines in a rich golden yellow color and, in this form, is known as Venus hair and is often used in jewelry. Blue or violet hues, on the other hand, are rare.

Chemical and Physical Properties

Rutile is insoluble in acid and cannot be melted with a blowpipe. In its pure form, it is weakly paramagnetic with a specific magnetic susceptibility (mass susceptibility) of 7.7·10−7 emu/Oe·mg, but if it also contains iron, it becomes antiferromagnetic[4].
Modifications and varieties.

Rutile is the most important and only modification of titanium dioxide that is stable at high temperatures. The other two are anatase and brookite.

Nigrin is the name given to an iron-bearing, black rutile.

Formation Conditions

Rutile forms as a high-temperature and high-pressure mineral, occurring both magmatically and metamorphically. It is a common accessory mineral in many types of rocks and is also found as a placer mineral in river sediments. Accordingly, rutile is often associated with a wide range of other minerals, including the other TiO₂ polymorphs anatase and brookite, as well as adularia, albite, apatite, calcite, chlorite, ilmenite, pyrophyllite, titanite, and quartz. Rutile also forms epitaxial intergrowths with hematite.

It occurs in eclogites and is the dominant titanium-bearing phase in garnet amphibolites.

Localities

As a common mineral species, rutile has been found in many locations worldwide, with around 5,900 occurrences documented as of 2019.

Notable for their exceptional specimens are the Graves Mountain Mine in Lincoln County, Georgia (USA), where crystals up to 15 cm in size have been found; the Cavradi Gorge near Sedrun in the Swiss canton of Graubünden; and the municipality of Ibitiara in Bahia, Brazil, both known for their striking rutile–hematite epitaxial intergrowths. In Ibitiara and in Itabira (Minas Gerais, Brazil), rutile inclusions in smoky quartz are also common. Large knee- or visor-shaped crystal twins up to about 7 cm have been found near Golčův Jeníkov and Soběslav in the Czech Republic. Crystals up to 3 cm in diameter and 5 cm in length were discovered in the Paragachay deposit on Mount Kapudschuk in the Nakhichevan Autonomous Republic, Azerbaijan.

In Germany, rutile has been identified in several regions, including the Black Forest (Baden-Württemberg), the Fichtelgebirge, Spessart, Bavarian Forest, and Upper Palatinate Forest (Bavaria), as well as in Hesse, Lower Saxony, the Siebengebirge (North Rhine-Westphalia), the Eifel (Rhineland-Palatinate), Saarland, the Ore Mountains (Saxony), Schleswig-Holstein, and Thuringia.

In Austria, the mineral has been found in Burgenland, in alpine clefts in many regions of Carinthia, Salzburg, and Styria, in parts of Lower Austria and Tyrol, as well as in Upper Austria and Vorarlberg.

In Switzerland, rutile occurs mainly in alpine clefts in the cantons of Graubünden, Ticino, and Valais.

Rutile has also been identified in rock samples from the Mid-Atlantic Ridge and the Southwest Indian Ridge, as well as on the Moon, specifically in the Fra Mauro Highlands.

Largest Producing Countries

Worldwide, the mining reserves for the most important titanium minerals, ilmenite and rutile, are estimated at 692.58 million tons, with the largest regional concentrations found in China (28.9%), Australia (17.0%), and India (13.3%) (as of November 2014).

Rutile, with a titanium content of about 60%, is the most important titanium mineral after ilmenite.

In its TiO₂ modification, rutile is used as a white pigment due to its high refractive index. It is also used, either alone or mixed with cellulose, as a coating material for electrodes in arc welding, where it improves or even enables the welding process.

Because of its semiconducting properties, rutile is employed in dye-sensitized solar cells (Grätzel cells). With a band gap of approximately 3.0 eV, it can absorb light with a wavelength shorter than about 400 nm.

As a Gemstone

Natural rutile is only occasionally cut and polished as a gemstone by collectors, as its crystals are typically too small. Synthetic rutile, however, has been produced since 1948 and marketed under the trade names Titania or Diamonite (not to be confused with Diamondite) as a diamond imitation. It even surpasses the brilliance of diamond, owing to its dispersion that is about six times higher.

Rutile needles included within other minerals, particularly quartz, are also popular in jewelry. These inclusions not only produce a golden luster, but also give rise to various optical phenomena, such as asterism (a star-like light reflection) and chatoyancy (the cat’s-eye effect).