manganocolumbite and cassiterite exsolution lamellae in ilmenite

Transcription

Hartmut Beurlen,
al.
Estudos Geológicos
v. 16 (2):et3-15
MANGANOCOLUMBITE AND CASSITERITE
EXSOLUTION LAMELLAE IN ILMENITE FROM THE
PITOMBEIRAS PEGMATITE (ACARI – RIO GRANDE DO
NORTE) IN THE BORBOREMA PEGMATITIC
PROVINCE, NE-BRAZIL.
Hartmut Beurlen*1
Rainer Thomas2
Marcelo R. Rodrigues da Silva3
Dailto Silva4
1 Programa de Pos-Graduação em Geociências, UFPE. * beurlen@ufpe.br
2 GeoForschungsZentrum Potsdam, Telegrafenberg B 120, D14473 Potsdam, Alemanha
3 Departamento de Geologia, UFPE. 4) Instituto de Geociências – UNICAMP.
RESUMO: Lamelas de exsolução de manganocolumbita, de cassiterita e de hematita
são observadas em cristais primários de ilmenita manganesífera, no pegmatito Pitombeiras
1, na Província Pegmatítica da Borborema, no município de Acari, Estado do Rio Grande do
Norte, Brasil. O pegmatito hospedeiro é heterogêneo e tem como encaixante o granito
porfirítico do batólito de Acari-Pau Pedra (580 Ma), por sua vez intrusivo nos biotitaxistos neoproterozóicos do Grupo Seridó. As exsoluções de hematita tem forma discoidal
em seção prismática e arredondada a irregular amebóide em seção pinacoidal da ilmenita
hospedeira, semelhante às numerosas ocorrências descritas na literatura. As exsoluções
de cassiterita são raras, com formas discoidais de até 4 por 15 m, com orientação em três
direções, fazendo 120° quando observadas na seção pinacoidal da ilmenita, relação similar
à descrita para exsoluções de rutilo em ilmenita na literatura, supostamente orientadas
segundo o romboedro II da mesma {hh2hl}. As exsoluções de ferrocolumbita são bem
mais freqüentes, enriquecidas em titânio e escândio, e se apresentam com formas tabulares muito finas (usualmente menores que 2 de espessura e 30 de comprimento), dispostas
muitas vezes em arranjos hexagonais, provavelmente orientadas segundo o romboedro III
da ilmenita {213l}, já que fazem um ângulo de aproximadamente 80° com as lamelas de
cassiterta. A composição da ilmenita hospedeira distingue-se por um conteúdo, em solução sólida, de 1,5 a 3,4 % peso de (Ta+Nb)2O5, e até 0,25 % peso de SnO2, indicando
reduzida solubilidade em condições de baixa temperatura. Já a solubilidade a altas temperaturas pode ser estimada em 3,0 a 4,5 % peso de (Nb+Ta)2O5, com base na composição
modal hóspede/hospedeiro.
Palavras chave: exsoluções de manganocolumbita; cassiterita; ilmenita hospedeira; pegmatito granítico; química mineral; Província Pegmatítica da Borborema.
Estudos Geológicos v. 16 (2), 2006
001 Estudos Geologicos Vol 16 - 2.p65
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Manganocolumbite and cassiterite exsolution lamellae in ilmenite from the pitombeiras pagmatie...
ABSTRACT: Exsolution lamellae of manganocolumbite, cassiterite and hematite are
observed in primary manganoan ilmenite crystals from Pitombeiras Pegmatite, in Borborema Pegmatitic Province, Acarí County in the State of Rio Grande do Norte-NE-Brazil. The
hosting heterogeneous pegmatite is intrusive in the porphyritic granite of the Acarí-Pau
Pedra batholith (0.58Ga), which intruded Neoproterozoic biotite-schists of the Seridó Group.
The hematite exsolutions are disc shaped in prismatic sections and rounded to irregular
ameboid shaped in pinacoidal sections, oriented along the ilmenite pinacoid, as described
from numerous other occurrences in the literature. The rare cassiterite exsolutions form
small, disc shaped (up to 4 by 15 m) lamellae arranged in three directions with angles of
120°, when observed in a pinacoidal section of the hosting ilmenite. A similar relation is
found in the literature for rutile exsolution lamellae in ilmenite, supposedly arranged along
the rhombohedron II {hh2hl} of the host. The much more frequent ferrocolumbite exsolutions are very thin tabular shaped, usually less than 2 m thick and 30 m long, commonly
forming hexagonal arrangements, supposedly oriented along the rhombohedron III {213l)}
of the ilmenite (because of the 80° angle observed with the cassiterite lamellae. The composition of the hosting ilmenite is distinguished by a content of 1.5 to 3.4 wt.% (Ta+Nb)2O5,
and less than 0.25 wt. % SnO2 in solid solution, indicating a low cassiterite and columbite
group mineral solubility at room temperature. At higher temperatures, the solubility of 3.0
to 4.5 wt. % (Ta+Nb)2O5 and 0.4% cassiterite can be estimated, based on the modal relationship between the hosting ilmenite and the exsolution lamellae.
Keywords: manganocolumbite exsolution lamellae; cassiterite; ilmenite host; mineral chemistry; granitic pegmatite; Borborema Pegamatitic Province.
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Hartmut Beurlen, et al.
INTRODUCTION
During the study of compositional
variations in columbite group minerals in
the Borborema Pegmatitic Province as possible tracers of the degree of fractionation
of different pegmatite types (Beurlen et al.
2007), an unique occurrence of manganoan
ilmenite with exsolution lamellae of titanian-scandian manganocolumbite, cassiterite and hematite was registered. The tabular
ilmenite crystals were found as main component in heavy mineral concentrates from
the Pitombeiras 1 pegmatite. The concentrate was provided by prospectors and,
according to their information, collected in
the Pitombeiras 1 pegmatite. Later, identical ilmenite crystals were found in blocks
of quartz collected by the authors in the
soil covering this pegmatite. The Pitombeiras 1 pegmatite may be reached leaving the
main Road from Acarí to Caicó in the State
of Rio Grande do Norte, 8.8 km southward
from Acarí, following to WSW by a secondary unpaved road for 1.9 km. The pegmatite is one of a group of small pegmatites
with nearly ESE strike and NNE dip, enclosed in the southern part of the Acarí-Pau
Pedra batolith (APPB). The group includes
the Canoa pegmatite, referred by Beurlen
et al. (2004) as the first occurrence of
"strüverite" in the BPP, located 1.9 km to
SW from Pitombeiras 1. Crystals of "strüverite" (this mineral, a high tantalian rutile,
was recently disabled by the IMA commission and should be considered as a high
tantalian rutile with up to 45wt% Ta2O5)
were now found also in the intermediate
zone of the Pitombeiras 1 pegmatite, identical in form and composition to that one
described from the Canoa pegmatite. As
some of the exsolution bearing ilmenite crystals of the heavy mineral concentrate are
partially included by strüverite crystals with
the same composition and intergrowths,
there is no doubt about the provenance of
this ilmenite. The registration and descrip-
tion of these manganocolumbite and cassiterite exsolutions seems to be of interest
because no similar intergrowths were referred in the pertinent literature (e.g. Rolff 1946,
Johnston Jr 1945, Ramdohr 1969, McLeod
& Chamberlain 1969, Uytboogart, 1951,
Antony et al. 1997, Uher et al. 1998, Cerný
& Ercit 1989, Craig & Vaughan 1981, Ixer &
Duller 1998)
GEOLOGY
The Canoa-Pitombeiras pegmatite
swarm occurs approximately. 1.5 km
westwards from the intrusive contact of the
hosting APPB with cordierite-sillimanitegarnet-biotite schists of the Seridó
Formation, top of the Neoproterozoic Serido
Group (Van Schmus et al. 2003). This location
is close to the western limit of the Borborema
Pegmatitic Province (BPP) (Fig. 1).
The APPB granitic facies in this area
is characterized by proeminent white K-feldspar phenocrysts and fine to medium grained biotite rich matrix, studied in detail by
Jardim de Sá et al. (1981, 1986). According
to geochemical data, discussed by Jardim
de Sá (1994) the calcalkaline to shoshonitic I
type magma source of the APPB was generated by the melting of a cca. 2.0 Ga old
granitic crust, with subordinated participation o juvenile (transition to A type) and/or
metassedimentary rocks. U/Pb in zircon
ages of 555 and 579 Ma were obtained for
the APPB by Legrand et al. (1991) and Jardim de Sá (1994), respectively. These ages
are much higher than the 480 to 533 Ma
supposed for the pegmatites of the BPP
(Ebert 1969, Almeida et al. 1970 and Araújo
et al. 2001, Baumgartner et al. 2006). According to da Silva et al. (1995), Araújo et al.
(2005) and Baumgartner et al. (2001) a more
likely source of the pegmatites of the BPP
are peraluminous pegmatitic granites, supposedly related to a late retrometamorphic
stage associated with an extensional-transpressional tectonic event at 525 Ma (Araú-
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jo et al. 2005). Baumgartner et al. (2006)
found an age of 528 Ma (U/Pb in monazite)
for one of these pegmatitic granites and
considered it too high to corroborate this
intrusions as source for the mineralized
pegmatites (509 to 514 Ma, U/Pb in columbite group minerals).
As in other pegmatites of the group
enclosed in the APPB, in Pitombeira 1, a
fast transition from a decimetric homogeneous steeply dipping pegmatite vein (80°
NE) to metric to decametric, gently (30 o to
60o NE) dipping lenses, with very incipient
zoned structure and subordinated discontinuous quartz cores, plunging to NW, is
observed (Fig. 2), contrasting with the usually well zoned larger pegmatites of the Parelhas-Equador area.
The poorly zoned part of the Pitombeiras pegmatite is formed by a medium to
coarse- grained wall zone (1 to 5cm grain
size), composed of quartz, K-feldspar and
subordinated albite, with sporadic biotite
plates growing perpendicular to the contact with the enclosing granite. Main accessory minerals are garnet, muscovite,
beryl, and magnetite. The wall zone locally
grades inwards into a blocky feldspar +
milky quartz zone with individual K-feldspar crystals reaching up to 1.5m also oriented perpendicular to the contact, showing
local replacement and overgrowths by albite. Discontinuous lenses of massive, milky
quartz, locally with masses of rose quartz in
the center (Fig. 2a). Black, saccharoidal tourmaline aggregates in thin veins crosscut the
quartz core and blocky feldspar (Fig. 2b), or
form decimetric, coarse grained, replacement
pockets at the transition to the quartz core.
Beryl crystals (usually 5 to 10 cm sized) are
Figure 1 – Borborema Pegmatitic Province on a simplified geologic base adapted from Brasil (1998,
2002), with the location of the Pitombeira pegmatite (number 3) discussed in the text.
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sporadically observed in all zones. Beryl crystals with yellowish-green colors predominate in the wall zone, while blue colored,
gem quality aquamarine may sometimes be
observed in the transition between the blocky feldspar and quartz core.
PETROGRAPHY
The samples of columbite-cassiterite
exsolution bearing ilmenite (type 1) were
found first as the main component in heavy
mineral concentrates provided by miners,
together with some larger ferrocolumbite
crystals. Some of the tabular ilmenite crystals (up to 8 mm large in composite grains
of the concentrate) are in direct single planar contact with crystals of strüverite, indicating simultaneous growth. The strüverite by its turn contains inclusions of ixiolite
(probable exsolution lamellae according to
Beurlen et al. 2007) and cassiterite. Another sample of the same ilmenite type was
found included in massive quartz blocks in
the soil covering the Pitombeiras 1 pegmatite. No sample of this ilmenite was found
in the pegmatite outcrops, but strüverite
crystals with the same composition and intergrowths with ixiolite were found in blocks of typical pegmatitic mineral association of the intermediate zone in the dumps of
this pegmatite and in the nearby Canoa
pegmatite, already described in detail by
Beurlen et al. (2004). In one of these pegmatite blocks a 5cm large strüverite crystal
is in direct contact with a 5cm large beryl
crystal, both idiomorfic against a large Kfeldspar crystal and massive milky quartz.
The primary pegmatitic origin of both, the
strüverite and this exsolution bearing ilmenite, is thus well established.
Figure 2 a View of the western end of the Pitombeira 1 pegmatite. At the surface the pegmatite is
homogeneous, less than 20cm thick, with a high dip of 8O° to north (right side of the foto), enclosed by
a porphyritic granodiorite of the APPB. Towards the depth, a fast increase of the thickness to more than
6m, a decrease of the dip to 45°, and an incipient zoned structure is observed, formed by a 2 to 3m thick
wall zone (W) with crystal size increasing towards the center and an up to 4m thick quartz core (Q) is
observed (now almost completely extracted, see the hole were the man is seated). b Detail of the northern
wall of the pegmatite excavation, with an incipient formation of intermediate zone (I) formed by meter
sized K-feldspar crystals oriented normally to the contact, at the transition between the wall zone (upper
part – W) and the quartz core (darker, lower part – Q).
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A second type of ilmenite was described
by Beurlen et al. (2004) as part of a late
hydrothermal alteration product of the
strüverite. This ilmenite 2 is always intergrown
with microlite and/or pyrochlore and some
niobian sphene, replacing the borders of
strüverite crystals. This ilmenite 2 usually has
a 30 to 60 mol% pyrophanite component – thus
being actually a pyrophanite in some cases-,
much higher than the Mn-content observed in
the type 1 ilmenite (up to 10 mol%). In addition,
in the type 2 ilmenite no exsolution bodies are
found, also distinguishing this generation from
type 1 ilmenite.
The type 1 ilmenite contains three different exsolved phases. The first one is titanian hematite forming disc shaped lamellae, up to 10 m by 30 m large, oriented
parallel to the pinacoid (0001), as can be
seen in prismatic sections of the hosting
ilmenite, and with a diameter of about 30 m
and rounded to ameboidal shapes shown
in the pinacoidal section. This type of exsolution bodies is the most common one
observed in ilmenite in many different types
of occurrences (Ramdohr 1969). The second
phase that occurs as product of exsolution
in this ilmenite type is cassiterite, in the form
of rare, randomly distributed, disc shaped
bodies, up to 4 m by 15 m large, oriented
along three directions with angles of 120°
when observed in pinacoidal sections of
the ilmenite. In the literature similar crystallographic relations are described for rutile exsolutions, supposedly oriented along
the rhombohedron II {hh2hl) of the hosting ilmenite according to Edwards (apud
Ramdohr 1969). The third exsolved phase
is a columbite-group mineral (or possibly
ixiolite), which occurs as thin, tabular shaped (< 2 m by 30 m) exsolution bodies frequently forming hexagonal arrangements in
a pinacoidal section of the hosting ilmenite
and in two directions when seen in prismatic
sections, in this case forming an angle of
about 30 to 40° with the hematite lamellae.
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These columbite lamellae are probably exsolved along the rhombohedron III
{2hh3hl) of the hosting ilmenite, because
they are oriented forming an angle of nearly 80° with those of the cassiterite lamellae.
The definitive identification as ixiolite or
columbite-group mineral in the present case
is not possible. This is because the composition obtained by EMPA in a few lamellae
larger than 2 m thick, and in a few even
larger grains formed by collection recristallization from the exsolution lamellae, is
very close to the upper limit of 10 wt%
(TiO2+SnO2+ Sc2O3) content admitted for
the, in this case disordered, columbite group
phases (Wise et al. 1998).
For all three of the exsolved phases,
the very homogeneous size, shape and distribution in the hosting ilmenite, and the
clearly established crystallographic host/
guest relationship for each phase are considered to assure its generation by a true
exsolution process (Fig. 3).
The modal proportion of the exsolution
lamellae was estimated roughly to be of 7.5%
hematite, 2.0% columbite and 0.2% cassiterite.
This primary ilmenite type 1 and its exsolutions are locally replaced along fractures
and grain borders by a myrmekitic intergrowth of titanian maghemite and niobian rutile,
probably formed by supergene alteration.
CHEMISTRY
Analytical methods
Semi quantitative chemical analyses of
the samples for preliminary mineral identification and imaging were obtained by SEM
at the University of Campinas São Paulo
using a SEM Leo 430i, Cambridge, EDS mod.
Cat. B, using the following working conditions: 20Kv, 30 seconds acquisition time, using
the following standards: Tao (Ta Mα), Nbo
(Nb Lα ), Sno(Sn Lα), Tio(Ti Kα), Vo (V Kα), Sbo
(SbLα), Bio (Bi Mα) Zro (Zr Lα), Uo (UMα), Hf o
(Hf Mα) PbF2 (PbMα), BCR2 (Fe Kα, Mn Kα,
Al Kα, Ca Kα, Na Kα, Si Kα, K Kα).
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Quantitative data were obtained by
electron microprobe analyses (EMPA) at the
GeoForschungsZentrum Potsdam-Germany,
using Cameca SX 50 and SX 100 EMPA equipments at 20 kV and 40 nA, with acquisition
times of 20 seconds for Fe Kα, Mn Kα, Ti Kα,
Sc KαMg K , Al Kα, Ca Kα, NaKα, Si Kα, K Kα,
30 seconds for Nb, Lα, SnLα, Sb Lα, Zr LαHf
Mα, Y Lα, Cs Lα, Ba Lα, and 50 seconds for
Ta Lα, Bi Mα, U Mα, PbMα, ThMα, Ce Lα, La
Lα, using the following standards: albite (Na),
ilmenite (Fe, Ti), cassiterite, orthoclase
(K,Al), titanite (Si,Ca,Ti,), zircon, Nbo, Tao,
Tho, Uo, vanadinite, BaSO4, CePO4, LaPO4,
YPO4, ScPO4, InSb, MgO, HfO2, MnTiO3,
Bi2S3 , pollucite.
The calculation procedure to obtain
the cationic composition in atoms per formula unit (APFU) listed in Table 1, was to
normalize them in a first step to a sum of
24 oxygen ions for most tantalates, to allow
an easier comparison. In a second step, in
cases where the obtained cation sum surpassed the corresponding theoretical value (12 cations for phases of the columbite, tapiolite, and ixiolite groups, and niobian rutile – strüverite, etc.), the Fe2+ content was partially converted to Fe3+ (according to Ercit et al. 1992c, 1992d) by trial
and error until the cation sum reached the
value of 12.000± 0.002 cations. In the case
of wodginite group minerals the normalization was made recalculating Fe2+ to Fe3+
cations as much as necessary to complete
Figure 3 a BSEI of a polished section with pinacoidal orientation of an ilmenite crystal showing three
different exsolved phases (:) scarce disk shaped, white cassiterite lamellae, frequent tabular light gray
manganocolumbite lamellae, both in three directions, and rounded, oval to irregular, ameboidal
shaped, medium gray, hematite exsolution bodies, in the dark gray ilmenite host. The hematite exsolution
bodies occur in two sizes, probably corresponding to two generations (scale bar 100 m). b Detail from
a, showing the hexagonal arrangement of the manganocolumbite lamellae (arrow) and the difference
in orientation between cassiterite and manganocolumbite lamellae (scale bar 50 m). c BSEI of a
section nearly orthogonal to the pinacoid of another ilmenite crystal, with the manganocolumbite
exsolution lamellae (light gray) in two directions in low angle with the hematite lamellae (medium
gray) along the pinacoid (NNE in the photo). A large idiomorphic manganocolumbite inclusion(
labeled with 7,in the lower part of the photo) and a large hematite lamella were formed by "collection
recrystallization" as indicated by the lack of the smaller exsolution lamellae in the area surrounding
the large inclusion (scale bar 100 m).
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the B+C-sites to 9.000 (for easier comparison with other minerals, a 24 oxygen formula was used instead of the 32 oxygen
formula required for the unit cell). The
effectiveness of the Fe3+ correction procedure was checked by repeating some calculations using the method proposed by
Droop (1987).
Mineral chemistry
Representative EMPA data are listed
in Table 1. The number of acceptable quantitative analyses of the exsolved phases is
not large because of the usually very small
size of the exsolutions, very close of the size
limit which warrantees the data to represent
a single phase result (about 4 or 5 m), avoiding mix-analyses. Many other semiquantitative SEM and EMPA data are not included but confirm the main characteristics
presented in Table 1 and Fig. 4a and b.
The data of the chemical composition
of the primary, exsolution bearing, type 1
ilmenite are very homogeneous. They have
a content of 9.0 to 12.0 wt % of MnO, corresponding to 18 to 26 mol% of pyrophanite {(Mn/(Mn+Fe) vary between 0.18 to .26}.
This Mn content is elevated in comparison
with that one observed in the usual granitic ilmenites (between 2.0 and 7.0 wt% MnO
according to Haggerty, 1976) and also higher than data from pegmatitic ilmenites
reported by Uher et al. (1998), with 3.0 to
6,5 wt% MnO. The content of dissolved
Nb2O5 + Ta2O5 in the ilmenite host varies
from 1.4 to 3.4 wt. % (or 1 to 3mol% of columbite, considering the 2/1 relation of
{(Nb+Ta}/{Fe+Mn} in the columbite group
minerals and the variation of (Ta+Nb) content between 0.08 and 0.23 APFU, as listed
in table 1}. The dissolved cassiterite content is of 0.03 to 0.25 wt% of cassiterite (ca.
0.1mol%). These values of dissolved columbite are below the maximal contents of
4.0 wt.% Nb2O5 + Ta2O5 in ilmenite referred
to by „erný & Ercit (1989).
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The alteration related, exsolution free,
type 2 ilmenite has similar dissolved contents of Nb2O5 + Ta2O5 (1.4 to 2.13 wt%),
than type 1 ilmenite, but smaller contents
of tin. The chemical difference to type 1
ilmenite is much more noticeable because
of the much higher contents of MnO in type
2 ilmenite (ranging from 15 to 29 wt% which
corresponds to a pyrophanite content varying from 32 to 63 mol%). Values in this
range are referred to by Haggerty (1976) to
be restricted to ilmenites in peralkaline rock
suites (7.0 to 30wt% MnO) or in Mn-ores.
The increasing Mn content in this ilmenite
in comparison with type 1 ilmenite, even
being higher than usual in granites or pegmatitic ilmenites, is in agreement with its
late stage formation, probably related to
hydrothermal alteration.
The deficit in Ti (always below the hypothetical stoichiometric value of 6.0 APFU, and
lower than the sum of Fe+Mn+Fe3+ which
always surpasses 6.0APFU) in both ilmenite
types, indicate that there is a dissolved excess of hematite in the ilmenite (incomplete
hematite exsolution in ferroan manganoan
hemoilmenite), typical for ilmenites formed
under high temperature conditions.
The composition of the columbite
group mineral exsolution lamellae indicates
a classification as manganocolumbite {Ta/
(Ta+Nb) ranging from 0.06 to 0.10}, but very
close to the limit between ferro- and manganocolumbite fields {(Mn/(Mn+Fe) ratio
between 0.54 and 0.58} in the columbite
group quadrangle (Fig. 4a). As the sum of
Ti+Sn+Fe3++Sc APFU ranges between 1,12
and 2.05 (or 9 to 18 mol% of dissolved
cassiterite+rutile+Sc+ Fe3+-bearing phases,
in a 12 APFU formula), it is not possible to
distinguish if the exsolved phase is a disordered titanian and scandian manganocolumbite or a scandian titanian ixiolite based
on mineral chemistry. This is because the
data overlap the limit between these two
phases as suggested by Wise et al (1998)
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Table 1 Representative EMPA data of ilmenite and its exsolution lamellae from Pitombeira pegmatite in the BPP. Mineral abbreviations: Mcl= manganocolumbite; Cst=cassiterite, Ilm=ilmenite; Ilm (Mn) = high manganoan ilmenite; Pyf = pyrophanite; Hmt = hematite; na = not analyzed; 0.00 = below detection
limit; others = W, Th, Hf, Al, Na, K, Le, Ce, Pb, Ba, Bi, Sb, P, Si, F.
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Figure 4 EMPA of ilmenite and its exsolution lamellae from the Pitombeira pegmatite in the BPP. a data
distribution in the "columbite quadrilateral" Ta/(Ta+Nb) versus Mn/(Mn+Fe) APFU plot. The primary, exsolution bearing ilmenite distinguishes easily by lower Mn and Ta numbers from the later,
secondary ilmenite. The exsolution lamellae of manganocolumbite have lower Ta and higher Mn
numbers than the hosting ilmenite, while the cassilterite exsolution lamellae have higher Ta and lower
Nb numbers; b in the ternary Nb,Ta / Ti,Sn / Fe,Mn APFU plot the position of the manganocolumbite
exsolution lamellae close to the 10 mol% (Sn,Ti) content is evidenced, that corresponds to the transition between disordered columbite and ixiolite according to Wise et al. 1998. The ilmenite contents in
the hematite exsolutions are shown to be 7 to 15 mol% (Ti,Sn) or 15 to 30% ilmenite in the hosting type
1 ilmenite (thus to be distisguished as a hemoilmenite composed by a ferrian ilmenite host and titan
hematite exsolutions according to the nomenclature by Buddington 1963). In comparison the secondary, type 2 ilmenite is distinguished by nearly endmember manganoan ilmenite, with no excess of iron
or manganese.
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as can bee seen in Fig. 4b. In two cases the
Sc + Ti contents surpass the maximal limit
of solubility for Sc and Ti admitted by these
authors for disordered columbite, while the
other three data are slightly below this limit.
An identification by conventional X-ray
diffraction after heating, which would be
indicated according to „erný et al. (1998),
is not possible in this case due to the small
size of the exsolution lamellae.
The hematite exsolution lamellae are
characterized by high Ti contents (between
0.76 and 1.77 APFU Ti, or up to 30 mol%
ilmenite) as only significant dissolved strange element, allowing to classify it as titanian
hematite. Such high Ti contents are also typical for hematite exsolution lamellae in plutonic or high temperature metamorphic rocks
(Haggerty 1976, Rumble III 1976). Unfortunately, the absence of coexisting tiatanomagnetite and the high Mn contents make temperature estimates impossible.
The EMPA data distribution in the "columbite quadrilateral" [Ta/(Ta+Nb) versus
Mn/(Mn+Fe) APFU plot] in Fig. 4a, allows
a clear distinction of the primary, exsolution lamellae bearing ilmenite type 1 by lower
Mn and Ta numbers (respectively Mn/
(Mn+Fe) and Ta/(Ta+Nb) from the later, secondary type 2 ilmenite in which no exsolution lamellae are observed. The exsolution
lamellae of manganocolumbite have lower
Ta and higher Mn numbers than the hosting ilmenite, while the cassiterite exsolution
lamellae have higher Ta and lower Nb numbers. This is a fractionation behavior similar
to that one observed between exsolutions
of orthorhombic columbite group minerals
in rutile host, in which Ta + Fe are fractionated in favor of the tetragonal rutile phase
while Mn and Nb are fractionated in favor of
the orthorhombic phase (Cerný et al. 1998).
The data distribution in the ternary
Nb,Ta / Ti,Sn / Fe,Mn APFU plot allow to
observe the position of the manganocolumbite exsolution lamellae close to the 10 mol%
(Sn + Ti) content which corresponds to the
transition between disordered columbite
(less than 10 mol %) and ixiolite according
to Wise et al. (1998), as discussed above.
In this diagram the ilmenite contents in the
hematite exsolutions are shown to be 7 to
15 APFU% Ti (plus minor Sn) which corresponds to 15 to 30 mol% ilmenite in the
hosting type 1 ilmenite. The composite crystals of type 1 ilmenite are thus to be distinguished as a hemo-ilmenite composed by a
ferrian ilmenite host (ilmenite with an excess of 5 to 10mol% hematite) and titanhematite exsolutions according to the nomenclature by Buddington & Lindsley
(1964). In comparison, the secondary, type
2 ilmenite is distinguished as nearly stoichiometric manganoan ilmenite, with a very
small excess of iron.
CONCLUSIONS
Two types of ilmenites are distinguished as accessory opaque minerals of the
Pitombeira pegmatite in the Borborema Pegmatitic Province. Type 1 ilmenite is distinguished by the unusual trellis like scandian
manganocolumbite and cassiterite exsolution lamellae in association with hematite exsolution lamellae. The (Ta+Nb)2O5 content
of the homogeneous primary magmatic ilmenite (precursor of the ilmenite with exsolutions) is estimated to vary form 3.0 to 4.5 wt.
%, very close to the values referred to by
Cerný
& Ercit (1989) as maximal contents in
Č
pegmatitic ilmenites. The Mn content of this
type 1 ilmenite is high (18 to 26 mol% pyrophanite) if compared with other ilmenite occurrences in pegmatites (Haggerty 1976,
Uher et al. 1998). Type 2 ilmenite is formed in
association with microlite and tantalian sphene as probable hydrothermal alteration product of "strüverite", and has higher Mn contents than type 1 ilmenite (up to 62mol%
pyrophanite).This ilmenite should therefore
be classified as pyrophanite, until now unknown in the BPP. The Ta number of type 2
13
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24/5/2007, 11:58
ilmenite is higher than that of type 1, which
is in accordance with the hypothesis of its
formation in late stages of crystallization.
Acknowledgments
This research was supported by CNPq
(Brazilian Research Council) Grants 470199/
01-6 and 352181/92-3, and CAPES grant AEX
0728/04-7. We are also indebted to Dr. W.
Heinrich of the GeoForschungsZentrum
Potsdam (GFZ) in Germany for the free use
of the Microprobe facility, Prof. Dr. Bernardino R. Figueiredo Instituto de Geociências of the University of Campinas, Brazil
(IGE-UNICAMP), and to O. Appelt and G.
Rehde of the GFZ for the technical support
during the microprobe analyses.
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manganocolumbite and cassiterite exsolution lamellae in ilmenite

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