on and niobium are replaced by varying amounts of manganese
and tantalum respectively, the general formula being (Fe, Mn) (Nb,
Ta)2O6. It was in this mineral that Charles Hatchett discovered,
in 1801, the element niobium, which he himself called columbium after
the country (Columbia or America) whence came the specimen in the
British Museum collection which he examined. The species has also been
called niobite. It crystallizes in the orthorhombic system, and the
black, opaque crystals are often very brilliant with a sub-metallic
lustre. Twinned crystals are not uncommon, and there is a distinct
cleavage parallel to the face marked _b_ in the figure. Hardness 6;
specific gravity 5.3. With increasing amount of tantalum the specific
gravity increases up to 7.3, and members at this end of the series are
known as tantalite (FeTa2O6). Specimens in which the iron is
largely replaced by manganese are known as manganocolumbite or
manganotantalite, according as they contain more niobium or more
tantalum. Columbite occurs as crystals and compact masses in granite and
pegmatite at Rabenstein in Lower Bavaria, the Ilmen Mountains in the
Urals, Haddam in Connecticut, and several other localities in the United
States; also in the cryolite of Greenland. Tantalite is from Finland,
and it has recently been found in some abundance in the deposits of
cassiterite in the tin-field of Greenbushes in the Blackwood district,
Western Australia.

[Illustration: Crystal of Columbite.]

Dimorphous with columbite and tantalite are the tetragonal minerals
tapiolite (= skogbolite) and mossite, so that the four form an
isodimorphous group with the general formula (Fe, Mn) (Nb, Ta)2O6.
Mossite is from a pegmatite vein near Moss in Norway, and tapiolite is
from Finland. All these minerals contain tin in small amount.
(L. J. S.)

COLUMBIUM, or NIOBIUM (symbol Cb or Nb, atomic weight 94), one of the
metallic elements of the nitrogen group, first detected in 1801 by C.
Hatchett in a specimen of columbite (niobite) from Massachusetts (_Phil.
Trans._ 1802, 49). It is usually found associated with tantalum, the
chief minerals containing these two elements being tantalite, columbite,
fergusonite and yttrotantalite; it is also a constituent of pyrochlor,
euxenite and samarskite. Columbium compounds are usually prepared by
fusing columbite with an excess of acid potassium sulphate, boiling out
the fused mass with much water, and removing tin and tun