Kongsberg mineralsymposium 1997
Norsk Bergverksmuseum Skrift, 12, 14-15(Nb,Fe)-substituted anatase from
Herrebøkasa, Østfold, NorwayGunnar Raade and Roy Kristiansen

Abstract
Fine-grained yellow anatase, forming pseudomorphs
after euxenite or polycrase, has a larger unit cell thannormal anatase and a composition close to(Tio.flb 0.2SFe 0.2J El.000 2'Sammendrag
Finkornet gul anatas som danner pseudomorfoser
etter euxenitt eller polykras, har en større celle ennvanlig anatas og en sammensetning nær(Tio.flb 0.2SFeO.25) El,(XP 2'Introductlon
A fine-grained yellow mineral- formed by the alterationof a metamict mineral (euxenite or polycrase) from theHerrebøkasa granite pegmatite in Østfold - gives an
X-ray powder pattern of anatase, but with distinctly
smaller spacing (Iarger unit cell). Since we have onlya 9 cm Debye-Scherrer film without any internal standard, we see no point in calculating the cell parameters. The aim of the present article is to report anddiscuss the results of a chemical analysis of thissubstance.
The material
The investigated specimen was collected by RK in1970. It measures 2x4.5 cm, consists mainly ofmicrocline and quartz and has a 1.8 cm long monazite
crystal perched on it. A light brown glassy mineral gavean X-ray powder pattern of zircon after heating to1OOO°Cfor 24 hrs. The dull yellow substance (anatase)forms pseudomorphs after prismatic crystals up to 0.5
cm long. Backscatter photomicrographs c1early show
the replacement of the original mineral by anatase (Fig.
1). An energy-dispersive spectrum shows Y, Nb, li, U,and Th to be the main constituents of the precursor.
From this composition and the crystal shape, the original mineral could have been euxenite or polycrase.
Nilssen (1970) described samarskite and a yellow
mineral surrounding it and fiIIing cracks in the mineralfrom Herrebøkasa (calledAspedammen in herpaper).
The li02 content of this samarskite is only 4.04 wt.%,
and it can hardly be regarded as the primary mineralof the pseudomorphs we describe here. The yellow
mineral investigatedby Nilssen (1970) has acompos~ion similarto that of its associated samarsk~e,being formed by leaching of Ca and U, some oxidation
and a further uptake of water. It is completely rnetamict
but gave a distinct powder pattern after being heated
to 1000°C in air. It is quite different from the yellow

14Fig. 1. Remnants of metamict mineral (light areas)
partly replaced by (Nb,Fe)-substituted anatase(dark areas). Herrebøkasa granite pegmatlte, østfold, Norway. Backscatter photomicrograph. Scalebar is 100 pm.
mineral we have encountered.
Chemicaldata

The anatase was analyzed with a Cameca Camebax
electron microprobe operated at 15 kV. Standards werewollastonite (Si), pyrophanite (li), hematne (Fe), Nband Ta metals. The analytical sums are variable due
to fluctuating beam currents (17.3-20.5 nA). Twoanalyses with sums near 100% are:
Si02
0.88
0.80
Ti02
44.06
44.46
Fep3
18.96
20.44
NbPs
33.06
31.11
Taps
1.61
1.61
Sum
98.57
98.42
All the 11 analyses performed show remarkably similar
atomic proportions, based on 2 oxygens (standard
deviations in parentheses):
Si
0.015(0.007)Ti
0.51(0.02 )
Fe3+
0.23(0.01)
Nb
0.24(0.02 )Ta
0.005(0.002)Sum
1.000The chemical formula of this anatase is thus very
close to (TiosNbo2leo2s) r.10P2 with about half of the Tiatoms replaced by Nb and Fe according to the substitution
scheme 2Ti4+= Nb5+ + Fe3+.This result makes it tempting
to postulate that Ti and (Nb,Fe) might be ordered in thestructure. A Rietveld refinement of powder data could
possibly give some indication, and will be attempted at alater date.
Discussion(Nb,Fe)-substitution is well known in the Ti02 polymorphrutile. IImenorutile with formula (Ti,Nb,Fe3+)Ps has a trirutile structure (but the name has also been used forniobian rutile). However, this substitution is not so well
documented in anatase. So-called 'Ieucoxene'- a yellow
alteration product of ilmenite, titanite, perovskite or otherTi-bearing minerals - often consists of anatase; it might
be Nb-bearing
if formed from a Nb-rich precursor.
'Nioboanatase' is a name given by Semenov to a yellow
alteration product of murmanite from Lovozero, Kola
Peninsula.Apartial analysis (of a mixture of 'nioboanatase'and an amorphous alteration product of murmanite rich inSi02) thegaveunit-cellTi0231.11,
Nbps17.80-21.61,Taps,.A,0.20
arewt.%;
parameters,
a = 3.73, c = 9.37
higher than for normal anatase (VIasov 1966).
A yellow alteration product of aeschynite from a greisenin central Kazakhstan was described in great detail as anew unnamed mineral by Podolskii
et al. (1988).
Microprobe
analysisgave the 'ideal'
formulaK(Nb,Ti)3Si(0,OH)10 1.5Hp. Interestingly, the X-ray study
showed the mineral to be tetragonal with cell parameterssomewhat larger than those for anatase: a = 3.81, c =9.60 A. It is, however, difficult to understand how this formula can be reconciled with an anatase structure. We havecalculated the Z value from the measured density (2.912.97 g/cm3) , the cell volume (139.35 A3) and the molecularweight of the empirical formula (933.60), and the result is
Z = 0.26-0.27. This clearly demonstrates the inconsistencyof the data set.
Acknowledgements
Hans-Jørgen Berg did the XRD work. Muriel Erambertassisted with the electron microprobe analyses. Brenda
B. Jensen kindly corrected the English text.
References
Nilssen, B. 1970: Samarskites. Chemical composition,
formula and crystalline phases produced by heating. Norsk Geologisk Tidsskrift 50, 357-373.
Podolskii,A. M., Grigorev, I. F., Ryabeva, E. G. & Yurkina,
K. V. 1988: Alteration of aeschynite from greisensin central Kazakhstan. Izvestiya Vysshikh
Uehebnykh Zavedenii, Geologya i Razvedka
1988(9),38-43 [in Russian].[Abstracted inAmerican
Mineralogist 75, 1213 (1990).]
VIasov, K. A. (Editor). 1966: Geoehemistryand mineralogyof rare elements and gene tie types of their deposits.
Volume Il. Mineralogyof rare elements. Israel Pro
gram for ScientificTranslations, Jerusalem, 945 pp.

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