Niobium (Nb) is not a chemical element that effortlessly
slips off your tongue when asked to quickly name 25 elements. However, it is
the 34th most common element in the earth’s crust. It
is less abundant than zinc, nickel, and copper but more abundant than cobalt or
molybdenum. That lack of familiarity is probably due to the fact that free
niobium is not found in nature; it is always combined with other elements in
minerals. Niobium is closely related to the element tantalum and the two are
often found together in minerals. In fact, at one time in the early 1800s
chemists believed columbium (former name of niobium) and tantalum were the same
element. That is one reason that columbite [(Fe,Mn)Nb2O6)]
and tantalite [(Fe,Mn)Ta2O6)]
are often confused in the field with each other and the moniker
columbite-tantalite (or coltan) is attached to these unknowns. I certainly know
that in the pegmatites of the Black Hills of South Dakota most rockhounds refer
to these dark minerals as columbite-tantalite since it is virtually impossible to determine if
tantalum or niobium is the dominant cation.
MinDat lists 115 minerals that contain essential
niobium, the majority of which are oxides and silicates: euxenite (Y-REM), fergusonite (Y-REM),
columbite, pyrochlore group, lueshite, latrappite, and limonorutile.
Commercially most niobium is extracted from ores of columbite-tantalite,
pyrochlore group, and euxenite with the largest resources located in Brazil and
Canada. The primary niobium ores are oxides of the pyrochlore group and located
in carbonatites (magmatic rocks containing more than 50% carbonate minerals)
and alkaline-syenite complexes. Carbonatites are usually associated with
crustal tectonic rift zones. (Virginia Department of Energy: Virginia Division
of Mineral Resources, Publication 115.)
A couple of years ago in Tucson, while rummaging
around in a dusty flat of minerals, I came across a specimen of latrappite. The
label was “older” and stated “compare to perovskite, same family, these are
better. From the Ray Collection.” MinDat (on my phone) informed me I was
looking at a mineral containing niobium. Older labels always attract my
attention, and often my money, and a
couple of days before I had picked up a specimen of perovskite with a
Mineralogical Research Company (CA) label. Now I was ready to examine their
relationship—it only took me two years.
Perovskite is a calcium titanium oxide [CaTiO3]
belonging to the Orthorhombic Crystal System; however, it is pseudo cubic and crystals
generally have a cubic habit. These small cubes come in a variety of colors
from black to brown to shades of yellow, orange and red. My cubes have an
adamantine to sub-adamantine luster, are fairly hard at ~5.5 (Mohs) and seem
partially translucent (ranges from transparent to opaque). At times some cubes have a metallic luster
and look similar to galena. Niobium is a common impurity in perovskite but not
an essential mineral. Anthony and others (2001-2005) noted perovskite forms as
an accessory mineral in alkaline mafic rocks, as nepheline syenites,
kimberlites, carbonatites, and can additionally form in calcium-rich skarns
(such as Magnet Cove, Arkansas), and is a common accessory mineral in calcium
and aluminum rich inclusions within carbonaceous chondrites.
In contrast to perovskite, latrappite is a mineral
where niobium is an essential element—Ca2NbFeO6. So,
what is the relationship between latrappite and perovskite? One simple
explanation is that the crystals of both minerals look similar to each
other—crystals are Orthorhombic but pseudo cubic and have a cubic outline,
cleavage is absent, the colors of the cubes are usually black to dark brown, a
hardness of ~5.5 (Mohs), a metallic luster, and generally are collected form
carbonatite complexes. A more “scientific” explanation is that the niobium
exceeds the titanium in latrappite type minerals. In fact, at one time
latrappite was known as niobium perovskite. Due to the analysis by Mitchell and
others (2017) the IMA redefined niobium perovskite as latrappite (niobium
greater than titanium) from a type locality (Carbonatite Complex),
Deux-Montagnes RCM, Laurentides, Quebec, Canada. But it also remains as a
mineral of the Perovskite Supergroup.
REFERENCES
CITED
Anthony, J.W. R.A Bichard, A. Bideaux, K.
W. Bladh, and M. C. Nichols, Eds., 2001-2005, Handbook of Mineralogy,
Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.
Mitchell, R., M.D. Welch, and A.R. Chakhmouradian, 2017,
Nomenclature of the perovskite supergroup: A hierarchical system of
classification based on crystal structure and composition. Mineralogical
Magazine: 81(3): 411-461. https://doi.org/10.1180/minmag.2016.080.156