Description:
Hydroxylbastnasite is closely related to its near namesake
bastnasite. Hydroxylbastnasite has in its formula and
structure hydroxyl ions, OH, that bastnasite lacks,
hence the name. The hydroxyl ion groups take the place
of most of the fluorines that are found in bastnasite.
X-ray studies confirm that the hydroxyl ion groups dramatically
increase the unit cells of hydroxylbastnasite structure
from that of bastnasite's structure. Crystallographically
and mineralogically this is significant. Significant
enough to proclaim Hydroxylbastnasite as a unique mineral.
A new proposed mineral has recently been named from
specimens that contain all hydroxides and no or very
few fluorines. The proposed mineral has a formula of
(Ce, La, Nd)CO3OH and is call hydroxylcarbonate-(Nd).
This mineral comes from the nickel rich bauxite deposits
at Nisi, Greece. The series is then complete with bastnasite
(having near 100% fluorines) and hydroxylcarbonate (with
near 100% hydroxides) as the two end members and hydroxylbastnasite
as the intermediate member. The structure of the three
minerals is made up of stacks of carbonate ion layers
and Ce(OH, F) layers. The Ce(OH, F) layers form flat
hexagonal sheets with each cerium bonded to three fluorines
and visa-versa. The carbonate layers are more complex
with angled carbonate triangular groups. The structure
is closely studied because it is one of the few rare
earth mineral structures that can accommodate variously
sized cations. Hydroxylbastnasite (also spelled hydroxyl-bastnasite)
has cerium, lanthanum and neodymium in its generalized
formula but officially the mineral is divided into three
minerals based on the predominant rare earth element
found in specimens. There is Hydroxylbastnasite-(Ce)
with a more accurate formula of (Ce, La)CO3(OH, F).
There is also Hydroxylbastnasite-(La) with a formula
of LaCO3(OH, F). And finally there is Hydroxylbastnasite-(Nd)
with a formula of (Nd, Ce, La)CO3(OH, F). There is little
difference in the three in terms of physical properties
and most hydroxylbastnasite is hydroxylbastnasite-(Ce).
Cerium in most natural hydroxylbastnasites usually far
and away dominates the others. Hydroxylbastnasite is
an ore of cerium, an important industrial metal, but
is far behind bastnasite and the phosphate mineral monazite
as a source of the rare earth element. Hydroxylbastnasite
is an important ore in the unusual karst, rare earth/nickel-rich/bauxite
deposits in Montenegro, Greece and other deposits in
the Balkans. These deposits are depleted in fluorine
and this the primary reason that hydroxylbastnasite
and not bastnasite forms. Hydroxylbastnasite is also
found to a lesser extent as a primary mineral in carbonate
rich igneous rocks called carbonatites. |