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. |
Color: |
commonly black but can be brown to brownish violet
if translucent |
Luster: |
vitreous to greasy |
Transparency: |
Crystals are commonly opaque but can be found translucent |
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: |
conchoidal |
Hardness: |
5.5 |
Specific Gravity: |
3.0 - 4.2 |
Streak |
black. |
Other Characteristics: |
Twinning may be seen as parallel grooves and allanite
has slight radioactivity |
Associated Minerals: |
quartz, feldspars, biotite, thorite, xenotime, monazite
and epidote |
Local Occurance: |
Saranac Mine, Gutz Farm, MacDonald Mine |
Best Field Indicators: |
crystal habit, color, radioactivity, luster and hardness. |
Description:
Allanite, also known as "orthite" in Europe,
is one of the most common rare earth minerals, which
is somewhat of an oxymoron. Rare 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 minerals: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 them.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 planes. 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 |
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