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Eur. J. Mineral., 32, 355–365, 2020
https://doi.org/10.5194/ejm-32-355-2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
Laurentthomasite, Mg2K(Be2Al)Si12O30: a new
milarite-group-type member from the Ihorombe region,
Fianarantsoa Province, Madagascar
Cristiano Ferraris1, Isabella Pignatelli2, Fernando Cámara3, Giancarlo Parodi1, Sylvain Pont1,
Martin Schreyer4, and Fengxia Wei5
1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590,
Muséum National d’Histoire Naturelle, CP 52, 61 rue Buffon, 75005 Paris, France
2Laboratoire GeoRessources, Université de Lorraine, Faculté des Sciences et Technologies, Rue Jacques Callot,
BP 70239, 54506 Vandœuvre-lès-Nancy, France
3Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano,
Via Mangiagalli, 34 – 20133 Milan, Italy
4Malvern Panalytical B.V., Lelyweg 1 (7602 EA), P.O. Box 13, 7600 AA Almelo, the Netherlands
5Institute of Materials Research and Engineering (IMRE),
2 Fusionopolis Way, Innovis, #08-03, 138634 Singapore
Correspondence: Cristiano Ferraris (ferraris@mnhn.fr)
Received: 12 February 2020 – Revised: 6 May 2020 – Accepted: 14 May 2020 – Published: 17 June 2020
Abstract. Laurentthomasite, ideally Mg2K(Be2Al)Si12O30, is a new milarite-group member found within
quartz-syenite pegmatites from the Ihorombe region, Fianarantsoa Province, Madagascar. It occurs as euhedral
{0001} hexagonal crystals, maximum 15 mm large and 5 mm thick. The crystals show a very strong dichroism
with cobalt blue and green-yellow colours when observed along [0001] and [1000], respectively. The mineral
is transparent, uniaxial (+) and its lustre is vitreous. The hardness is about 6 (Mohs scale), showing a poor
{0001} cleavage, irregular to conchoidal fracture, and a measured density of 2.67(8)g cm−3. Laurentthomasite
is hexagonal, space group P6/mcc (no. 192), with a=9.95343(6)Å, c=14.15583(8)Å, V=1214.54(1)Å3
and Z=2. The strongest nine lines in the X-ray powder diffraction pattern [din Å – (I) – hkl] are 3.171 – (10)
– 211, 4.064 – (8) – 112, 2.732 – (8) – 204, 4.965 – (6) – 110, 2.732 – (4) – 204, 3.533 – (3) – 004, 7.055 – (2)
– 002, 4.302 – (2) – 200 and 3.675 – (2) – 202. Chemical analyses by electron microprobe and several spectro-
scopies (inductively coupled plasma, ICP; optical emission, OES; mass, MS; and Mössbauer) give the following
empirical formula based on 30 anions per formula unit: (Mg0.86 Sc0.54 Fe2+
0.35 Mn0.26)P=2.01(K0.89 Na0.05 Y0.02
Ca0.01 Ba0.01)P=0.98[(Be2.35 Al0.50 Mg0.11 Fe3+
0.03)P=2.99(Si11.90 Al0.10)O30]; the simplified formula is (Mg,
Sc)2(K, Na)[(Be, Al, Mg)3(Si, Al)12O30]. The crystal structure was refined to an Rindex of 1.89 % based on
430 reflections with Io>2σ (I ) collected on a four-circle diffractometer with CuKα radiation. By comparison
with the general formula of the milarite group, A2B2C[T(2)3T(1)12O30](H2O) x(0 < x < n, with n < 2 pfu,
per formula unit), the laurentthomasite structure consists of a beryllo-alumino-silicate framework in which the
T(1) site is occupied by Si and minor Al and forms [Si12O30] cages linked by the T(2) site mainly occupied
by (Be +Al). The Aand Csites occur in the interstices of the framework while the Bsite is vacant. The origin
of the strong dichroism is related to a charge transfer process between Fe2+and Fe3+in octahedral Asites and
tetrahedral T(2) sites, respectively.
Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC.
356 C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30
1 Introduction
Besides economic reasons, Madagascar’s pegmatites are
amongst the best research fields for mineral collectors be-
cause of both the size and the aesthetic characteristics of
many different mineral species. Although there are more than
370 valid mineral species from approximately 1000 different
localities in Madagascar (Mindat, 2020a), including a rela-
tively large number of new mineral species containing the
light elements Li, Be or B (Table 1), no new species belong-
ing to the milarite group have been described. Milarite and
osumilite have been reported in the central and southern re-
gions of Amoron’i Mania and Anosy (Pezzotta, 2005; Holder
et al., 2018), but laurentthomasite, Mg2K(Be2Al)Si12O30, is
the first mineral species belonging to the milarite group for
which Madagascar is the type locality.
Laurentthomasite Mg2K(Be2Al)Si12O30 is the anhydrous
Mg-dominant analogue of milarite Ca2K(Be2Al)Si12O30 and
was approved as a new mineral species in April 2019 by the
Commission on New Minerals, Nomenclature and Classifi-
cation (CNMNC) of the International Mineralogical Associ-
ation (IMA) (2018–157) as a member of the milarite group
(9.CM.05 in the classification of Strunz and Nickel, 2001).
The general formula of minerals belonging
to the milarite group (Forbes et al., 1972) is
VIAIX
2BXII
2C[IVT(2)IV
3T(1)12O30](H2O)xwith x=0−n
(n < 2 pfu – Gagné and Hawthorne, 2016a), with the follow-
ing known site occupancies: A= Al, Fe3+, Sn4+, Mg, Zr,
Fe2+, Ca, Na, Y or Sc; B=, Na, K or H2O; C=, Na, K or
Ba; T(1) =Al or Si; and T(2) =Li, Be, B, Mg, Al, Si, Mn,
Fe2+or Zn. Milarite-group minerals are double-ring silicates
having maximum topological symmetry corresponding to
space group P6/mcc, although cation ordering may lead to
lower symmetry.
Together with the recent description of aluminosugilite
(Nagashima et al., 2020), laurentthomasite attests to, once
more, the great compositional flexibility of the milarite struc-
ture type; at the present time 25 members belong to this
group, with the number constantly increasing from 15 in
1991 (Hawthorne et al., 1991) up to 23 in 2016 (Gagné and
Hawthorne, 2016a).
Laurentthomasite captured our attention whilst analysing
a set of gemstones coming from the south of Madagascar.
Its dichroism going from deep blue to green yellow together
with prismatic hexagonal crystals was a source of perplexity
for collectors, since it could be confused with both corun-
dum sapphire and/or cordierite, mineral species much more
common in the Madagascan southern provinces. Both sap-
phire and cordierite were rejected based on hardness mea-
surements: 6 vs. 7.5 and 9 (Mohs scale), respectively. On the
other hand, the first results of chemical investigations show-
ing the presence of elements such as Sc, K and Be stimu-
lated our interest. Scandium-rich minerals are very rare: only
15 terrestrial mineral species have scandium as an important
constituent, while another five are of extraterrestrial origin
(Mindat, 2020b)
The name laurentthomasite honours Laurent Thomas, born
1971 in Tours (Centre-Val de Loire, France). He has been a
very active geologist, prospector and mineral dealer since the
early 1990s, especially for African areas such as Madagas-
car. He is the one who first brought to the public knowledge
some new species from Madagascar such as pezzottaite from
Mandrosonoro, as well as new localities such those of grandi-
dierite from Tranomaro, euclase from Itasy and chrysoberyl
from Tsitondroina (see Lefevre and Thomas, 1998; Forner et
al., 2001). The new mineral species and its name were ap-
proved by the Commission on New Minerals, Nomenclature
and Classification, International Mineralogical Association
(IMA 2018–157).
The laurentthomasite description is based on one holo-
type and two cotype specimens, which are deposited
in the collections of the Muséum National d’Histoire
Naturelle (MNHN) of Paris (France); catalogue num-
bers are MNHN_MIN_218.1_a for the holotype and
MNHN_MIN_218.1_b and MNHN_MIN_218.1_c for co-
types.
2 Geological settings
The studied samples were collected by Laurent Thomas
in the Ihorombe region (Fianarantsoa Province) at about
80 km north-east of the village of Betroka (23◦1600300 S,
46◦0504900 E), in southern Madagascar. The area is located
in the north-eastern part of the Ranotsara shear zone in the
Antananarivo block, one of the five tectonic units into which
Madagascar is divided (Collins et al., 2000; Collins and
Windley, 2002). The Antananarivo block consists of 2550–
2500 Myr granitoids tectonically interlayered with granites,
syenites and gabbros (Tucker et al., 1999; Kröner et al.,
2000). The whole of the Antananarivo block was ther-
mally and structurally reworked between ∼750 and 500Myr
(Collins and Windley, 2002), with pre-existing rocks being
metamorphosed to granulite facies and with the development
of gneissic fabrics. Magmatism at circa 550 Myr produced
granitoids bodies metres to kilometres across that are char-
acteristic of this tectonic unit. Laurentthomasite was found
within a pegmatite (coeval with these granitoid bodies) in
a series of biotite- and amphibole-rich gneisses as well as
migmatites and a set of pyroxene-rich alkali syenites (Be-
sairie, 1959; Tucker et al., 1999; Kröner et al., 2000).
Laurentthomasite is associated with orthoclase, massive
quartz, rare pale green apatite crystals, phenakite, beryl, al-
bite, magnetite, thortveitite and cheralite (Fig. 1): at the mi-
croscopic scale unknown phases (currently under investiga-
tion) containing different ratios of Nb, Ta and W are also
present. Due to the extremely weathered conditions of the
few outcrops, pegmatite samples containing laurentthoma-
site are rare and deeply weathered into laterite; so far only
Eur. J. Mineral., 32, 355–365, 2020 https://doi.org/10.5194/ejm-32-355-2020
C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 357
Table 1. Approved mineral species containing Li, Be and/or B from Madagascar.
Mineral Formula Reference∗
Béhierite (Ta5+, Nb5+)(BO4) Fleischer (1962)
Bityite LiCaAl2(AlBeSi2O10)(OH)2Lacroix (1908)
Fluor-liddicoatite Ca(Li2Al)Al6(Si6O18)(BO3)3(OH)3F Dunn et al. (1977)
Grandidierite (Mg,Fe2+)(Al,Fe3+)3(SiO4)(BO3)O2Lacroix (1902)
Laurentthomasite Mg2K(Be2Al)Si12O3This study
Londonite (Cs,K,Rb)Al4Be4(B,Be)12O28 Simmons et al. (2001)
Manandonite Li2Al4(Si2AlB)O10(OH)8Lacroix (1912)
Pezzottaite Cs(Be2Li)Al2(Si6O18) Laurs et al. (2003)
Schiavinatoite (Nb,Ta)(BO4)Demartin et al. (2001)
Vránaite Al16B4Si4O38 Cempírek et al. (2016)
∗References are those of the “The New IMA List of Minerals” updated to March 2020, available on the website
(http://cnmnc.main.jp/, last access: 6 May 2020) of the Commission on New Minerals, Nomenclature and
Classification (CNMNC) of the International Mineralogical Association (IMA).
Figure 1. (a) Blue hexagonal crystal of laurentthomasite in par-
tially weathered pegmatite mainly composed of quartz, orthoclase,
phenakite and secondary apatite. (b) Enlargement of (a).
one crystal in a matrix has been found, whilst isolated speci-
mens are more common.
The genesis of the new mineral laurentthomasite is related
to crystallization of Sc-enriched pegmatites like those occur-
ring at the Tørdal pegmatite field in southern Norway (Stef-
fenssen et al., 2019). In contrast to the Tørdal bodies, the
Madagascan pegmatite where laurentthomasite was found
does not have any garnet that could be a major host for scan-
dium.
3 Appearance and physical properties
The sample material consists of tabular euhedral {0001}
hexagonal crystals with maximum width and thickness val-
ues of 15 and 5 mm, respectively (Fig. 1a–b); all observed
laurentthomasite crystals are characterized by dark micro-
metric inclusions of elongated crystals of an Fe2+-rich phase
as confirmed by Mössbauer spectroscopy. Observed forms
are {1010} and {0001}. Crystals show a very strong dichro-
ism with cobalt blue and green-yellow colours when ob-
served along [0001] and [1000], respectively (Fig. 2a–b).
Figure 2. Images showing the strong dichroism of laurentthoma-
site crystals. Under white light, crystals are deep cobalt blue along
[0001] and yellow-green perpendicular to [0001].
The origin of dichroism lies in the presence of the transi-
tion metal ions Fe2+and Fe3+, located in octahedral Asites
and tetrahedral T(2) sites, respectively, and inter-valence
charge transfer between them. Twinning was not observed.
The mineral has a light blue streak and is transparent and
non-fluorescent. Its lustre is vitreous. The hardness is about
6 (Mohs scale) based on scratching tests, with poor cleavage
parallel to {0001}. The tenacity is brittle with irregular to
conchoidal fracture. Laurentthomasite has a measured den-
sity of 2.67(8) g cm−3(hydrostatic balance) and a calculated
density of 2.66(4) g cm−3using the empirical formula.
In our 23 µm thick thin section, the mineral is colourless
and pleochroism is not visible; it is optically uniaxial (+),
with ω=1.540(2) and ε=1.545(3) (λ=589 nm) measured
by refractometry; and dispersion was not observed. The com-
patibility index (1 – KP/KC)=0.028 is rated as excellent
(Mandarino, 1981).
https://doi.org/10.5194/ejm-32-355-2020 Eur. J. Mineral., 32, 355–365, 2020
358 C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30
Figure 3. Deconvoluted Mössbauer spectrum of laurentthomasite.
4 Analytical methods
For elements heavier than carbon, wavelength-dispersive X-
ray spectroscopy (WDS) 40-point analyses were performed
with an electron microprobe CAMECA SX100 (Service
d’Analyse Microsonde Camparis – Université Pierre et Marie
Curie, Paris), at an accelerating voltage of 15kV, a probe
current of 10 nÅ and a beam diameter of 5µm. Analysed ele-
ments and standards were Si, Mg and Ca (diopside); Al and K
(orthoclase); Sc (synthetic Sc2O3); Ti and Mn (pyrophanite);
Fe (hematite); Zn (sphalerite); Ba (baryte); and Na (albite).
The polished section of the analysed crystal was free of in-
clusions, at least to the depth reached by the electron beam.
Both beryllium and trace elements (including rare earth el-
ements, REEs) were analysed using about 0.1 g of pow-
dered sample by inductively coupled plasma optical emission
spectrometry (ICP-OES) and mass spectrometry (ICP-MS)
at the SARM laboratory of CRPG-CNRS. The amount of
Fe3+within the structure was deduced from Mössbauer spec-
troscopy (Fig. 3). The miniaturized Mössbauer spectrome-
ter (MIMOS II) developed by Klingelhöfer et al. (1996) at
the LCPME laboratory (Université de Lorraine, France) was
used to investigate the oxidation state of iron in laurentthom-
asite samples.
Raman spectra (Fig. 4) were obtained using a modified
Princeton Instruments spectrometer at the IMPMC (Insti-
tut de minéralogie, de physique des matériaux et de cos-
mochimie, Université Pierre et Marie Curie, Paris, France).
The instrument is synchronized with a nanosecond pulsed
diode pumped solid state (DPSS) laser operating at 532 nm
with a 1.5 ns duration for the pulse, a 10 to 2000 Hz repetition
rate and up to 1 mJ output energy per pulse. The spectrome-
ter was synchronized and optimized to collect the signal only
during the laser pulse in order to minimize the luminescence
background.
Figure 4. Raman spectrum of laurentthomasite compared to those
of milarite and osumilite.
Powder diffraction X-ray data were collected between 4
and 100◦(CuKα1-2-circle θ-θgoniometer) using a Bragg-
Brentano X’pert Pro MPD diffractometer at the IMPMC.
A single-crystal (39.6 µm ×28.7µm ×16.9µm) was anal-
ysed using an X-ray diffraction four-circle Agilent Super-
Nova instrument at the Institute of Materials Research and
Engineering of Singapore (CuKα radiation); the data were
collected in the 2θrange 0.279 to 113.785◦at 10 s exposure
for each frame (0.029◦width) and integrated by the software
CrysAlisPro; empirical absorption correction was applied.
5 Results
5.1 Chemical analyses
Tables 2 and 3 show the chemical composition of laurent-
thomasite. The deconvoluted experimental Mössbauer spec-
trum shows the presence of two types of octahedral Fe2+:
the blue and the green curves belong to the laurentthomasite
and the inclusions mentioned above, respectively (Fig. 3). As
shown in Table 4 the Fe2+and Fe3+relative contents of lau-
rentthomasite are 92.0 % and 5.8 %, respectively. Thus, af-
ter normalizing to 100 %, Fe2+constitutes 94 % of the Fe
in laurentthomasite itself. The violet doublet represents the
tetrahedral Fe3+contained within laurentthomasite; the cen-
tre shifts (CS), quadrupole splitting (QS) and relative areas
are reported in Table 4.
In Fig. 4 the Raman spectrum of laurentthomasite is com-
pared to those of milarite and osumilite (Lafuente et al.,
2015). The spectra are very similar in having the most intense
Eur. J. Mineral., 32, 355–365, 2020 https://doi.org/10.5194/ejm-32-355-2020
C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 359
Table 2. WDS and ICP-OES-MS chemical analyses of laurentthom-
asite.
Constituent Wt. % Range SD Probe standard
SiO273.10 71.29–74.90 0.41 Diopside
Al2O33.11 3.03–3.20 0.05 Orthoclase
Sc2O33.78 3.70–3.86 0.05 Synthetic Sc2O3
Y2O3∗0.22
TiO20.02 0.02–0.019 0.01 Pyrophanite
FeO 2.69 2.50–2.67 0.07 Hematite
Fe2O3∗∗ 0.19
MnO 1.91 1.77–2.04 0.07 Pyrophanite
MgO 4.01 3.80–4.09 0.07 Diopside
ZnO 0.04 0.01–0.07 0.03 Sphalerite
CaO 0.07 0.06–0.08 0.01 Diopside
BaO 0.16 0.09–0.23 0.04 baryte
BeO* 6.02
Na2O 0.15 0.10–0.18 0.02 albite
K2O 4.30 3.92–4.67 0.03 orthoclase
Total 99.77
∗Measured by ICP-MS; the analytical uncertainty for both Y and Be contents is <
5 %. ∗∗ The amount of Fe2O3was calculated starting from analysed FeO and the
results of Mössbauer spectroscopy.
peak around 480 cm−1and comparable to the known spectra
of other milarite-group minerals like brannockite, chayesite,
or poudretteite (Lafuente et al., 2015) or friedrichbeckeite,
and almarudite (Lengauer et al., 2009). Differences in Ra-
man shifts of individual bands are due to the chemical varia-
tions, bond geometries and bond strengths for different min-
eral species. Figure 4 clearly shows that the laurentthomasite
spectrum shares more affinities with the osumilite than with
milarite; the most pronounced difference between the three
spectra is the splitting of bands around 280 cm−1, with an
evident separation into two components with maxima at 263
and 288 cm−1for laurentthomasite.
The empirical formula based on 30 anions pfu (per for-
mula unit) is (Mg0.86 Sc0.54 Fe2+
0.35 Mn0.26)P=2.01 (K0.89
Na0.05 Y0.02 Ca0.01 Ba0.01)P=0.98 [(Be2.35 Al0.50 Mg0.11
Fe3+
0.03)P=2.99 (Si11.90 Al0.10) O30]. The simplified formula
is (Mg, Sc)2(K, Na)[(Be, Al, Mg)3(Si, Al)12O30], while the
ideal formula is Mg2K(Be2Al)Si12O30, requiring K2O 4.96,
BeO 5.27, MgO 8.49, Al2O35.37 and SiO275.92 for a total
of 100 wt. %.
5.2 Crystallographic data
The cell parameters obtained from Rietveld refinement of
the X-ray powder diffraction data (HighScore suite program,
Degen et al., 2014) (Fig. 5; Table 5) are a=9.95343(6) Å,
c=14.15583(8) Å, V=1214.54(1) Å3and Z=2. Parame-
ters used for the Rietveld refinement were (a) the 0 shift fixed
at 0, (b) the background modelled using a flat and a 1/X
background as well as five parameters of a Chebyshev func-
tion, (c) isotropic displacement parameters for each atomic
positions, (d) atomic occupancies for the non-oxygen posi-
tions, (e) the peak shape modelled using a pseudo-Voigt func-
Table 3. Trace element contents of laurentthomasite.
Elem. µg g−1Uncertainty
As 36.6 <10 %
Bi 0.07 <20 %
Cd 0.07 <20 %
Co 3.49 <20 %
Cr 5.5 <10 %
Cs 82.9 <5 %
Cu 3.0 <20 %
Ga 37.3 <10 %
Ge 2.15 <10 %
Hf 6.59 <10 %
In 7.72 <15 %
Mo n.d*. <D.L.**
Nb 2.99 <10 %
Ni 15.3 <5 %
Pb 0.85 <20 %
Rb 1405 <1 %
Sb 0.57 <20 %
Sn 2.36 <20 %
Sr 21.2 <15 %
Ta 2.02 <10 %
Th 0.87 <20 %
U 0.70 <20 %
V 3.4 <15 %
W n.d* <D.L.**
Y 1754 <1 %
Zn 176 <5 %
Zr 7.81 <15 %
La 1.50 <20 %
Ce 3.62 <15 %
Pr 0.749 <20 %
Nd 3.88 <15 %
Sm 3.07 <10 %
Eu 0.239 <15 %
Gd 5.56 <10 %
Tb 2.32 <15 %
Dy 29.4 <10 %
Ho 12.8 <10 %
Er 91.2 <5 %
Tm 43.4 <5 %
Yb 922 <1 %
Lu 321 <1 %
∗n.d: not detected. ∗D.L.: detection limit.
tion with Finger–Cox–Jephcoat (FCJ) asymmetry (Finger et
al., 1994) rather than a fundamental parameter approach so
the microstructural parameters like size and strain were not
refined, and (f) the preferred orientation was not refined.
Single crystal X-ray diffraction investigations show that
laurentthomasite belongs to the hexagonal crystal sys-
tem with space group P6/mcc (no. 192) and unit cell
parameters a=9.95800(7) Å, c=14.114916(11) Å, V=
1215.081(15) Å3and Z=2 (Table 6). The structure of lau-
rentthomasite was refined using SHELXL-2012 (Sheldrick,
https://doi.org/10.5194/ejm-32-355-2020 Eur. J. Mineral., 32, 355–365, 2020
360 C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30
Figure 5. Rietveld refinement of the X-ray powder diffraction pattern of laurentthomasite.
Table 4. Parameters from fitting of the Mössbauer spectrum.
Element CS (mm s−1) QS (mm s−1) RA (%)
Fe(II) octaLaur 1.21 2.31 92.0
Fe(III) tetraLaur 0.25 0.71 5.8
Fe(II) octaIncl 0.85 1.9 2.2
Incl: inclusions. Laur: laurentthomasite. CS: centre shift. QS: quadrupole splitting.
RA: relative areas.
2015) starting from the atom coordinates of oftedalite
(Cooper et al., 2006). Scattering curves of fully ionized
species were used at cation sites (Rossi et al., 1983;
Hawthorne et al., 1995): for the T(1) site ionized Si was re-
fined vs. neutral Si; according to information from chemical
analyses and also the fact that Fe and Mn have very similar
scattering factors, Fe2++Mn2+(0.61 apfu, atoms per for-
mula unit) was constrained to be equal to Sc3+(0.54 apfu)
and then refined versus Mg2+at the Asite. Neutral vs. ion-
ized scattering curves were used at oxygen sites (Rossi et al.,
1983; Hawthorne et al., 1995). The Fourier difference map
did not reveal any maximum above 0.29 e−Å3. Anisotropic
full-matrix least-squares refinement on F2
oyielded R1=
1.89 % [430 reflections with Io>2σ I ] and Rall =1.89 %
(431 reflections). Experimental details are reported in Ta-
ble 6. Refined atom coordinates and equivalent isotropic dis-
placement parameters are reported in Table 7. Selected in-
teratomic distances and bond angles are given in Table 8. A
Crystallographic Information File (CIF) and list of observed
and calculated structure factors are available as electronic
material. The structure is isotypic with minerals of the mi-
larite group (Gagné and Hawthorne, 2016a).
6 Discussion
As discussed by Gagné and Hawthorne (2016a), the struc-
ture of the milarite-group minerals is a beryllo-alumino-
silicate framework consisting of a four-connected three-
dimensional net.
The majority of the minerals belonging to the milarite
group can be described with space-group symmetry P6/mcc
(Gagné and Hawthorne, 2016a), which is reduced to P62c
and Pnc2 because of cation ordering in respectively roed-
derite and armenite (Armbruster, 1989, 1999).
The main feature of the laurentthomasite structure is a
[T(1)12O30] unit consisting of double six-membered rings
of corner-linked SiO4tetrahedra (Fig. 6). These rings are
stacked along the caxis, thus establishing a channel sys-
tem along [0001]. These units are interconnected by shar-
ing corners with T(2)O4tetrahedra. These obtained struc-
tural blocks are arranged in groups of three around one cen-
trally located AO6octahedron. The Csite is situated in the
centre of the channel between two [T(1)12O30] units (Fig. 7).
Laurentthomasite presents the charge arrangement [10]
of Gagné and Hawthorne (2016b). It is related to of-
tedalite (ScCaKBe3Si12O30; Cooper et al., 2006) by the
(AScT (2)Be)−1(AMgT (2)Al) and ACaA
−1Mg substitutions,
to milarite (Ca2KBe2AlSi12O30) by the ACaA
−2Mg2sub-
stitution, and to friedrichbeckeite (Mg2NaKBe3Si12O30;
Lengauer et al., 2009) by the (BNaT (2)Be)−1(BT (2)Al) sub-
stitution.
Hawthorne et al. (1991) observed < T -O > distances
of 1.609–1.612 Å for several milarite crystals with T (1)=
(Si11.89-Si12): mean value of 1.611Å. Anyway, the observed
< T (1)–O >distance, 1.608(2) Å (Table 8), is compatible
with the full occupancy of Si at the T(1) site, and it compares
well, within experimental error, with the distances observed
Eur. J. Mineral., 32, 355–365, 2020 https://doi.org/10.5194/ejm-32-355-2020
C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 361
Table 5. X-ray powder diffraction data for laurentthomasite.
I d-obs [Å] d-calc [Å] h k l
2 7.055 7.078 0 0 2
1 5.457 5.470 1 0 2
6 4.965 4.977 1 1 0
2 4.302 4.310 2 0 0
8 4.064 4.071 1 1 2
2 3.675 3.681 2 0 2
3 3.533 3.539 0 0 4
1 3.269 3.274 1 0 4
1 3.253 3.258 2 1 0
10 3.171 3.175 2 1 1
1 2.956 2.960 2 1 2
8 2.881 2.884 1 1 4
1 2.870 2.873 3 0 0
4 2.732 2.735 2 0 4
1 2.678 2.681 2 1 3
1 2.486 2.488 2 2 0
1 2.388 2.391 3 1 0
1 2.345 2.348 2 2 2
1 2.131 2.133 3 1 3
1 1.980 1.981 3 1 4
1 1.880 1.881 4 1 0
1 1.863 1.865 4 1 1
2 1.825 1.827 3 1 5
1 1.822 1.823 3 0 6
1 1.817 1.818 4 1 2
1 1.768 1.769 0 0 8
1 1.723 1.724 5 0 0
1 1.711 1.712 2 2 6
1 1.660 1.661 4 1 4
1 1.566 1.567 4 1 5
1 1.506 1.507 3 0 8
1 1.501 1.502 3 3 4
1 1.441 1.442 2 2 8
1 1.411 1.412 4 2 5
1 1.407 1.408 6 0 2
1 1.380 1.380 5 2 0
1 1.373 1.374 5 2 1
1 1.331 1.331 6 0 4
1 1.313 1.314 3 1 9
1 1.191 1.191 5 2 6
1 1.004 1.004 1 0 14
in other T (1)Si members of the milarite group, as for instance
1.609 Å in sugilite (Armbruster and Oberhänsli, 1988) or
1.610 Å in aluminosugilite (Nagashima et al., 2020). There-
fore, even if the empirical formula of laurentthomasite shows
Si =11.90 apfu vs. Al =0.10 apfu, the scattering curve of Al
was not taken into account during refinement. The tetrahe-
dral quadratic elongation value (TQE) (Table 8), as defined
in Robinson et al. (1971), shows that the T(1) site is rather
regular (7.39◦2), whereas the T(2) sites is much more dis-
torted [see the tetrahedral angle variance (TAV=196.13◦2)],
in agreement with a mixed occupancy site.
Figure 6. The double ring of T(1) tetrahedra, obtained with VESTA
3.0 (Momma and Izumi, 2011).
Figure 7. The A−T(2) layer forming 12 rings. These rings are
aligned with six-membered double Si rings along [0001]. Csites
are located at the centre of these channels. T(2) sites are shown in
green; Asites are shown in pink. Diagram obtained with VESTA
3.0 (Momma and Izumi, 2011).
The observed < T (2)–O >distance of 1.674(1) Å (Ta-
ble 8) is slightly larger than the predicted value of 1.637Å
(Gagné and Hawthorne, 2016b) for a pure [4]Be−O bond
length. As, according to the chemical analysis, the T(2) oc-
cupancy, besides the presence of Be2+(2.35 apfu) and Al
(0.50 apfu), also shows the presence of Mg (0.11 apfu) and a
very small amount of Fe3+determined by Mössbauer spec-
troscopy. Considering thus a [4]Mg−O of 1.939 Å (Gagné
and Hawthorne, 2016b), and the larger ionic radius of Al3+
with respect to Be2+, one can easily justify the slightly higher
value (1.674 vs. 1.637 Å). The refined occupancy for Be
is 0.74(1) atoms per site (i.e. 2.22 apfu) (Table 7), which
is comparable to the one resulting from chemical analy-
ses (2.35 apfu), and the refined occupancy of Al (inclusive
of Mg and Fe), at T(2) of 0.26(1) (i.e. 0.78 apfu). The re-
fined site scattering at T(2) is therefore 18.82 epfu (electrons
per formula unit), and compared to the scattering calculated
from chemistry (18.0 epfu), this indicates that, in the crys-
tal studied by single crystal X-ray diffraction, Mg2+at T(2)
https://doi.org/10.5194/ejm-32-355-2020 Eur. J. Mineral., 32, 355–365, 2020
362 C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30
Table 6. Crystal data and structure refinement for laurentthomasite.
Temperature 296(2) K
Space group P6/mcc
Crystal size (mm) 0.40 ×0.29 ×0.17
Unit cell dimensions a(Å) =9.95800(10) Å
c(Å) =14.14920(10) Å
V(Å3)=1215.085(19)
Dcalc.2.66(4) g cm−3
Dobs.2.67(8) g cm−3
Radiation Cu Kα (1.54184 Å)
Monochromator mirror optics
Diffractometer SuperNova, Dual, AtlasS2
Scan type ωscan
Absorption correction CrysAlisPro (Oxford Diffraction/Agilent
Technologies UK Ltd, Yarnton, England)
µ(mm−1)5.269
θmin (◦) 5.13
θmin (◦) 72.72
Reflections collected 13424
Independent reflections 431
Rint (%) 6.42
Rsig (%) 1.35
Index limits −12 ≤h≤12, −12 ≤k≤12, −17 ≤l≤17
Completeness to θ=72.72◦100.0 %
Refinement on F2using SHELXL-2012 (Sheldrick, 2015)
Data/restraints/parameters 431/0/50
Goodness of fit on F21.224
R1(%) 1.89
wR2(%) 4.76
Extinction coefficient 0.0038(2)
1ρmax (e Å−3)0.300 at 0.78 Å from T(1)
1ρmin (e Å−3)−0.219 at 2.10 Å from C
Table 7. Atomic coordinates, chemical site occupancies (occ.), electrons per site and isotropic displacement parameters Ueq for laurentthom-
asite. Wyck. stands for Wyckoff positions.
Site Wyck. x Y z occ. s.s. XRD/EMP e.p.s.∗Ueq
C2a0 0 1/4 K+1.005(8) 19.1(2)/19 0.0224(5)
A4c1/3 2/3 1/4 Sc3+0.285(4) +Fe2+18.56(8)/18.51 0.0113(3)
0.285(4) +Mg2+0.430(9)
T(1) 24m0.09704(5) 0.35423(5) 0.10773(3) Si4+0.27(5) +Si00.73(5) 14/14 0.0065(2)
O1 24m0.1146(2) 0.4129(2) 0 O=0.66(8) +O00.34(8) 8/8 0.0160(4)
O2 12l0.20991(14) 0.28450(15) 0.12663(9) O=0.67(6) +O00.33(6) 8/8 0.0163(3)
O3 24m0.13114(13) 0.48954(13) 0.18109(8) O=0.92(5) +O00.08(5) 8/8 0.0099(3)
T(2) 6f0 1/2 1/4 Be2+0.745(7) +Al3+0.255(7)∗∗ 6.30(6)/5.88 0.0103(7)
∗s.s. is site scattering, XRD is X-ray diffraction, EMP is electron microprobe and e.p.s. is electrons per site. ∗∗ The Al also accounts for Mg.
might be lower (or even [4]Fe3+higher up to 0.06apfu). Bet-
ter agreement could be obtained with only Al and Be at
T(2), although this is not supported by chemical analyses
nor supported by the mean bond length at T(2). Allocation of
0.03 apfu Fe3+at this site seems to be in agreement with the
larger size observed. On the basis of the assumed mixed oc-
cupancy of T(2), the calculated value [using bond-distances
from Shannon, 1976 and Gagnè and Hawthorne, 2016a] is
1.665 Å, in reasonable agreement with the observed value
of 1.674 Å.
The refinement yields a Csite fully oc-
cupied by K (19.1 epfu), in good agree-
ment with the chemical analysis that shows
0.89 K +0.05 Na +0.02 Y +0.01 Ca +0.01 Ba =0.98 apfu,
Eur. J. Mineral., 32, 355–365, 2020 https://doi.org/10.5194/ejm-32-355-2020
C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 363
Table 8. Bond lengths (Å) for laurentthomasite.
T(1)-O3 1.5971(9) T(2)-O3 ×4 1.6742(11)
T(1)-O1 1.6103(6) TQE 1.0538
T(1)-O2 1.6116(16) TAV (◦2)196.13
T(1)-O2 1.6130(15)
< T (1)-O>1.608 A–O3 ×6 2.1364(10)
TQE 1.0019 OQE 1.0521
TAV (◦2)7.39 OAV (◦2)166.92
C-O2 ×12 3.0856(13)
OAV: octahedral angle variance; OQE: octahedral quadratic elongation;
TAV: tetrahedral angle variance; TQE: tetrahedral quadratic elongation, as
defined in Robinson et al. (1971).
corresponding to a calculated site scattering of 19.0 epfu.
The < C–O >distance, 3.086(1) Å (Table 8), is in good
agreement with the [12]K–O bond length (3.095 Å) predicted
by Gagné and Hawthorne (2016b) of 3.095 Å, considering
the presence of about 10 % smaller cations (Na, Ca and Ba).
The Asite is a strongly distorted octahedron as shown
by the octahedral angle variance (OAV =166.92◦, Table 8);
the observed < A–O >distance of 2.136(1) Å (Table 8) is
slightly longer than predicted for a pure [6]Mg–O bond length
(2.089 Å) (Gagné and Hawthorne, 2016b). Considering the
Asite population obtained by chemical analyses (PA2+=
1.47 apfu) and taking into account the quite significant pres-
ence of Sc (0.54 apfu), one could justify the derived longer
value, given the [6]Sc–O bond length (2.121Å: Cong et al.,
2010; bixbyite-type Sc2O3). Nevertheless, the presence of
0.61 apfu of Fe2+and Mn2+(0.35 Fe2+0.26 Mn) also con-
tributes to the lengthening of the < A–O >mean bond dis-
tance, in closer agreement with the observed value. How-
ever, this is not yet enough because the calculated mean bond
length is 2.123(1) Å (using the ionic radii of Shannon, 1976)
(Table 8). The Asite is therefore larger than expected; this
is probably due to (i) the Aoctahedron sharing three edges
with three T(2) tetrahedra as well as to (b) an increased
charge at the T(2) site due to some replacement of Be by
Al (and minor Fe3+). All these features induce an increase
of the A–O(3) bond distance. Incidentally, the refined ion-
ization of O3 shows almost completely ionized oxygen at
that site (92 %), compared with ionization refined at O1 and
O2 sites (ca. 65 %, Table 7). This should be ascribed to the
strong incidence of charge at the O3 site, which is threefold
coordinated with one T(2) site, one T(1) site and one Asite,
whereas the O1 site is twofold bonded just to two Si4+at the
T(1) site, and the O2 site is threefold bonded to two Si4+at
the T(1) site and has a long bond to K+at the Csites. It is
therefore more plausible to have a more covalent bond at the
O1 and O2 sites, in full agreement with refinement. The re-
fined scattering (37.12 epfu – Table 7) of the Asite is in very
good agreement with the chemical analysis (37.2 epfu). The
Fourier difference map clearly indicates that the Bsites are
vacant in laurentthomasite.
The milarite group has yet again demonstrated an extraor-
dinary versatility in its crystal chemistry.
Data availability. The data used in this paper can be found in the
Supplement.
Supplement. The supplement related to this article is available on-
line at: https://doi.org/10.5194/ejm-32-355-2020-supplement.
Author contributions. CF and GCP designed the experiments. SP,
IP and FW carried them out. FC and MS developed the model and
performed the refinements. CF prepared the manuscript with con-
tributions from all co-authors.
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. The authors wish to thank Mustapha Abdel-
moula for the Mössbauer data and Olivier Beyssac for the Raman
facilities. The authors would also like to thank Thomas Armbruster,
Edward Grew, and an anonymous referee for their constructive crit-
icism and fruitful suggestions that substantially helped to improve
the quality of the manuscript, as well David Smith for revising
both the grammar and English style of an earlier version of the
manuscript.
Review statement. This paper was edited by Sergey Krivovichev
and reviewed by Thomas Armbruster and one anonymous referee.
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