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The Canadian Mineralogist
Vol. 38, pp. 1409-1418 (2000)

CESIAN BAZZITE AND THORTVEITITE FROM CUASSO AL MONTE, VARESE, ITALY:
A COMPARISON WITH THE MATERIAL FROM BAVENO, AND INFERRED ORIGINCARLO MARIA GRAMACCIOLI
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy

VALERIA DIELLA
Centro CNR per la Geodinamica alpina e quaternaria, Via Botticelli 23, I-20133 Milano, Italy

FRANCESCO DEMARTIN
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano,
Via Venezian 21, I-20133 Milano, Italy

PAOLO ORLANDI
Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria, 53, I-56100 Pisa, Italy

ITALO CAMPOSTRINI
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy

ABSTRACTThe occurrence of thortveitite, Sc2Si2O7, and bazzite, Be3Sc2Si6O18, in the miarolitic cavities of the granophyre of Cuasso al
Monte, in Varese, Italy, is reported for the first time, together with additional data for the corresponding species from the granite
of Baveno. Bazzite from both these localities is Mg-poor and contains notable amounts of cesium (up to 2.3 wt% Cs2O), which
is not encountered in the corresponding samples from Alpine fissures, and which has been overlooked in the type material (from
Baveno). These discoveries confirm the similarity between the Baveno and Cuasso al Monte occurrences, and suggest the possibility that scandium minerals are more widespread in granitic rocks than is inferred at present. The composition and paragenesisof these Sc-rich species indicate the importance of complexes in enhancing the different geochemical behavior of scandium withrespect to the REE, and that of equilibria with species such as feldspars, where aluminum cannot be replaced by transition
elements.
Keywords: scandium, thortveitite, bazzite, Baveno, Cuasso al Monte, Italy.

SOMMAIRE
Nous signalons pour la première fois la présence de thortvéitite, Sc2Si2O7, et de bazzite, Be3Sc2Si6O18, dans les miaroles d’un
granophyre à Cuasso al Monte, dans la province de Varese, en Italie, et nous fournissons des observations nouvelles sur lesmêmes espèces dans le granite de Baveno. A chaque endroit, la bazzite a une faible teneur en Mg et est enrichie en césium
(jusqu’à 2.3% de Cs2O, en poids), ce qui n’avait pas été signalé dans la bazzite de la localité-type (Baveno). Ce ne sont pas les
caractéristiques d’échantillons de ces espèces récupérés des fentes alpines. Ces nouvelles observations confirment la ressemblancede Baveno et de Cuasso al Monte, et fait penser que les minéraux de scandium pourraient bien être plus répandus dans les roches
granitiques que l’on ne le soupçonne présentement. La composition et la paragenèse des espèces à Sc montre l’importance decomplexes aqueux à distinguer le comportement géochimique du scandium par rapport aux terres rares, et d’équilibres ioniques
avec des espèces telles que les feldspaths, dans lesquels l’aluminium ne peut pas être remplacé par des éléments de transition.
Mots-clés: scandium, thortvéitite, bazzite, Baveno, Cuasso al Monte, Italie.

§

E-mail address: carlo.gramaccioli@unimi.it

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THE CANADIAN MINERALOGISTINTRODUCTION
The discovery of bazzite, Be3Sc2Si6O18, in miarolitic
cavities of the Baveno granite, in the Lago Maggiorearea of Italy, by Artini (1915) followed closely
Eberhard’s (1908, 1910) discovery of thortveitite in the
Iveland region in Norway as the first known mineral inwhich scandium is present as an essential constituent.
The original find at Baveno remained the only one there
until about 1980, although several occurrences ofbazzite have been discovered in Alpine fissures since
1939 (see Hänni 1980 for references). At the same time,
additional scandium-rich species have been observed at
Baveno: for instance, thortveitite Sc2Si2O7 occurs invery unusual specimens as tiny greyish or deep blue
crystals, in miarolitic cavities (Orlandi 1990). Other
important discoveries are the new minerals cascandite
CaScSi3O8(OH) (Mellini & Merlino 1982, Mellini et al.
1982), jervisite NaScSi2O6 (Mellini et al. 1982), andscandiobabingtonite Ca 2 (Fe 2+ ,Mn)ScSi 5 O 14 (OH)
(Orlandi et al. 1998). For a detailed history of all suchdiscoveries, see Gramaccioli et al. (1998).
In recent years, the scandium-rich species and theirparagenesis have received attention. Such an increaseof interest is witnessed by a number of recent discoveries of scandium minerals (besides the literature concerning Baveno, see also Bergstøl & Juve 1988, Juve &Bergstøl 1990, Armbruster et al. 1995, Liferovich et al.
1997, 1998, Bernhard et al. 1998, Raade & Erambert1999); furthermore, the geochemical behavior of Sc,
which is distinct with respect to the rare-earth elements(REE), has been shown not to be due only to the differences in ionic radii. Recent observations have emphasized the importance of complexes of Sc in thedepositing solutions (Gramaccioli et al. 1999a, b, 2000).
On considering the granophyre from Cuasso al
Monte, Lombardy, Italy, and from the nearby localityof Carona in Canton Ticino, Switzerland, and its relationships with the Baveno granite (Bakos et al. 1990,
Boriani et al. 1992), scandium minerals were expected
to occur here for some time. However, in 1992 an interesting specimen was noticed by Mr. R. Appiani in aquarry (Cava Laghetto) at Cuasso al Monte: it consistsof a group of very tiny (maximum length 0.1 mm) greyish blue hexagonal prismatic crystals with flat terminations and similar to bazzite from Baveno, perched upon“orthoclase” (microcline) in a miarolitic cavity, associated with quartz, albite, and a black biotite-like mineral.
Furthermore, in 1994, another mineral collector, Mr.
F. Anderbegani, submitted to our attention a certain
number of specimens of another mineral he had foundin the same quarry, showing very tiny groups of whitish
aggregates of crystals. These aggregates are perchedupon “orthoclase”, are associated with tourmaline, fluorite, quartz and albite, and are similar to greyish
thortveitite from Baveno.
A preliminary energy-dispersion analysis showed thepresence of notable amounts of scandium in both cases.

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We thus decided to carry out a more detailed investigation of these minerals from Cuasso al Monte. At thesame time, one cotype specimen of bazzite in good crystals and samples of thortveitite from Baveno had become available to us, thereby providing the possibilityof obtaining improved crystallographic and compositional data on these minerals from both localities.

X-RAY DATA
BazziteA fragment of a tiny blue, bazzite-like crystal from
Cuasso al Monte was examined using a Nonius CAD–4
single-crystal X-ray diffractometer. Although the poor
quality of the crystal prevented the possibility of refining the structure, the data were sufficient for deducing
the Laue symmetry 6/mmm and the unit-cell parameters;
these results are reported in Table 1. On this basis, theidentity of the mineral from Cuasso al Monte withbazzite is confirmed; final proof was provided by quantitative electron-microprobe analysis (see below). The
large unit-cell dimensions constitute a peculiarity ofbazzite from this new locality; the high value of c may
relate to the presence of notable amounts of divalentions, such as Fe2+, substituting for Sc3+, but the highvalue of a cannot easily be explained.

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CESIAN BAZZITE AND THORTVEITITE, CUASSO AL MONTE, ITALY

The cotype specimen of bazzite from Baveno examined is a part of the old Bazzi collection, which is now
preserved in the mineralogical collection of Dipartimento di Scienze della Terra, University of Milan. From
this sample, a very tiny fragment of a crystal was selected, and very good data were obtained using a CAD–
4 single-crystal diffractometer, permitting accurate
refinement of the structure (Demartin et al. 2000). The
unit-cell values (Table 1) are in line with recent values
obtained for bazzite from other localities, but are notably different with respect to the previous set determinedby Peyronel (1956) on type (or cotype) material as well.
Such a difference is almost certainly due to the poor
accuracy of the older measurements, especially reflectedin the anomalously small value of c.

persion spectrometer; the results are reported in Tables
5 and 6. The system was operated at an accelerating
voltage of 15 kV, a sample current on brass of 10 nA,
and a counting time of 10 or 20 seconds on the peaks,
and 5 or 10 seconds on the backgrounds. Kaersutite (forMg, Na,Ti, K), spessartine (for Fe and Mn), omphacite(for Si and Ca), pollucite (for Cs), and zircon (for Zr)

Thortveitite
For the other scandium-bearing mineral from Cuasso
al Monte and all thortveitite specimens from Baveno,
no single crystals providing reliable X-ray data could
be found. The X-ray powder-diffraction patterns of two
different samples of this mineral from Cuasso al Monte,
obtained using a Gandolfi camera, are reported in Table
2, together with those of a sample from Norway and ofpure synthetic thortveitite for comparison. The corresponding patterns obtained from four different samplesof thortveitite from Baveno are reported in Table 3, andthe unit-cell parameters, obtained from a least-squares
fit of these powder data, are reported in Table 4. They
do not differ significantly, and agree satisfactorily withthe corresponding data on “classic” occurrences in Norway and Madagascar (Bianchi et al. 1988), as well withthose for a pure synthetic sample of Sc2Si2O7.
The composition of thortveitite is variable; in somecases, the mineral is almost pure, and in others it contains notable quantities of yttrium, zirconium, and theheaviest REE. As was shown by several authors (Smolin& Shepelev 1970, Voloshin et al. 1983, 1985, Bianchi
et al. 1988, Gramaccioli et al. 1999b, Foord et al. 1993),
the unit-cell parameters a and b increase appreciably on
replacing Sc by Y and the REE, whereas c and ␤ remain
almost constant. By following only this criterion, andconsidering the data reported in Table 4, the first specimen from Cuasso al Monte and the blue specimens from
Baveno represent a Y- (and REE-)poor variety ofthortveitite (see below). For the second specimen from
Cuasso al Monte, the data were insufficient to deduce
the unit-cell parameters, and the X-ray powder-diffraction pattern (Gandolfi) only supports the identification.

CHEMICAL COMPOSITION
In addition to a series of preliminary EDS analyses,
quantitative chemical analyses were performed on polished samples using an Applied Research Laboratories
electron microprobe fitted with six wavelength-dispersion spectrometers and a Tracor Northern energy-dis-1409 38#6-déc.00-2203-08

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THE CANADIAN MINERALOGIST

were employed as standards; with respect to Sc,Y, Zr,
Hf, and the REE, the lines, the standards used and theprocedures followed to account for interferences aredescribed in detail in Bianchi et al. (1988), Demartin etal. (1991) and Mannucci et al. (1986). The results were
processed for matrix effects using a modified version of
the MAGIC IV program.
BazziteOur results are reported in Table 5, together withsignificant data from other localities taken from the literature. There is a noticeable compositional difference
between the samples from Alpine fissures and thosefrom the other locations; the former contain importantamounts of magnesium, and the latter contain instead
non-negligible amounts of heavy alkalis, especially cesium. This element was overlooked in previous investigations of the type material (Bertolani 1948); its1409 38#6-déc.00-2203-08

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presence was first noticed on examining EDS spectra oftype and cotype specimens (Gramaccioli et al. 1998).
Bertolani (1948) used spectrographic analysis todetect the presence of Y and REE in type (or cotype)specimens. These elements have not been detected with
our electron microprobe, as with material from otherlocalities (see, for instance, Hänni 1980); such a discrepancy is certainly due to the higher sensitivity of a spectrographic analysis, which, on the other hand, shouldhave revealed the presence of cesium; the early volatilization of Cs in the emitting source with respect to theother elements considered is a probable reason for thiserror.
As in beryl (Hänni 1980, Aurisicchio et al. 1988,
Artioli et al. 1993), the charge balance in bazzite to account for the presence of divalent ions like Mg or Fe2+
in the 4(c) Sc (or Al) site can be achieved mainly owing
to the presence of alkali metal ions (Na+ and Cs+) in thechannels; a similar observation could also be drawn for

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results of the crystal-structure refinement; however,such procedures may not be entirely correct. The actual
contents of beryllium were determined by Chystyakova
et al. (1966) for bazzite from Kazakhstan, by Juve &Bergstøl (1990) for bazzite from the Heftetjern pegmatite, Tørdal, Norway, and by Della Ventura et al. (2000)
for type stoppaniite. In these instances, an excess of Be

the newly discovered species stoppaniite, the Fe-richmember of the beryl group (Ferraris et al. 1998, Della
Ventura et al. 2000). If all the iron in the specimen from
Baveno is assumed to occupy the 4(c) sites in the formof Fe2+, the charge of the alkali metals in the channels
would be balanced almost exactly on replacing the Sc3+
ions by the 2+ (and 4+) ions present in the structure.
The same, however, does not happen for the specimen
from Cuasso al Monte, and on these grounds, the presence of some Fe3+ instead of Fe2+ might be assumed aswell (Aurisicchio et al. 1988, Artioli et al. 1993).
Similarly, bazzite from Alpine fissures contains
more magnesium than necessary to balance the charges
in this way, even without considering a possible role ofiron. Therefore, in agreement with Armbruster et al.
(1995), the presence of some 2+ ions in the channels
cannot be ruled out, at least in some cases. Furthermore,other ways of balancing the charges cannot be excluded:
these possibilities are the presence of OH– groups in the
structure, or the partial substitution of Si4+ by Be2+ (seebelow) or even of Be2+ by Li+. Therefore, in view ofthese possibilities and in lack of additional information,
no attempt has been made to derive the Fe2+:Fe3+ ratio.
The low analytical total in some cases (including bazzite
from Cuasso al Monte) is almost certainly due to neglect of the presence of H2O, which might also bepresent in the channels.
An adequate crystal-chemical characterization of all
the minerals in the beryl group would imply an accurate
determination of the contents of the lightest elements,
such as Be, Li, or H, on a significant number of specimens. In view of the lack of such information, most data
concerning Be and H reported in Table 5 have been
derived from simple assumptions (e.g., by imposing avalue of 0.5 to the atomic ratio Be/Si), or using the1409 38#6-déc.00-2203-08

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THE CANADIAN MINERALOGIST

was interpreted as evidence for partial replacement of
Si by Be, corresponding to the proportions Si5.93Be0.07,
Si5.7Be0.3, and Si5.53Be0.44 at the Si site, respectively; forstoppaniite, such a replacement is in agreement with the
crystallographic results obtained by Ferraris et al.
(1998). This possibility was considered in our crystalstructure refinement of the sample from Baveno and ofanother sample from Tørdal, Norway; however, no definite conclusions in this respect could be achieved using
X-ray data only (Demartin et al. 2000).

The geochemical behavior of scandium is peculiar andapparently contradictory: in igneous rocks, Sc is usually concentrated in ferromagnesian minerals, and forthis reason it usually prefers mafic assemblages, butnotable exceptions are found in some carbonatites,
where the accessory minerals are instead the major hostsof Sc (Eby 1973).
Other important concentrations of this element occur in hydrothermal or pneumatolitic veins in granitic
rocks, in association with fluorine-rich minerals, such

Thortveitite
The electron-microprobe data for thortveitite fromboth Cuasso al Monte and Baveno (Table 6) confirm
the results obtained from X-ray diffraction, the analyzed
specimens being reasonably pure. Instead, the EDS
spectrum of sample #2 from Cuasso al Monte shows thepresence of non-negligible amounts of Y, the REE, andZr; unfortunately, no suitable fragments were available
for a quantitative chemical analysis.
In view of the extreme scarcity of the thortveititefrom Baveno, as well as of the very small dimensions
and powdery nature of the samples, an electron-microprobe analysis could not be performed on the same
specimens from which X-ray data were obtained. However, the almost identical EDS spectra provided by aseries of different samples of the blue variety suggestthat our X-ray data should indeed correspond to thechemical composition indicated in Table 6. Further evidence in this respect is provided by the constancy of theunit-cell parameters of all the samples of the blue variety here examined, in view of the evident relationships
between such parameters and the Y (and REE) content.
Since the main substituents of Sc in thortveitite are
generally Y and the REE, on the whole the poorest
samples in these elements should be the richest in Sc. In
the analyzed specimen from Baveno and in the sample
from Kola studied by Voloshin et al. (1991), the Y content is the lowest on record; owing to replacement by
notable amounts of Ca and especially of Mn, the Sc
content for the former is appreciably lower than that for
the latter. Since Mn3+ is an intense absorber (Rossman1988), as in a number of deep blue or violet Mn-bearing
minerals such as purpurite, piemontite or “violane”, thepeculiar blue color of the unusual variety of thortveititefrom Baveno could be ascribed to the presence of thision, thereby providing a possible clue to its origin(Gramaccioli et al. 1999b; see also below). For the
specimen from Cuasso al Monte, notable amounts of Fe
substitute for Sc (up to nearly 3 wt% Fe2O3).

DISCUSSION
A general picture concerning the formation of scandium-rich minerals and their geological significance has
not been established yet, owing to the scarcity of reliable physical–chemical data and of natural examples.

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CESIAN BAZZITE AND THORTVEITITE, CUASSO AL MONTE, ITALY

as zinnwaldite, fluorite, and topaz [see, for instance,
Phan (1963), and references therein]; the miarolitic cavities at Baveno and Cuasso al Monte are related to thiskind of occurrence. It should be remarked that Sc, together with other elements that commonly occur in theseveins, especially Zr, Be, Al, Fe and the REE, forms very
stable fluoride complexes.
In order to account for the origin of specific Sc minerals, since scandium is rare, and as its properties are
similar to those of Y and the REE, a process of very
selective enrichment of this element is needed, and therole of formation and disruption of such complexes may
be essential (Gramaccioli et al. 1999 a, b, 2000).
Here, some thermodynamic data are available. For
instance, the first stability constants, K = [MeF(n–1)+]/[Men+][F–], of such complexes (as a decimal logarithm,
at room temperature) range from 9.80 (Zr) to 7.65 (Th),
7.10 (Al), 7.08 (Sc), 6.04 (Fe3+), 5.89 (Be), 5.76 (Mn3+),4.85 (Sn), 4.81 (Y), 4.48 (Yb), 3.56 (La), etc.; for thesubsequent stability constants concerning complexes
richer in fluorine, K = [MeF2(n–2)+]/[MeF(n–1)+][F–], etc.
the trend is similar, i.e., those of Zr being the moststable, followed by those of Th, etc.; an exception tothis trend is the couple Al–Sc, where the stability commonly is reversed (Lourié 1975), thereby enhancing thepossibility of complexing scandium.
In view of the great differences in the stability ofsuch complexes, in the presence of adequate concentrations of the fluoride ion in the depositing solutions, even
if the concentrations of all these elements in the solutions were comparable, the values of the activity of thecorresponding free ions could easily differ by several
orders of magnitude. Accordingly, the chemical behavior of scandium in these conditions should notably differ from that of yttrium and the REE on one hand andfrom that of zirconium on the other (Gramaccioli et al.
1999a, b, 2000).
If the activity of the fluoride ion in a solution is reduced, a dramatic increase in the activity of the “free”
cations necessarily follows, owing to the disruption of
the fluoride complexes. For instance, such a case canoccur as a consequence of the deposition of fluorinerich minerals (such as fluorite or micas such as zinnwaldite, etc., which are abundant in the miarolitic cavities at
Baveno and Cuasso al Monte); this deposition may result from variations of temperature (or pressure), or may
also be due to mixing with an externally derived fluid
phase.
On examining the present results obtained forCuasso al Monte and Baveno, evidence in favor of sucha process is available. First of all, in spite of the chemical similarity among Sc, Y and the REE, bazzite,
cascandite, jervisite, and scandiobabingtonite are always
pure (Mellini & Merlino 1982, Mellini et al. 1982,
Orlandi et al. 1998), and thortveitite is commonly very
pure.
Whereas the structures of the other Sc minerals appear to be selective, that of thortveitite in particular can

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host notable quantities of other elements such as Y, Zr,
and the REE (Table 6) without extensive rearrangement.
This low selectivity is further confirmed by the existence of well-known synthetic and natural REE- or Zrdominant equivalents of this mineral (Smolin &Shepelev 1970, Voloshin et al. 1983, 1985, RoelofsenAhl & Peterson 1989). For this reason, the relative purity of some samples of thortveitite cannot be entirely
ascribed to crystal-chemical reasons, and must reflect
the importance of the chemistry of the depositing solutions.
Since the same process leading to enrichment of Sc
in minerals necessarily leads to its depletion in the depositing solutions, with consequent relative enrichmentof Zr, Y or of the REE, this accounts for the existence inthe same locality of pure and impure varieties ofthortveitite formed at various stages. If other more selective Sc minerals are being formed, the same processleads to the eventual deposition of specific REE minerals, such as complex REE oxides, gadolinite, or evenzircon, of which a Sc-rich variety occurs at Baveno(Gramaccioli et al. 1998).
On these grounds, in the presence of a sufficientconcentration (activity) of fluorides or other ligands inthe depositing solutions, we might expect that thechemical behavior of Sc and of the other elements
should more or less follow the range of the stability
constants of the corresponding complexes. It is interesting to note that in the range of stability constants shown
above, the elements ranging from Al to Be are just thoseoccurring (besides Si) in bazzite, and at least many of
those ranging from Sc (Zr) to Yb are the most significant components of thortveitite or of the others Sc-richspecies from Baveno.
However, in spite of the stability of its complexes,
aluminum does not seem to follow the other elements;
it always remains a minor component of these scandium-rich minerals, although it is difficult to believe that
it is not available in the depositing solutions. Whereas
the selectivity of the crystal structure can account formost instances, bazzite in particular should not be too
selective, its Al-dominant counterpart being beryl.
The remarkably limited content of Al in bazzite wastaken as an indicator of partial miscibility occurring
between this mineral and beryl (Hänni 1980, Weibel etal. 1990, Della Ventura et al. 2000); such a conclusion
is in agreement with early observations by Frondel &Ito (1968), who found that substitution of Fe, Cr, etc.
for Sc, but not for Al, can be extensive but does notextend to the Sc-free end-member composition. Although the existence of a miscibility gap would not contradict our theory concerning the formation of Sc
minerals, the presently available data are not, in our
opinion, sufficient to draw a definitive conclusion, andother possibilities should be considered.
For instance, at thermodynamic equilibrium or nearequilibrium, the activity of the components in bazzite
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nected with the activity of the corresponding chemical
species in the accompanying minerals. Among theseminerals, feldspars are always in close association, andlocally are intergrown with bazzite, as in the Norwegian occurrence (Juve & Bergstøl 1990).
A possible equilibrium between beryl (here considered as a component of a certain mineral of its group)
and an alkali feldspar can be expressed as follows:
Be3Al2Si6O18 + 2K+ + 4 H+ → 2KAlSi3O8+ 3Be2+ + 2H2O
If the activity of KAlSi3O8 is considered to be constant,and the solution is dilute, then:K = exp(–⌬G°/RT ) = [aBe2+]3/([aK+]2[aH+]4[␥.x Be3Al2Si6O18])
where a represents the activities of the corresponding
species in the depositing solution. For the beryl compound Be3Al2Si6O18, the product ␥.x between its activity coefficient and the molar fraction in the crystal has
been used in lieu of the activity. On these grounds, sucha molar fraction depends very strongly upon the pH, and
it is instead (formally) independent of the Al activity inthe depositing solution: in particular, a low value of pH
in the depositing solution prevents the formation of an
Al-rich member of the beryl group, and vice versa.
Therefore, for a low pH, even if a non-negligible
concentration of the Be2+ ion were present in these solutions, no deposition of “common” beryl would takeplace, and if this mineral were present already, it would
be destroyed to deposit feldspar. However, as an alternative, the deposition of another beryl-group mineralcontaining Be3Al2Si6O18 as a minor component could
equally occur, provided the pH and the activity of theBe2+ ion are not too low, the activity of alkali ions is not
too high, and “free ions” of elements replacing Al arepresent in sufficient concentration. Such chemical parameters might not differ extensively in most of theobserved localities yielding bazzite, thereby accounting
for its almost constant composition.
Among these “free ions”, the most important for our
purposes are Sc3+ and Fe3+, which can derive from dissociation of their fluoride complexes and cannot enter
the feldspar structure in substantial amounts. In otherwords, the corresponding equilibrium reaction between
Sc-substituted (or Fe-substituted) beryl:
Be3Sc2Si6O18 + 2K+ + 4 H+ → 2KScSi3O8+ 3Be2+ + 2H2O
cannot proceed beyond a certain point, because the feldspar structure can hardly accommodate scandium (oriron); this crystal-chemical detail involving an accompanying mineral, together with the particular stabilityof the beryl structure, which captures scandium, may
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of other scandium-rich minerals; moreover, this is another reason (apart from the values of the ionic radii)
for explaining the presence of notable amounts of Fe(instead of Al) in all natural specimens of bazzite(Table 5).
The highest Fe content (up to nearly 7 wt% Fe2O3)
observed in bazzite might be interpreted as corresponding approximately to the limit of solubility of this element in the mineral. However, in view of theisomorphism between Sc3+ and Fe3+ in octahedral coordination, it is difficult to imagine that there are limits
for such a substitution. In any case, Frondel & Ito (1968)
synthesized a series of Sc-doped compositions, but they
could not obtain the pure end-member Be3Fe3+2Si6O18.
The synthetic bazzite obtained by these authors contains
a maximum of 1/3 Fe (atomic), although they argued
that the series “may extend to about 50% Fe3+ or somewhat over”. On the contrary, this end-member was actually synthesized by Hänni (1980) from chemical
reaction of the solutions with the steel container, and amineral of a similar composition, stoppaniite, has been
discovered recently (Ferraris et al. 1998, Della Ventura
et al. 2000).
A possible explanation of the conflicting views concerns the oxidation state of iron, which was apparently
not checked by Frondel & Ito in their products. If mostFe is present as Fe2+ instead of Fe3+ (as seems to be thecase with many natural specimens) and only considering the requirements of charge balance, it is quite reasonable to imagine that a limit to the Mg or Fe2+ content
in beryl or bazzite should exist, thereby providing a reason for the limited Fe (and Mg) content of bazzite. This
limit may vary with temperature, or with the presenceof alkali ions in the structure, which favor charge balance. It should be remarked, however, that such a process cannot proceed beyond a certain limit, because ahigh concentration of the alkali ions in the solution(which should be in equilibrium with the corresponding
concentration in the crystal) shifts the above-describedchemical equilibrium in favor of feldspar.
Another possibility might be that the complexes ofFe2+ are less stable than those of Fe3+, and therefore in
reducing conditions the position of iron in the series ofstability constants is not so close to that of scandium. In
any case, the question is still open.The very likely presence of the easily hydrolyzable Mn3+ in the blue varietyof thortveitite is another point in favor of the presenceof notable amounts of fluorides in the depositing solutions (Gramaccioli et al. 1999b, 2000).
A paragenetic sequence of the minerals deposited in
the miarolitic cavities at Baveno and Cuasso al Montehas been reported by Pezzotta et al. (1999) and is in
substantial agreement with our present theory. Following this sequence, at Baveno the deposition of fluoriderich minerals such as especially zinnwaldite led tobreakdown of the Be complexes, enhancing the activityof the free Be2+ ion in the solution and allowing thedeposition of beryllium minerals, such as bavenite,

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CESIAN BAZZITE AND THORTVEITITE, CUASSO AL MONTE, ITALY

bertrandite, gadolinite, or gugiaite; a subsequent deposition of fluorite led to the breakdown of the most stablecomplexes, such as those of Sc, thereby allowing forthe formation of bazzite first (when the concentrationof the Be ion was still reasonably high in the solution),and finally of thortveitite or of the other scandium minerals, such as cascandite, jervisite, or scandiobabingtonite, of which Baveno is the type locality.
According to the same authors, at Cuasso al Montemany late-stage minerals, including Be silicates, were
deposited by late hydrothermal fluids, thereby altering
the sequence observed at Baveno. It should be remarked
that these beryllium minerals at Cuasso al Monte are
not exactly the same found at Baveno: for instance,
bavenite, for which Baveno is the type locality, is altogether absent at Cuasso al Monte.

ACKNOWLEDGEMENTS
We are grateful to the Editor, Prof. Robert F. Martin, as well to Dr. A.U. Falster and an anonymous referee, for a number of substantial suggestions for
improving our manuscript; Professor Henry A. Hänni
kindly provided us with a copy of his Inaugural Dissertation. We are also grateful to Mr. G. Danini, the Curator of Museo Insubrico di Scienze Naturali at Induno
Olona (Varese) for having permitted the use of theunique bazzite specimen from Cuasso al Monte (sample
No. MISN 1300) for our investigation; in addition,
thanks go to Mr. F. Anderbegani, Mr. R. Appiani, Dr.
Paolo Biffi, Dr. Massimo Sbacchi and Mr. Emanuele
Sinelli for having helped in the quest for specimens, andMr. Luigi Saibene for technical help.

1417

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Received February 9, 2000, revised manuscript accepted November 3, 2000.

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