(Fe,Mn)(Nb,Ta)2O6– (Fe,Mn)(Ta,Nb)2O6 Minerals at the columbite end of this coltan series are niobium-rich, and those at the tantalite end are tantalum-rich. Either iron or manganese is nearly always the other major element, in which case the name is prefixed ‘ferro’
or ‘mangano’. Coltan minerals are brown or black, massive, or form tabular or short
prismatic crystals. They occur in granite pegmatites and in
detrital deposits.
THESE ORESof niobium and tantalum are mined from granite pegmatites in South Dakota, USA.
thin tabular crystal
typical submetallic lustre
Pyrochlore
(Ca,Na)2Nb2O6F
Pyrochlore is orange, brownish red, brown, or black. Crystals are octahedral with modified faces, or in granular or massive aggregrates. Pyrochlore forms in carbonatites, pegmatites, and is an accessory mineral in alkaline rocks. It also accumulates in some detrital deposits. As it often contains traces of uranium and thorium, it may be radioactive.
COMPOSITIONOxide.
CRYSTAL SYSTEMOrthorhombic.
CLEAVAGE/FRACTUREDistinct/
Subconchoidal or uneven.
LUSTRE/STREAKSubmetallic to vitreous/Red, brown, or black.
HARDNESS/DENSITY6–6.5 / 5.17–8.0.
KEY PROPERTIES Dense, dark minerals.
two intergrown octahedra
COMPOSITIONOxide.
CRYSTAL SYSTEMCubic.
CLEAVAGE/FRACTUREIndistinct/Uneven, splintery.
LUSTRE/STREAKVitreous to resinous/Yellow or brown.
HARDNESS/DENSITY5–5.5 / 4.45–4.9.
KEY PROPERTIES Shape; slight radioactivity.
SECTION SHOWN
uneven surface
where broken modified
octahedron THIS PYROCHLORE
comes from the Dande-Doma carbonatite in Zimbabwe.
SECTION SHOWN pyrimidal crystals
CARBONATITE from Katete, northwestern Zimbabwe, is a source of monazite.
uneven fracture
TRACEuranium in xenotime is used to date unfossiliferous sedimentary rocks such as the Precambrian rocks in Kimberley, Western Australia.
Xenotime-(Y)
YPO4
The yttrium phosphate xenotime-(Y) is brown, red, yellow, or grey, and crystals are prismatic or equant, often with pyramidal terminations. These may form radiating or rosette-shaped aggregates. Xenotime-(Y), an important ore
of yttrium, is an accessory mineral in granitic and alkaline intrusive
igneous rocks, gneisses, alpine-type hydrothermal veins,
and clastic sediments. It may contain traces of radioactive
uranium or thorium.
Monazite-(Ce)
(Ce,La,Nd,Th)PO4
Cerium-rich monazite-(Ce) is the most common of the four minerals known as monazite. It is brown, pink, or grey.
Crystals are tabular, prismatic, or wedge-shaped, or form granular or massive aggregates. Sources include carbonatites, pegmatites, gneisses, and fissure veins, and monazite also accumulates in rich sand deposits. It is radioactive if thorium is present.
waxy lustre
COMPOSITIONPhosphate.
CRYSTAL SYSTEMTetragonal.
CLEAVAGE/FRACTUREGood/Uneven or splintery.
LUSTRE/STREAKVitreous or resinous/Pale brown or yellow.
HARDNESS/DENSITY4–5 / 4.4–5.1.
KEY PROPERTIES Shape; slight radioactivity.
splintery fracture
COMPOSITIONPhosphate.
CRYSTAL SYSTEMMonoclinic.
CLEAVAGE/FRACTUREDistinct/Conchoidal or uneven.
LUSTRE/STREAKResinous, waxy, vitreous or adamantine/White or pale brown.
HARDNESS/DENSITY5–5.5 / 4.98–5.43.
KEY PROPERTIES Crystal shape; radioactivity.
translucent brown crystal
resinous lustre on broken surface
THIS PEGMATITE
sample from Ytterby, Sweden, contains black gadolinite-(Y) with biotite and pale-coloured feldspar.
Gadolinite-(Y)
Y2Fe2+Be2Si2O10
Gadolinite-(Ce) contains more cerium than yttrium but the opposite applies to the more common gadolinite-(Y). It forms black, greenish black, or dark brown prismatic crystals and massive aggregates, and has thin splinters that are translucent green. Gadolinite-(Y) is mainly found in granites and granitic pegmatites. Traces of uranium and thorium can make it slightly radioactive.
black massive gadolinite-(Y)
rock matrix COMPOSITIONRing silicate.
CRYSTAL SYSTEMMonoclinic.
CLEAVAGE/FRACTURENone/Conchoidal or splintery.
LUSTRE/STREAKVitreous or greasy/
Greenish grey.
HARDNESS/DENSITY6.5–7 / 4.36–4.77 KEY PROPERTIES Translucent green splinters.
COMPOSITIONDisilicate.
CRYSTAL SYSTEMMonoclinic.
CLEAVAGE/FRACTUREImperfect/Conchoidal to uneven.
LUSTRE/STREAKVitreous, resinous, or submetallic/Greyish brown.
HARDNESS/DENSITY5.5–6 / 3.5–4.2.
KEY PROPERTIES Shape, colour, and lustre.
A DARKlayer rich in brown allanite-(Ce) from Namaqualand, South Africa.
Allanite-(Ce)
(Ca,Ce,La)2(Al,Fe2+,Fe3+)3(Si3O12)(OH)
Allanite used to be called orthite; it is now divided into allanite-(Ce), if cerium is dominant, and rarer allanite-(Y), if yttrium is dominant. Both can contain radioactive trace elements. Allanite-(Ce) forms tabular or prismatic brown or black crystals, but also occurs as bladed or massive aggregates or embedded grains. It is found in granites, syenites, pegmatites, and certain metamorphic rocks.
Important localities are found in Norway, Sweden, Finland, and Greenland.
well-formed crystals
rather resinous lustre mass of elongate black crystals
conchoidal fracture SECTION SHOWN
DARKchromite-rich bands in anorthosite, a cumulate deposit at Dwars River, Eastern Bushveld, South Africa.
Chromite
Fe2+Cr2O4
This most important ore of chromium, chromite is opaque black or dark brown and has octahedral crystals, often in granular or massive aggregates. It forms large deposits in layered basic and ultrabasic igneous intrusions, and is preserved when these rocks are metamorphosed to form serpentinites.
Platinum
Pt
This precious metal is opaque, silvery grey, and markedly dense. The cubic crystals are rarely seen, and platinum usually occurs as disseminated grains associated with chromium and copper ores in layered basic and ultrabasic igneous rocks. When these are weathered, platinum accumulates as grains and nuggets in the resulting placer deposits. It often forms an alloy with
iron and so may be magnetic.
PLATINUMis mined at the Merensky Reef deposit of South Africa, one of the world’s most important sources of this metal.
serpentinite matrix COMPOSITIONOxide.
CRYSTAL SYSTEMCubic.
CLEAVAGE/FRACTURENone/Uneven.
LUSTRE/STREAKMetallic or submetallic/Brown.
HARDNESS/DENSITY5.5 / 4.5–4.8.
KEY PROPERTIES Weakly magnetic;
resembles magnetite (p.126).
cube-shaped crystal
platinum nugget
COMPOSITIONElement.
CRYSTAL SYSTEMCubic.
CLEAVAGE/FRACTURENone/Hackly.
LUSTRE/STREAKMetallic lustre/Silvery metallic.
HARDNESS/DENSITY4–4.5 / 14–19.
KEY PROPERTIES Very dense, silvery, malleable metal; lacks secondary alteration.
granular chromite
rounded weathered crystals