Description:
Anatase is a polymorph with two other minerals.
The minerals rutile and brookite as well as anatase
all have the same chemistry, TiO2, but they have different
structures. At higher temperatures, about 915 degrees
Celsius, anatase will automatically revert to the
rutile structure. Rutile is the more common and the
more well known mineral of the three, while anatase
is the rarest. Anatase shares many of the same or
nearly the same properties as rutile such as luster,
hardness and density. However due to structural differences
anatase and rutile differ slightly in crystal habit
and more distinctly in cleavage.
Anatase and rutile have the same symmetry, tetragonal
4/m 2/m 2/m, despite having different structures.
In Rutile, the structure is based on octahedrons of
titanium oxide which share two edges of the octahedron
with other octahedrons and form chains. It is the
chains themselves which are arranged into a four-fold
symmetry. In anatase, the octahedrons share four edges
hence the four fold axis. Crystals of anatase are
very distinctive and are not easily confused with
any other mineral. They form the eight faced tetragonal
dipyramids that come to sharp elongated points. The
elongation is pronounced enough to distinguish this
crystal form from octahedral crystals, but there is
a similarity. In fact anatase is wrongly called "octahedrite"
in spite of the difference in forms. Of course "tetragonal
dipyramidite" does not sound right either! Nice
specimens of anatase are associated with quartz and
are considered classics in the mineral world. The
good luster, well formed crystal shape and interesting
character make anatase a popular mineral for collectors. |
