The new tourmaline supergroup mineral dutrowite,
Na(Fe
The mineralogy of the Apuan Alps (Tuscany, Italy) has been studied for more than 150 years, leading to the identification of more than 300 different mineral species, among which 44 have here their type locality (Table 1). The first new mineral species to be discovered was the Cu–Pb sulfosalt meneghinite, described by Bechi (1852) from the Pb–Zn–Ag ore deposit exploited at the Bottino mine. Since then, another 24 minerals belonging to this mineral group have been identified following the mineralogical studies done on the small hydrothermal ore deposits occurring in the Apuan Alps. In these same localities, new oxide minerals were discovered, as well as a suite of secondary species related to the alteration of primary sulfides (mainly pyrite – e.g. Biagioni et al., 2020). Conversely, silicate minerals associated with these ore deposits were usually neglected, the only exception being allanite-(La) by Orlandi and Pasero (2006). It is worth noting that several ore deposits embedded in the Paleozoic basement of the Apuan Alps are spatially associated with tourmalinite bodies (e.g. Benvenuti et al., 1989) and tourmaline-bearing Permian metarhyolite rocks (Vezzoni et al., 2018). However, few data on tourmaline supergroup minerals from the Apuan Alps are available (Benvenuti et al., 1991; Mauro et al., 2022).
New mineral species from the Apuan Alps (Tuscany, Italy).
In the framework of an ongoing study of the crystal chemistry of non-pegmatitic tourmaline supergroup minerals from Tuscany (Italy), tourmaline samples from Permian metarhyolite exposed in the Fornovolasco area (southern Apuan Alps) were studied. Preliminary chemical analyses indicated the occurrence of a Ti-rich tourmaline supergroup mineral deserving additional investigations. Further studies confirmed the preliminary results, and the mineral dutrowite, as well as its name, were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMNC-IMA) under voting number 2019-082. The name honours Barbara Lee Dutrow (born 1956), Adolphe G. Gueymard professor at Louisiana State University, for her contributions to the understanding of the chemical variability of tourmaline supergroup minerals, as well as on the petrologic significance of staurolite. Holotype material is deposited in the mineralogical collection of the Museo di Storia Naturale, University of Pisa, Via Roma 79, Calci (Pisa), under catalogue number 19890.
This paper describes the new mineral species dutrowite, discussing its crystal chemistry and its possible petrologic significance.
Dutrowite was identified in a sample from the outcrop of the Fornovolasco
Metarhyolite Formation (Pieruccioni et al., 2018) close to the Boscaccio
locality (44
Dutrowite was found as chemically homogeneous domains within anhedral to
subhedral crystals up to 1 mm in size (Fig. 1); domains of dutrowite can
reach up to 0.5 mm across. Colour is brown, with a light-brown streak.
Dutrowite is transparent, with a vitreous lustre. It is brittle, with an
irregular fracture. Hardness was not measured, but it should be about
7–7.5, by analogy with other members of the tourmaline
supergroup. Calculated density, based on the empirical formula and unit-cell
parameters from single-crystal X-ray diffraction, is 3.203 g cm
Photomicrograph of dutrowite in plane-polarized light. Symbols after Warr (2021): Ap – “apatite”; Bt – “biotite”; Dtw – dutrowite; Kfs – K-feldspar; Odrv – oxy-dravite; Qz – quartz. “Biotite” is partially altered in “chlorite” and rutile, whereas K-feldspar is sericitized. Red box corresponds to the area where the grain used for single-crystal X-ray diffraction was extracted. Type material. Catalogue number 19890, Museo di Storia Naturale, Università di Pisa.
In type material, dutrowite is associated with quartz, altered feldspars (both albite and K-feldspar), Mg-rich annite (hereafter “biotite”), Fe-rich clinochlore, muscovite, and minor “apatite”, ilmenite, rutile and zircon. Moreover, it is associated with domains of Fe-rich oxy-dravite.
Micro-Raman spectra were obtained on the sample of dutrowite shown in Fig. 1
in nearly back-scattered geometry with a Jobin-Yvon Horiba XploRA Plus
apparatus, equipped with a motorized
Quantitative chemical analyses of dutrowite and associated oxy-dravite were
done using a Superprobe JEOL JXA 8200 electron microprobe (WDS mode, 15 kV,
10 nA, 1
Chemical data (in wt %) for dutrowite and associated oxy-dravite.
Note:
The
Mössbauer parameters for tourmaline (dutrowite and associated oxy-dravite) collected at room temperature.
Intensity data were collected using a Bruker Apex II diffractometer equipped
with a Photon II CCD area detector and graphite-monochromatized
Mo
Summary of parameters describing data collection and refinement for dutrowite.
The micro-Raman spectrum of dutrowite is displayed in Fig. 2. The Raman
shift of the observed bands is shown, as obtained through fit profile using
Fityk (Wojdyr, 2010). In the region between 100 and 1200 cm
Raman spectrum of dutrowite
Watenphul et al. (2016a, b) discussed the relations between band positions
and crystal chemistry of the studied tourmalines. In particular, the
vibrational modes of
Fractional atom coordinates and displacement parameters of dutrowite are
reported in Table 5, and selected bond distances are given in Table 6.
Following Henry et al. (2011), the empirical ordered formula of dutrowite is
Mössbauer spectrum of tourmaline supergroup minerals
(dutrowite and associated oxy-dravite). Fitted absorption doubled assigned
to Fe
Sites, Wyckoff positions, site occupancies (s.o.), fractional atom
coordinates, and isotropic (
Selected bond distances (in Å) for dutrowite.
The empirical structural formula of dutrowite was optimized using the method
of Wright et al. (2000), distributing cations among the
Refined and calculated site-scattering (s.s., in electrons per formula unit) and proposed site population (in atoms per formula unit) for dutrowite.
Table 7 compares the site scattering and the proposed site populations.
Bond-valence calculations, weighted according to the optimized site
populations and obtained using the bond-valence parameters of Brese and
O'Keeffe (1991), are reported in Table 8. There is an excellent match
between observed and refined site scattering, i.e. 142.63 vs. 142.20
electrons per formula unit, respectively. Moreover, the
Weighted bond valence (in valence units) in dutrowite.
Note: left and right superscripts indicate the number of equivalent bonds involving anions and cations, respectively.
X-ray powder diffraction data (
Owing to the small size of the available material, no X-ray powder diffraction pattern of dutrowite was collected. The calculated X-ray powder diffraction pattern, based on the structural model given in Table 5, is reported in Table 9.
Dutrowite is the first tourmaline supergroup mineral having Ti (
Henry and Dutrow (1996), reviewing the contents of minor chemical
constituents in tourmaline supergroup minerals, reported up to 4.07 wt %
TiO
Titanium-rich tourmaline supergroup minerals have been identified in the
Třebíč pegmatites, Manjaka and Řečice, in the Czech
Republic, with Ti contents up to 0.55, 0.59, and 0.64 apfu, respectively
(Novák et al., 2011; Gadas et al., 2019). Konzett et al. (2012) measured
up to 3.42 wt % TiO
Titanium was reported by Dutrow and Henry (2022) in tourmaline from an
(anhydrite–gypsum)-bearing meta-evaporite sampled in the Arignac Gypsum
Mine, France; one spot analysis, having Ti
Several substitution mechanisms have been proposed to explain the occurrence
of Ti in the crystal structure of tourmaline supergroup minerals. Some
authors have argued the Ti substitution at the
In addition to chemical data reported in Table 1, collected on the area
where the grain of dutrowite used for the single-crystal X-ray diffraction
study was extracted, other spot analyses were done on the other domains
having brown and blue absorption colours in transmitted light microscopy
(Fig. 1), respectively. All spot analysis data are deposited in the
Supplement. Brown domains are usually enriched in Ti, in
agreement with previous results obtained, for instance by da Fonseca-Zang
et al. (2008). The blue domain, whose average chemical composition is given
in Table 2, has the following empirical ordered formula:
Figure 4 shows the X-ray maps for selected elements in dutrowite and
associated oxy-dravite. Whereas Na is homogeneously distributed between
these two phases, Ca is depleted in the core of the crystal; the X-ray map
collected using Ca
Chemical variability of dutrowite and associated oxy-dravite is shown in
Fig. 5. In agreement with Novák et al. (2011), considering Al
X-ray maps collected on dutrowite and associated oxy-dravite (see Fig. 1).
Relations between chemical constituents in dutrowite (brown circles) and associated oxy-dravite (blue circles).
These substitution mechanisms should favour an increase in the unit-cell
parameters, indeed, in agreement with the ionic radii of Shannon (1976),
Dutrowite occurs in the groundmass of the porphyritic metarhyolite belonging
to the Fornovolasco Metarhyolite Formation. Actually, it was identified in
an unusual sample, characterized by the presence of “biotite”. This is a
rare mineralogical feature of the Fornovolasco Metarhyolite, since the
relics of “biotite” phenocrysts are usually completely replaced by Fe-rich
clinochlore, quartz, and rutile (Vezzoni et al., 2018). Textural data
indicating the relations of dutrowite with oxy-dravite are limited to a few
observations, and the relations between these two phases are not clear. For
instance, some zoned crystals, cut orthogonal to the
As discussed above, “biotite” is a host for Ti in metarhyolite and may
have played some role in the crystallization of dutrowite. Some authors
(e.g. Dini et al., 2008; Gadas et al., 2019; Nabelek, 2021) stressed the
role of this mica as a source of Ti for Ti-enriched tourmaline. Boron
metasomatism and crystallization of tourmaline as a replacement of earlier
magmatic silicates is well-known during the late-magmatic and
early-hydrothermal evolution of several granitic intrusions (e.g. Woodford
et al., 2001; Dini et al., 2008). These processes affected also the
Fornovolasco Metarhyolite and could be related to the pre-Alpine
tourmalinization and sericitization processes described by Vezzoni et al. (2020). Biotite from metarhyolite has
Vereshchagin et al. (2022), on the basis of synthesis experiments, suggested that low-P conditions are favourable to Ti enrichment in tourmalines. As regards temperature conditions, a comparison with the Ti enrichment in other silicates can be proposed. For instance, “biotite” is enriched in Ti at low-P (as for tourmaline) and high-T (Henry et al., 2005). If so, dutrowite could be the result of the high-T/low-P replacement of “biotite” in the Fornovolasco Metarhyolite during the late-magmatic/hydrothermal evolution of this Permian intrusive rock.
The oxy-nature of both dutrowite and oxy-dravite does not necessarily imply
an oxidizing geological environment, which would not be in accord with the
local precipitation of pyrite and other sulfides in metarhyolite. As
observed in some oxy-tourmalines (e.g. Bačík et al., 2013), the
deprotonization reaction could be simply due to local charge-balance
requirements related to high-charged cations (e.g. Al
Dutrowite is the first tourmaline supergroup mineral with Ti as a species-forming chemical constituent. Its finding and description improve the knowledge of the crystal chemistry of this important group of cyclosilicates and give some insights into the enrichment of Ti in these minerals, suggesting substitution mechanisms and the role of “biotite” as a source of Ti during the late-stage evolution of the magmatic–hydrothermal system associated with the emplacement of the Fornovolasco Metarhyolite Formation.
Available data also encourage a more accurate petrological study on this recently described geological formation (Pieruccioni et al., 2018), focusing on the tourmaline supergroup minerals, which show wide chemical variability. Their characterization may help in deciphering the evolution of this sector of the northern Apennines during both the Permian magmatic–hydrothermal history and the subsequent Alpine tectono-metamorphic events.
The Crystallographic Information File data of dutrowite are available in the Supplement. Additional chemical data of dutrowite and associated oxy-dravite are also made available.
The supplement related to this article is available online at:
CB collected preliminary data. CB and DM carried out single-crystal X-ray diffraction and micro-Raman spectroscopy; CB and FB examined crystal–chemical data. FZ collected electron microprobe data. HS collected Mössbauer data. AD contributed to the geological background. CB, DM, and FB wrote the paper, with inputs from the other authors.
At least one of the (co-)authors is a member of the editorial board of
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This article is part of the special issue “New minerals: EJM support”. It is not associated with a conference.
Edward Grew is acknowledged for useful discussion. Yuri Galanti is thanked for field assistance during dutrowite sampling. The University Centrum for Applied Geosciences (UCAG) is thanked for the access to the E.F. Stumpfl electron microprobe laboratory. Frank Hawthorne and an anonymous reviewer are thanked for their comments.
This research received support from the Ministero
dell'Istruzione, dell'Università e della Ricerca through the project
PRIN 2020 “HYDROX – HYDRous- vs. OXo-components in minerals: adding new
pieces to the Earth's H
This paper was edited by Sergey Krivovichev and reviewed by Frank Hawthorne and one anonymous referee.