LATRAPPITE: A RARE NIOBIUM MINERAL

 

Niobium (Nb) is not a chemical element that effortlessly slips off your tongue when asked to quickly name 25 elements. However, it is the 34^th 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.

Above photomicrographs: cubes of perovskite ~.6 mm along margins. Collected Perovskite Peak, San Benito County, California.

Photomicrograph, cube of perovskite ~ 2mm on edge. Some overgrowth of calcite. Collected from Malenco Valley, Valtellina, Sondrio Province, Lombardy, Italy.

In contrast to perovskite, latrappite is a mineral where niobium is an essential element—Ca₂NbFeO_6. 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. 

A cube of latrappite ~2.4 mm on edge. Collected from the Type Locality, Bond Zone, Oka, Deux-Montagnes RCM, Laurentides, Quebec, Canada.
 

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