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Laurentthomasite, Mg 2 K(Be 2 Al)Si 12 O 30 : a new milarite-group-type member from the Ihorombe region, Fianarantsoa Province, Madagascar


Abstract and Figures

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 15mm large and 5mm 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 (C) 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 [d in 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 spectroscopies (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)S2.01(K0.89 Na0.05 Y0.02 Ca0.01 Ba0.01)S0.98[(Be2.35 Al0.50 Mg0.11 Fe3+0.03)S2.99(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 R index of 1.89% based on 430 reflections with Io>2sigma.I / collected on a four-circle diffractometer with CuKalpha radiation. By comparisonwith 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 (BeCAl). The A and C sites occur in the interstices of the framework while the B site is vacant. The origin of the strong dichroism is related to a charge transfer process between Fe2+ and Fe3+ in octahedral A sites and tetrahedral T (2) sites, respectively.
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Eur. J. Mineral., 32, 355–365, 2020
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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 (
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 cm3. 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 Curadiation. 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
3T(1)12O30](H2O)xwith x=0n
(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-
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 (231600300 S,
460504900 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
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
(, 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-
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 cm3(hydrostatic balance) and a calculated
density of 2.66(4) g cm3using 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). 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
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 (Curadiation); the data were
collected in the 2θrange 0.279 to 113.785at 10 s exposure
for each frame (0.029width) 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
C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 359
Table 2. WDS and ICP-OES-MS chemical analyses of laurentthom-
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
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 cm1and 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 cm1, with an
evident separation into two components with maxima at 263
and 288 cm1for 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
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 g1Uncertainty
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, 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 s1) QS (mm s1) 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 eÅ3. 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
stitution, and to friedrichbeckeite (Mg2NaKBe3Si12O30;
Lengauer et al., 2009) by the (BNaT (2)Be)1(BT (2)Al) sub-
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
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 (7.392), whereas the T(2) sites is much more dis-
torted [see the tetrahedral angle variance (TAV=196.132)],
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 AT(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]BeO 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]MgO 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) 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
Dcalc.2.66(4) g cm3
Dobs.2.67(8) g cm3
Radiation Cu (1.54184 Å)
Monochromator mirror optics
Diffractometer SuperNova, Dual, AtlasS2
Scan type ωscan
Absorption correction CrysAlisPro (Oxford Diffraction/Agilent
Technologies UK Ltd, Yarnton, England)
θmin () 5.13
θmin () 72.72
Reflections collected 13424
Independent reflections 431
Rint (%) 6.42
Rsig (%) 1.35
Index limits 12 h12, 12 k12, 17 l17
Completeness to θ=72.72100.0 %
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)
max (e Å3)0.300 at 0.78 Å from T(1)
min (e Å3)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
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. The supplement related to this article is available on-
line at:
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
Review statement. This paper was edited by Sergey Krivovichev
and reviewed by Thomas Armbruster and one anonymous referee.
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Full-text available
The milarite structure-type has an uncommonly large number of distinct mineral species (23 at present). Here we explore this structure type from the point of view of possible root-charge arrangements and endmember compositions. Enumeration shows that there are 34 distinct root-charge arrangements with Si ¼ 12 apfu and 39 distinct root-charge arrangements with Si ¼ 8-11 apfu. A priori bond-valence calculations for all root-charge arrangements allows evaluation of lattice-induced strain as a function of chemical composition for all arrangements for which the detailed atomic arrangement has been refined. Analysis of localized strain indicates that the B site has the highest amount of strain in the structure, and in accord with this finding, milarite-group minerals with vacancies at the B site are more common than milarite-group minerals with fully occupied B sites. The a priori bond-valence calculations suggest that many other compositions are possible for the milarite structure-type. Examination of synthesis results reveals 20 synthetic compounds with the milarite-type structure that have distinct (dominant) endmember compositions. Examination of ~350 chemical analyses from the literature reveals 29 distinct endmember compositions, six of which deserve to be described as new mineral species. Two additional analyses could lead to new minerals, but require confirmation of site populations by structure refinement.
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The crystal sructures of six natural milarites (A_2B_2C[T2_3Tl_(12)O_(30)] (H_2O)_(x,)) P6/mcc, ɑ ~ 10.40, c ~ 13.80 Å, Z=2) covering the range of known compositions have been refined to R indices of ~ 3% using MoKɑ X-ray intensity data. Previously proposed split-atom models for the A and B sites are confirmed. Polarized spectroscopy in the infrared region shows H_2O to be the only significant H-bearing group. Band polarization characteristics show the H-H vector to be ⊥c and the H_2O plane to be ‖c; this corresponds to the type II H_2O found in the related structures of beryl and cordierite. There is a quantitative correlation between band intensity and H_2O content, and the observed molar absorptivities are similar to those measured in cordierite. Milarite has a beryllo-alumino-silicate framework structure. The framework is a four connected three-dimensional net, one of a series of seven simple nets with prominent double rings of tetrahedra previously derived by Hawthorne and Smith (1986). There are l5 minerals with this net as the basic tetrahedral arrangement; in general, Si is strongly to completely ordered into the ring tetrahedra, and the other tetrahedral cations (Li, Be, B, Al, Mg, Fe^(2+), Fe^(3+), Mn^(2+), and Zn) are strongly to completely ordered into the ring-linking tetrahedra. The important substitution in normal milarite is Na^B + Be^(T2) = □^B + Al^(T2) with the amount of Be varying between approximately 1.5 and 2.5 apfu; in (Y,REE)- bearing milarite, the important substitution is (Y,REE)^A + Be^(T2) = Ca^A + Al^(T2) with the amount of Be varying up to 3.0 atoms pfu. The mean bond lengths of the polyhedra involved in these substitutions vary accordingly, and linear models of this behavior are developed. In addition, there is chemical evidence of a small amount of Al ⇌ Si substitution at the Tl site. Similar linear models are developed for the whole group of milarite type minerals. Milarite is found in a variety of environments, from vugs in plutonic rocks through diverse pegmatites to hydrothermal ore deposits and alpine veins; a review of specific parageneseiss given. Milarite is a low-temperature hydrothermal mineral crystallizing in the range of 200-250 ºC at low pressures. Associated minerals suggest hat milarite crystallizes from alkaline fluids with Be being transported as fluor-complexes and carbonate complexes. Other members of the milarite group occur in peralkaline rocks, meteorites, peraluminous volcanic rocks, and high-grade metamorphic rocks, as well as other geochemically more unusual rocks, and form under a wide variety of P-T conditions.
We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930 °C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. The Eu/Eu* in monazite reflects changes in fO2 relative to QFM related to the growth and breakdown of spinel. The Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (< 29 ± 8 Ma) suggests that a thin mantle lithosphere (< 80 km) or advective heating may have also been important in the formation of this high‐temperature, low‐pressure terrane. This article is protected by copyright. All rights reserved.
The system B2O3-Al2O3-SiO2 (BAS) includes two ternary phases occurring naturally, boromullite, Al9BSi2O19, and boralsilite, Al16B6Si2O37, as well as synthetic compounds structurally related to mullite. The new mineral vránaite, a third naturally occurring anhydrous ternary BAS phase, is found with albite and K-feldspar as a breakdown product of spodumene in the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar. Boralsilite also occurs in this association, although separately from vránaite; both minerals form rare aggregates of subparallel prisms up to 100 μm long. Optically, vránaite is biaxial (–), nα = 1.607(1), nβ = 1.634(1), nγ = 1.637(1) (white light), 2Vx(calc.) = 36.4°, X ≈ c; Y ≈ a; Z = b. An averaged analysis by EMP and LA-ICP-MS (Li, Be) gives (wt%) SiO2 20.24, B2O3 11.73, Al2O3 64.77, BeO 1.03, MnO 0.01, FeO 0.13, Li2O 1.40, Sum 99.31. Raman spectroscopy in the 3000–4000 cm−1 region rules out the presence of significant OH or H2O. Vránaite is monoclinic, space group I2/m, a = 10.3832(12), b = 5.6682(7), c = 10.8228(12) Å, β = 90.106(11)°; V = 636.97(13) Å3, Z = 1. In the structure [R1 = 0.0416 for 550 Fo > 4σFo], chains of AlO6 octahedra run parallel to [010] and are cross-linked by Si2O7 disilicate groups, BO3 triangles, and clusters of AlO4 and two AlO5 polyhedra. Two Al positions with fivefold coordination, Al4 and Al5, are too close to one another to be occupied simultaneously; their refined site-occupancy factors are 54% and 20% occupancy, respectively. Al 5 is 5-coordinated Al when the Al9 site and both O9 sites are occupied, a situation giving a reasonable structure model as it explains why occupancies of the Al5 and O9 sites are almost equal. Bond valence calculations for the Al4 site suggest Li is likely to be sited here, whereas Be is most probably at the Al5 site. One of the nine O sites is only 20% occupied; this O9 site completes the coordination of the Al5 site and is located at the 4th corner of what could be a partially occupied BO4 tetrahedron, in which case the B site is shifted out of the plane of the BO3 triangle. However, this shift remains an inference as we have no evidence for a split position of the B atom. If all sites were filled (Al4 and Al5 to 50%), the formula becomes Al16B4Si4O38, close to Li1.08Be0.47Fe0.02Al14.65B3.89Si3.88O36.62 calculated from the analyses assuming cations sum to 24. The compatibility index based on the Gladstone – Dale relationship is 0.001 (“superior”). Assemblages with vránaite and boralsilite are inferred to represent initial reaction products of a residual liquid rich in Li, Be, Na, K and B during a pressure and chemical quench, but at low H2O activities due to early melt contamination by carbonate in the host rocks. The two BAS phases are interpreted to have crystallized metastably in lieu of dumortierite in accordance with Ostwald Step Rule, possibly first as “boron mullite”, then as monoclinic phases.
Presents top research Provides state-of-the-art methods and techniques Addresses mineralogists, crystallographers and materials scientists Aims and Scope "Highlights in Mineralogical Crystallography" presents a collection of review articles with the common topic: structural properties of minerals and synthetic analogues. It is a valuable resource for mineralogists, materials scientists, crystallographers, and earth scientists. This book includes: An introduction to the RRUFF database for structural, spectroscopic, and chemical mineral identification. • A systematic evaluation of structural complexity of minerals. • ab initio computer modelling of mineral surfaces. • Natural quasicrystals of meteoritic origin. • The potential role of terrestrial ringwoodite on the water content of the Earth's mantle. • Structural characterization of nanocrystalline bio-related minerals by electron-diffraction tomography. • The uniqueness of mayenite-type compounds as minerals and high-tech ceramics.
X-ray single-crystal structure refinements on natural roedderite from Bellerberg (Eifel district, FRG), (Mg4.66Fe0.12Mn0.10Ti0.01)Na1.15K1.0Si12O30, lead to an ordered Na arrangement which reduces the space group from P6/mcc to P62c. All other structural positions maintain a P6/mcc pseudo-symmetry. -Author