Anatase

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Anatase Rutile

Anatase is a metastable mineral form of titanium dioxide (TiO2). In its natural form, the mineral is often found as a black solid, although the pure material is colorless or white. TiO2 also occurs in two other mineral forms: brookite and rutile. Anatase typically forms small, isolated, and sharply developed crystals, and, like rutile, it crystallizes in the tetragonal system.

Anatase is classified as an oxide mineral and has the chemical formula TiO2. It belongs to the tetragonal crystal system and the ditetragonal dipyramidal crystal class, with an H-M symbol of (4/m 2/m 2/m) and a space group of I41/amd. The unit cell dimensions are a = 3.7845, c = 9.5143 [Å], with Z = 4. The formula mass of anatase is 79.88 g/mol. The mineral can be found in a range of colors, including black, reddish to yellowish brown, dark blue, and gray.

Anatase is metastable at all temperatures and pressures, with rutile being the thermodynamically stable polymorph. However, anatase often forms first in many geological processes due to its lower surface energy, transforming to rutile at elevated temperatures. Despite having the same degree of symmetry as rutile, there is little relation between their interfacial angles except in the prism zone of 45° and 90°.

The most common pyramid of anatase, which features perfect cleavage along its faces, has an angle over the polar edge of 82°9′, whereas the corresponding angle in rutile is 56°52½’. This difference results in a steeper pyramid for anatase compared to rutile. Anatase has perfect cleavage on [001] and [011] and a sub-conchoidal fracture. It is a brittle mineral with a Mohs hardness of 5.5–6. The luster of anatase can vary from adamantine to splendent or metallic, and its streak is pale yellowish white.

Crystal Habit

Anatase crystals can be found in two distinct growth habits. The more common type occurs as simple, acute double pyramids with an indigo-blue to black color and a steely luster. These types of crystals are frequently found in Le Bourg-d’Oisans in Dauphiné, France, where they are associated with rock crystal, feldspar, and axinite in crevices within granite and mica-schist formations. Similar anatase crystals, often of microscopic size, can also be found in sedimentary rocks, including sandstones, clays, and slates. The thermodynamically stable {101} plane is the most commonly exposed facet in natural and synthetic anatase crystals.

The second type of anatase crystals features numerous pyramidal faces, often resulting in flatter or prismatic shapes. These crystals are generally honey-yellow to brown in color, closely resembling xenotime in appearance. Historically, such crystals were mistakenly identified as a distinct species named “wiserine.” They are typically found attached to the walls of crevices in the gneiss formations of the Alps, particularly in the Binnenthal near Brig in the canton of Valais, Switzerland. Naturally occurring pseudomorphs of rutile after anatase are also known.

Anatase crystals can exhibit twinning, although it is rare, occurring on the {112} plane. The mineral has a specific gravity ranging from 3.79 to 3.97 and is transparent to nearly opaque. It has uniaxial (−) optical properties and can be anomalously biaxial in deeply colored crystals. The refractive index values are nω = 2.561 and nε = 2.488, with a birefringence of δ = 0.073. Anatase exhibits weak pleochroism.

Anatase Under the Microscope

Watch this detailed analysis of anatase crystals under the microscope:

Anatase Meaning and Healing Properties

The following section is based on cultural beliefs and is not scientifically proven.

Anatase is believed to help individuals view challenges from multiple perspectives, which may lead to finding clear and effective solutions. In alternative healing, the stone is said to aid in treating sinus problems and is considered more beneficial for mental and spiritual well-being rather than physical ailments.

Frequently Asked Questions

Where is anatase mineral found?

Anatase can be found as hard, brilliant crystals of tetragonal symmetry in a range of colors. It is present in veins within igneous and metamorphic rocks and is also commonly found in placer deposits of detritus. Significant vein deposits are found in regions of the Alps, while placer deposits are common in Minas Gerais and Bahia, Brazil.

What is the difference between anatase and rutile?

Rutile is generally deep red, whereas anatase is yellow to blue. Rutile is known for its higher absorbance properties. Both anatase and rutile are used in the production of white pigments for paints, paper, and ceramics due to their unique properties.

Why is anatase a better photocatalyst than rutile?

Anatase tends to exhibit higher photocatalytic activity compared to rutile. In thin films, anatase shows increased activity up to a thickness of 5 nm, whereas rutile reaches its maximum activity at around 2.5 nm. This suggests that in anatase, charge carriers excited deeper within the bulk of the crystal are more capable of contributing to surface reactions compared to rutile.

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