|
Although thorite is the most common thorium mineral, it is not the primary ore of thorium. The
phosphate mineral monazite has an average of about 6% thorium in its composition that includes several
rare earth elements. Enough monazite is mined to supply most of the current thorite demand. If monazite deposits
become scarce or the demand for thorium increases, then thorite and to lesser extents other thorium bearing
minerals such as thorianite, thorogummite , huttonite and thorutite will gain greatly in importance.
Thorite is currently an important ore of uranium. A variety of thorite often called "uranothorite" is particularly
rich in uranium and has been a viable uranium ore at Bancroft, Ontario, Canada. Other varieties of thorite include
"orangite", an orange variety; "calciothorite" an impure variety with trace amounts of calcium and "freyalite" actually
a discredited variety containing cerium (it turned out to be an altered form of the mineral melanocerite).
There is a closely related mineral to thorite called thorogummite that was once considered a variety
of thorite but is a product of alteration to thorite. Thorogummite has some of the SiO4 tetrahedrons
replaced by four hydroxides for a formula that looks like:
(Th, U)2(SiO4)(2 - X)(OH)4X
The X represents the conversion of one silicate tetrahedron with a negative four charge (-4) to four
hydroxides with a negative one charge (-1) each. So that if half the silicate tetrahedrons are replaced
by the four hydroxides, where X = 1, the formula would look like this:
(Th, U)2SiO4(OH)4
Thorite also is related to a mineral called huttonite. Thorite and huttonite are dimorphs. The two minerals have the
same chemistry, they just have different structures, di means two and morph means shape. A similar situation occurs
with the dimorphism of diamond and graphite, both of whom are composed of carbon, but have very different
structures. Huttonite ironically belongs in the Monazite Group and is related structurally to its members.
Because thorite is highly radioactive, specimens are often 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. Thorogummite is believed to be formed from thorite by hydration which is facilitated by metamictation.
Specimens of thorite generally come from igneous pegmatites and volcanic extrusive rocks, hydrothermal
veins and contact metamorphic rocks as well as small grains found in detrital sands. Crystals are rare, but
when found can produce nicely shaped short prismatic crystals with pyramidal terminations. Remember, this
is a 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 !
Physical Characteristics:
Color is normally black, but also brownish black, orange, yellowish-orange and dark green.
Luster is resinous.
Transparency: Crystals are opaque.
Crystal System is tetragonal; 4/m 2/m 2/m
Crystal Habits include short prismatic crystals with a square cross-section and simple pyramidal terminations. Also massive, embedded irregular grains and reniform.
Cleavage is poor, in two directions lengthwise, but is rarely seen.
Fracture is conchoidal.
Hardness is 4.5 - 5
Specific Gravity is 4.1 - near 7.
Streak is orange to brown.
Other Characteristics: Nearly always metamict and always strongly radioactive.
Associated Minerals include quartz, feldspars, biotite and betafite.
Notable Occurrences are found at Langesundfjord, Norway; Bancroft, Ontario, Canada; Eifel District, Germany and Arizona, USA.
Best Field Indicators are color, luster, crystal habit, fracture and especially radioactivity.
| 
)
