THE MINERAL ALLANITE
- Chemistry: (Ca, Ce, La, Y)2(Al, Fe)3(SiO4)3(OH),
Calcium Cerium Lanthanum Yttrium Aluminum Iron Silicate Hydroxide.
- Class: Silicates
- Subclass: Sorosilicates
- Group: Epidote
- Uses: As a source of rare earth metals and mineral specimens.
Allanite, also known as "orthite" in Europe, is one of the
most common rare earth minerals, which is somewhat of an oxymoron.
earth elements include many unusual and valuable metals. Up to
20% of allanite's weight could be composed of these rare earth elements,
making allanite a potentially valuable ore. Because of these rare earth
metals, especially thorium, which are frequently radioactive, allanite
is frequently slightly radioactive to no one's great surprise.
The radioactivity, as in other radioactive minerals, can manifest itself
in a couple of ways. Embedded crystals of allanite are frequently seen
with a "halo" or dark ring; evidence of the radioactive effects
on nearby minerals. Allanite can also become metamict. This is a condition
found in radioactive minerals and results from the destructive effects
of its own radiation on its crystal lattice. The effect can destroy a crystal
lattice completely while leaving the outward appearance unchanged. The
complete destruction of the allanite structure will produce a glassy hydrated
substance. The hydration is facilitated by the metamictation.
If just most of the elements that can be found in allanite were shown,
then the formula would be written as (Ca, Ce, Y, La, Th, Na, K)2(Al,
Fe, Be, Mn, Mg)3(SiO4)3(OH),
or Calcium Cerium Yttrium lanthanum Thorium Sodium Potassium Aluminum
Iron Beryllium Manganese Magnesium Silicate Hydroxide. A mineral like
allanite is sometimes referred to as a "trash can mineral" because
it can possess elements that other minerals just seem to not want (i.e.
the "trash") or is it because it can attract every stray element
like a trash can attracts stray cats. whatever the case, the shorter formula
version is used here and is more indicative of its general chemistry but
does not reflect the broad chemical possibilities that allanite can produce.
Allanite in the strictest sense is broken into three officially recognized
Allanite-(Ce) the cerium rich allanite, also the most common
and in general the one most often referred to as just allanite or orthite.
Allanite-(La) the lanthanum rich allanite.
Allanite-(Y) the yttrium rich allanite.
These minerals share the same structures and vary slightly in only a
few of their properties. Chemical test would be required to differentiate
Allanite, like other Epidote
Group minerals, has some structural complexity in that it has both
single silicate tetrahedrons, SiO4, and double silicate
tetrahedrons, Si2O7. The formula
of allanite could be expressed in a such a way so as to reflect this organization;
(Ca, Ce, La, Y)2(Al, Fe)Al2O(SiO4)(Si2O7)(OH).
The Al2O group represents the parallel chains
of AlO6 and AlO4(OH)2
octahedra that are the heart of the Epidote Group structure. The silicate
groups and extra ions connect the chains together. Since the chains are
parallel, the crystals tend to be prismatic. The chains are arranged in
parallel planes and allanite's cleavage breaks the bonds between these
Allanite is found as an accessory mineral in several igneous rocks such
as granites, syenites, diorites and their pegmatites and in a few metamorphic
rocks as small embedded grains. It forms tabular crystals that can have
excellent form and character. Remember, this is a slightly radioactive
mineral and should be stored away from other minerals that are subject
to damage from radioactivity and of course human exposure should
be limited !
Color is commonly black but can be brown to brownish violet
Luster is vitreous to greasy.
Transparency: Crystals are commonly opaque but can be found
Crystal System: Monoclinic; 2/m
Crystal Habits include long, somewhat prismatic or tabular crystals
with a typically dominant pinacoid that the crystal is often flattened
against. The terminations are usually wedge shaped, more rarely tapered
pyramids. Also massive or as is most commonly the case, as embedded grains.
Cleavage poor in one direction, lengthwise.
Fracture is conchoidal.
Hardness is 5.5
Specific Gravity is 3.0 - 4.2
Streak is black.
Other Characteristics: Twinning
may be seen as parallel grooves and allanite has slight radioactivity.
Associated Minerals include quartz,
Notable Occurrences are widespread and include the Ural Mountains
of Russia; Falun, Ytterby and Sheppsholm, Sweden; Trimouns, France; Madagascar;
the Eifel District of Germany; Spain; Otter Lake, Quebec and Madawaska,
Ontario, Canada and Amelia Court House, Virginia; Barringer Hill, Texas;
California; Franklin, New Jersey; Edenville and even New York, New York
and New Mexico, USA.
Best Field Indicators are crystal habit, color, radioactivity,
luster and hardness.