Rutile ((Anatase, Brookite, Riesite, Akaogiite) is one of the five forms of titanium dioxide found in nature. Rarer polymorphs include Brookite and Anatase, both which also form unique and distinctive crystals. Akaogiite and Riesite are the other forms of titanium dioxide but are rarely found in nature. Rutile is TiO2 but may contain up to 10% iron, occasionally with some minor niobium or tantalum.
Rutile comes in a surprising contrast of distinct habits and colors, making it a very interesting mineral. It has multiple unique crystal forms as well as several distinctive colors, styles, and associations. Rutile can range from mirror-like metallic-looking crystals, to dark reddish sub-metallic crystals, to bright golden-yellow needles. Even the opaque metallic-looking forms are somewhat translucent on edge under backlighting, with a dark red translucent tinge.
Rutile is well known for its habit of forming needle-like inclusions within other minerals, especially Quartz, in the form of long and slender yellow straw-like crystals. These inclusions can range from scattered needles to dense parallel fibers within a host mineral. This combination is known as Rutilated Quartz and is used both as a collector’s mineral and gemstone.
Rutile inclusions are also responsible for the asterism or chatoyancy effects on some gemstones, such as Star Sapphire and Star Ruby. The thin, parallel Rutile fibers that formed within the host mineral provide these unique optical effects. In other gems, one direction of parallel crystals will form a line of light on the surface of the gem known as a “cat’s-eye”. The phenomenon that produces a cat’s-eye is known as “chatoyance”. The best-known gem for its chatoyance is cat’s-eye chrysoberyl.
FORMS, HABITS, AND SYSTEM OF RUTILE
Rutile belongs to the Tetragonal Crystal System. Hardness is 6 – 6½ on Mohs scale. Color is dark-red, metallic-gray, brownish-red, orange-red, reddish-black, golden-yellow, straw yellow and black if high in Nb or Ta. Luster is adamantine to metallic.
Crystal Habit: Rutile forms as long and slender, straight prismatic crystals, often deeply striated and with steep complex terminations. Also, in short prismatic and thick stubby crystals. Twinning is very common, with various forms including sixlings, eightlings (both in the form of endemic rutile twins), knee-shaped twins, and v-shaped twins.
Rutile often forms in capillary needles and dense reticulated forms, in acicular habit, in delicate snowflake-like aggregates, and in star-shaped formations of dense needle groupings. Thin acicular crystals and needles are commonly frozen within other minerals, especially Quartz. May also be grainy, massive, in veins, and in rounded water worn pebbles.
WHERE RUTILE IS FOUND
Rutile is found in several different environments, including plutonic and intrusive igneous rocks and granites, metamorphic gneiss and schists, carbonatites, regional metamorphic schists, and hydrothermal replacement deposits (including veins in alpine cavities). Also, in detrital river and beach deposits.
Noteworthy Localities: Rutile is found in a host of different mineral deposits in hundreds of localities worldwide, but its occurrence is usually limited to small and isolated crystals. The following deposits are famous for producing excellent crystals.
Switzerland produces classic specimens in many of the mountainous alpine environments. These localities include the Binn Valley, Wallis (especially at the Lengenbach Quarry and at Lerchentini); the St. Gotthard pass, Ticino; and the Cavradi gorge, Curnera Valley, Grisons. Another important alpine European occurrence is the Vizze Valley, Bolzano, Italy.
The varieties Struverite and Ilmenorutile are found in large crystals in the Madagascar pegmatites at Antsirabe and Betafo, Antananarivo Province. Lustrous Rutile crystals come from Ribaue, in the Alto Ligonha pegmatite, Mozambique.
Brazil has several outstanding Rutile occurrences, including those that contain the most prolific Rutilated Quartz sources. The most exceptional of the Brazilian localities is Novo Horizonte (formerly Ibitiara), Bahia, which produces striking golden-yellow acicular and star-shaped formations associated with mirror-like Hematite crystals. Freestanding crystals and v-shaped twins occur in many of the Brazilian gem pegmatites of Minas Gerais; especially noteworthy is Diamantina, in the Jequitinhonha valley.
The United States has several noteworthy sources of Rutile. The most famous is Graves Mountain, Lincoln County, Georgia, where stubby, lustrous, mirror-like crystals were once found. Beautiful dark red and etched prismatic crystals come from Hiddenite and Stony Point, Alexander County, North Carolina; and interesting sixlings and other twinned forms were well-known from the soils of Parkesburg, Chester County, Pennsylvania, especially as floater crystals. Good Rutile crystals come from Magnet Cove, Garland County, Arkansas, including Rutile pseudo morphs after Brookite. In California, two localities that have produced exceptionally lustrous stubby crystals are the Champion Mine, White Mountains, Mono County; and Darwin, Inyo County.
Anatase is always found as small, isolated, and sharply developed crystals, and like rutile, it crystallizes in a tetragonal system. Anatase is often the first titanium dioxide phase to form in many mineral forming processes due to its lower surface energy, then will transform (pseudo morph) to Rutile at elevated temperatures.
Two crystal habits of Anatase are common. The more common occurs as simple acute octahedra with an indigo-blue to black color and steely luster. Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit. Their color is honey-yellow to brown.
Brookite is the titanium dioxide mineral that belongs to the Orthorhombic Crystal System. Brookite crystals are typically tabular, elongated and striated parallel to their length. They may also be pyramidal, pseudo-hexagonal or prismatic. Brookite is usually brown in color, sometimes yellowish or reddish brown, or even black. Beautiful, deep red crystals are also known.
By Bill Jones, Sidewinder Minerals