Femtosecond Laser Ablation-ICP-Mass Spectrometry and CHNS Elemental Analyzer Reveal Trace Element Characteristics of Danburite from Mexico, Tanzania, and Vietnam

Abstract

Danburite is a calcium borosilicate that forms within the transition zones of metacarbonates and pegmatites as a late magmatic accessory mineral. We present here trace element contents obtained by femtosecond laser ablation-inductively coupled plasma (ICP)-mass spectrometry for danburite from Mexico, Tanzania, and Vietnam. The Tanzanian and Vietnamese samples show high concentrations of rare earth elements (∑REEs 1900 µg∙g⁻¹ and 1100 µg∙g⁻¹, respectively), whereas Mexican samples are depleted in REEs (<1.1 µg∙g⁻¹). Other traces include Al, Sr, and Be, with Al and Sr dominating in Mexican samples (325 and 1611 µg∙g⁻¹, respectively). Volatile elements, analyzed using a CHNS elemental analyzer, reach <3000 µg∙g⁻¹. Sr and Al are incorporated following Ca²⁺ = Sr²⁺ and 2 B³⁺ + 3 O²⁻ = Al³⁺ + 3 OH⁻ + □ (vacancy). REEs replace Ca²⁺ with a coupled substitution of B³⁺ by Be²⁺. Cerium is assumed to be present as Ce⁴⁺ in Tanzanian samples based on the observed Be/REE molar ratio of 1.5:1 following 2 Ca²⁺ + 3 B³⁺ = Ce⁴⁺ + REE³⁺ + 3 Be²⁺. In Vietnamese samples, Ce is present as Ce³⁺ seen in a Be/REE molar ratio of 1:1, indicating a substitution of Ca²⁺ + B³⁺ = REE³⁺ + Be²⁺. Our results imply that the trace elements of danburite reflect different involvement of metacarbonates and pegmatites among the different locations.

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minerals
Article
Femtosecond Laser Ablation-ICP-Mass Spectrometry
and CHNS Elemental Analyzer Reveal Trace Element
Characteristics of Danburite from Mexico, Tanzania,
and Vietnam
Le Thi-Thu Huong 1, *ID , Laura M. Otter 2, 3, *ID , Michael W. Förster 3,
Christoph A. Hauzenberger 1, Kurt Krenn 1, Olivier Alard 3,4, Dorothea S. Macholdt 2,
Ulrike Weis 2, Brigitte Stoll 2and Klaus Peter Jochum 2ID
1NAWI Graz Geocentre, University of Graz, 8010 Graz, Austria;
christoph.hauzenberger@uni-graz.at (C.A.H.); kurt.krenn@uni-graz.at (K.K.)
2Climate Geochemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany;
d.macholdt@mpic.de (D.S.M.); ulrike.weis@mpic.de (U.W.); Brigitte.stoll@mpic.de (B.S.);
k.jochum@mpic.de (K.P.J.)
3Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia;
michael.forster@hdr.mq.edu.au (M.W.F.); olivier.alard@mq.edu.au (O.A.)
4Géosciences Montpellier, UMR 5243, CNRS & UniversitéMontpellier, 34095 Montpellier, France
*Correspondence: thi.le@uni-graz.at (L.T.-T.H.); laura.otter@hdr.mq.edu.au (L.M.O.)
Received: 7 May 2018; Accepted: 28 May 2018; Published: 29 May 2018


Abstract:
Danburite is a calcium borosilicate that forms within the transition zones of metacarbonates
and pegmatites as a late magmatic accessory mineral. We present here trace element contents
obtained by femtosecond laser ablation-inductively coupled plasma (ICP)-mass spectrometry for
danburite from Mexico, Tanzania, and Vietnam. The Tanzanian and Vietnamese samples show
high concentrations of rare earth elements (
∑
REEs 1900
µ
g
·
g
−1
and 1100
µ
g
·
g
−1
, respectively),
whereas Mexican samples are depleted in REEs (<1.1
µ
g
·
g
−1
). Other traces include Al, Sr, and Be,
with Al and Sr dominating in Mexican samples (325 and 1611
µ
g
·
g
−1
, respectively). Volatile elements,
analyzed using a CHNS elemental analyzer, reach <3000
µ
g
·
g
−1
. Sr and Al are incorporated following
Ca
2+
= Sr
2+
and 2 B
3+
+3O
2−
= Al
3+
+ 3 OH
−
+

(vacancy). REEs replace Ca
2+
with a coupled
substitution of B
3+
by Be
2+
. Cerium is assumed to be present as Ce
4+
in Tanzanian samples based
on the observed Be/REE molar ratio of 1.5:1 following 2 Ca
2+
+ 3 B
3+
= Ce
4+
+ REE
3+
+ 3 Be
2+
.
In Vietnamese samples, Ce is present as Ce
3+
seen in a Be/REE molar ratio of 1:1, indicating a
substitution of Ca
2+
+ B
3+
= REE
3+
+ Be
2+
. Our results imply that the trace elements of danburite
reflect different involvement of metacarbonates and pegmatites among the different locations.
Keywords:
danburite; trace elements; REE; femtosecond LA-ICP-MS; CHNS elemental analyzer;
pegmatites; skarn
1. Introduction
Danburite crystallizes in the orthorhombic system and has the formula CaB
2
Si
2
O
8
. Its structure
consists of a tetrahedral framework with boron and silicon orderly distributed in different tetrahedral
sites. The framework of corner-sharing Si
2
O
7
and B
2
O
7
groups are interconnected by Ca atoms [
1
,
2
].
According to previous studies [
3
,
4
], the structural unit of danburite contains two tetrahedrally coordinated
cations (T1: B and T2: Si), one calcium, and five oxygen atoms, among which O1, O2, and O3 are bonded
to both B and Si, while O4 and O5 are bridging oxygens of the Si2O7and B2O7groups, respectively.
Minerals 2018,8, 234; doi:10.3390/min8060234 www.mdpi.com/journal/minerals

Minerals 2018,8, 234 2 of 15
Danburite is one of the few boron minerals that are valued as gemstones. After its discovery in
Danbury, Connecticut, USA, colorless gem-quality danburite has been subsequently found in Japan,
Mexico, Russia, Sri Lanka, and Switzerland [
5
]. Exceptionally rare is yellow danburite, which so far
has been reportedly found only in Madagascar, Tanzania, Myanmar, and Vietnam [
6
,
7
]. The important
geological environments that are known to have produced gem-quality danburite specimens include
pegmatites and metacarbonates associated with hydrothermal activity [8–10].
Previous studies presenting danburite compositions were generally limited to its major element
geochemistry (e.g., [
11
,
12
]) due to the lack of microanalytical reference materials for boron minerals.
While Huong et al. [
7
] overcame this issue by applying femtosecond laser ablation-inductively coupled
plasma-mass spectrometry (LA-ICP-MS), which allows virtually matrix-independent calibration,
their study was confined to a regional scale. Here, we present state-of-the-art femtosecond LA-ICP-MS
determination of major and trace element concentrations in danburite from three distinct worldwide
distributed occurrences (Tanzania, Vietnam, and Mexico). The aims of this study are (1) to investigate
the geochemical differences of danburites from different locations (Tanzania, Vietnam, and Mexico)
and rock types (pegmatites and skarn), (2) to elucidate their potential for further provenance
discrimination, and (3) to understand the incorporation of trace elements into the danburite structure.
2. Materials and Methods
2.1. Sample Material
For this study, we selected 6 danburite samples from 3 deposits in Mexico, Tanzania, and Vietnam
(2 samples from each deposit), representing different geological environments. The Mexican danburites
were collected from the polymetallic skarn deposit (sulfides of Ag, Pb, Cu, and Zn) in the
Charcas mining district, San Luis Potosi. The area is characterized by marine siliciclastic and volcaniclastic
rocks, with 2 domains (east and west) separated by a regional fault [
9
]. In the region of San Luis Potosí,
numerous volcanic systems and igneous rocks are associated with different mineral deposits. The Charcas
deposit has a large Ca–B metasomatic envelope composed of early datolite and later danburite.
Other minerals associated with danburite include calcite, apophyllite, stilbite, chalcopyrite, sphalerite,
and citrine. The samples appear as colorless, transparent, prismatic euhedral crystals and are up to 6 cm
in length.
The Tanzanian danburite originates from the central zone of a pegmatite mostly as yellowish,
fine-grained, massive, opaque aggregates, but occasionally also as larger single crystals with color and
transparency. The mine is referred to by the locals as “Munaraima” and is situated in Eastern Tanzania,
at the edge of the Uluguru Mountains [
10
] near the village of Kivuma. The region around Kivuma
is dominated by a metasedimentary sequence including metapelites, gneisses, and spinel and
ruby-bearing marbles, which underwent granulite facies metamorphism during the East African
Orogen at ~640 Ma [
13
,
14
]. Tonalitic dikes and pegmatites, commonly found in this area, intruded the
basement rocks during slow cooling of the whole area. The contact zone of marble and pegmatite
is dominated by a mineral assemblage consisting of microcline (variety amazonite), blue quartz,
kyanite, and dravite, while the core complex is mainly composed of massive quartz and schörl [
10
].
For this study, small, anhedral, transparent yellow danburite crystals ranging from 0.5 to 1 cm in size
were selected.
The Vietnamese danburite samples (1–1.5 cm) appear as yellow, transparent, broken, and slightly
rounded crystals and have been found in a placer deposit (Bai Cat) in the Luc Yen mining area,
Yen Bai province, Northern Vietnam [
7
,
15
]. The geology of Luc Yen is dominated by metamorphic
rocks, mainly granulitic gneisses, mica schists, and marbles, which are associated with the large-scale
Ailao Shan–Red River shear zone. Locally, aplitic and pegmatitic dykes occur [
16
]. Danburite crystals
are associated with ruby, sapphire, spinel, topaz, and tourmaline in the Bai Cat placer deposit, which is
surrounded by marble units. While the primary formations of ruby, sapphire, and spinel in Luc Yen are
associated with metamorphosed limestones, those of tourmaline and topaz originate from pegmatite

Minerals 2018,8, 234 3 of 15
bodies. Besides tourmaline and topaz, these pegmatites contain orthoclase, smoky quartz, lepidolite,
and beryl. Danburite crystals have not yet been discovered in situ, hence their genetic relationship
with the Luc Yen pegmatites is not verified. However, fluid inclusion studies of Luc Yen danburites
indicate a pegmatitic origin [15].
2.2. Analytical Methods
Chemical data for major elements were obtained by electron microprobe at the
Institute of Geosciences, Johannes Gutenberg University Mainz, by laser ablation-inductively coupled
plasma-mass spectrometry (LA-ICP-MS) at the Max Planck Institute for Chemistry, Mainz, and by
CHNS Elemental Analyzer at Macquarie University.
Electron probe micro-analysis (EPMA) was performed at the University of Mainz with a
JEOL JXA 8200 Superprobe instrument equipped with 5 wavelength-dispersive spectrometers,
using 15 kV acceleration voltage and 12 nA filament current. Calcium and silicon were analyzed with
wollastonite as a standard material.
LA-ICP-MS data for a total of 55 elements were obtained using an NWRFemto femtosecond laser
operating at a wavelength of 200 nm in combination with a ThermoFisher Element2 single-collector
sector-field ICP mass spectrometer (see Table 1). Pre-ablation cleaning was performed using a spot
size of 65
µ
m, 80
µ
m/s scan speed, and 50 Hz pulse repetition rate at 100% energy output to remove
any superficial surface residue. Thereafter, samples were ablated using line scans of 300
µ
m length at
a spot size of 55
µ
m and a scan speed of 5
µ
m/s. These parameters resulted in an energy density of
ca. 0.51 J/cm
2
at the sample surface, and the pulse repetition rate was set to 50 Hz. Since there is no
matrix-matched calibration material for Ca–B silicates available, we applied a laser device that produces
pulses at 150 fs, enabling virtually matrix-independent calibration [
17
]. The glass microanalytical
reference material NIST SRM 610 was used as calibration material in the evaluation process, where
43
Ca
was used as internal standard. Reduction of data and elimination of obvious outliers were performed
following a programmed routine in Microsoft Excel described in Jochum et al. [18].
All samples were additionally analyzed for their H, C, N, and S contents in a vario EL cube
elemental analyzer (Elementar, Langenselbold, Germany). For analysis, 50 to 100 mg samples were
packed in Sn-foils (no flux added) and were ignited in an oxygen–He gas atmosphere furnace at
around 1150
◦
C. The produced gases were then trapped and released in a set of chromatographic
columns for the sequential analysis of N (no trapping), then C, H, and S. Each sample was measured
for 9 min, and released gases were sequentially analyzed with a thermal conductivity detector.
Sample measurements were repeated 3 times for each sampling location, and all values were calibrated
against the reference materials BAM-U110, JP-1, and CRPG BE-N (Table 2). Analytical uncertainties
were evaluated from reference material values, which were found to lie within 16% and 25% for C and
H of the data tabulated in the GeoReM database [19].
Table 1.
Operating conditions of the femtosecond laser ablation-inductively coupled plasma-mass
spectrometry (fs-LA-ICP-MS) system.
Operating Conditions of NWRFemto200 Laser System
Wavelength λ(nm) 200
Fluence (J·cm−2)0.51
Pulse length (fs) 150
Pulse repetition rate (Hz) 50
Laser energy output (%) 100
Spot size (µm) 55
Line length (µm) 300
Scan speed (µm·s−1)5
Warm-up time (s) 28
Dwell time (s) 60
Washout time (s) 30

Minerals 2018,8, 234 4 of 15
Table 1. Cont.
Operating Conditions of the Element2 Mass Spectrometer
RF power (W) 1055
Cooling gas (Ar) flow rate (L·min−1)16
Auxiliary gas (Ar) flow rate (L·min−1)1.19
Additional gas (He) flow rate (L·min−1)0.7
Sample gas (Ar) flow rate (L·min−1)0.7
Sample time (s) 0.002
Samples per peak 100
Mass window (%) 10
Time per pass (s) 2
Scan mode (Escan/Bscan) both
Mass resolution 300
Table 2. Reference materials for CHNS analyzer.
BE-N Altered Basalts (SARM) H
TCD
C
TCD
N
TCD
S
TCD
S
IR
n14 20 17 21 8
Average (µg·g−1)2771 ±534 2301 ±147 197 ±42 301 ±37 298 ±23
RSD % 19 6 21 12 8
BAM-U110
n13 18 18 17 –
Average (µg·g−1)12,258 ±1758 72,340 ±2640 4237 ±165 9114 ±1082 –
RSD % 14 4 4 12 –
JP-1 Peridotite massif (JGS)
n4 12 14 14 14
Average (µg·g−1)3195 ±170 763 ±82 91 ±23 27 ±14 26 ±7
RSD % 5 11 26 51 27
ndenotes the number of measurement performed; average refers to arithmetic means of the nvalues measured;
RSD
%
is relative standard deviation expressed in %. “TCD” refers to the thermal conductivity detector and “IR” to
the infrared detector devices.
3. Results
The chemical composition of the danburite samples from Mexico, Tanzania, and Vietnam are
presented in Table 3. The major element mass fractions of B, Ca, and Si are close to the stoichiometric
composition, i.e., 28.32 wt % B
2
O
3
(calculated from 87,890 ppm B obtained by ICP-MS), 22.81 wt % CaO,
and 48.88 wt % SiO2, respectively.
The trace elements Li, Sc, Ga, Se, Rb, Zr, Nb, Ag, Cd, Sn, Cs, Hf, Ta, W, Ir, Pt, Au, Tl, Bi, and U
have concentrations below the detection limit in all samples (see detection limits in footnote of Table 3).
A C1-chondrite–normalized plot of rare earth element (REE) mass fractions (normalizing data from [
20
])
displays a strong enrichment of light rare earth elements (LREEs: La, Ce, Pr, Nd, Sm, Eu) compared to
heavy rare earth elements (HREEs: Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in danburite from Tanzania and
Vietnam, while the samples from Mexico are mostly below detectability (Figure 1). Europium shows a
negative anomaly of the same order for samples from Tanzania and Vietnam, while no other strong
anomaly is observed (e.g., Ce). Total lanthanide content of Tanzanian samples is up to 1900
µ
g
·
g
−1
,
hence the mass fractions of LREE exceed those of HREE by a 500-fold enrichment. The Vietnamese
samples are different, with a total REE content of around 1000
µ
g
·
g
−1
, hence the mass fractions of
LREE exceed those of HREE by a 200-fold enrichment. The Mexican danburites appear to be REE-poor,
with total REE contents below 1
µ
g
·
g
−1
for La and Ce, while the remaining REEs from Nd to Lu are
below the detection limits. Possible quadrivalent trace elements such as Ti, Hf, and Zr were also
below the detection limits. Thorium and Pb show low mass fractions of 0.1 and 11
µ
g
·
g
−1
in the
Mexican danburites, and 0.6 and 8
µ
g
·
g
−1
in the Vietnamese and Tanzanian danburites, respectively.

Minerals 2018,8, 234 5 of 15
The trivalent element Al shows highly varying concentrations among the different deposits and is
anti-correlated with
∑
REE, thus also with Be (see Figure 2A,B), while no correlation was observed
between Al and Sr (Figure 2C). The highest mass fractions of Al are found in the Mexican samples
(325
µ
g
·
g
−1
) and the lowest in the Tanzanian samples (84
µ
g
·
g
−1
) (Table 3). Strontium is highest
(1611
µ
g
·
g
−1
) in the Mexican samples and lowest (66
µ
g
·
g
−1
) in the Vietnamese samples (Figure 2D).
Manganese yields up to 18
µ
g
·
g
−1
in the Vietnamese danburites, while the Mexican and Tanzanian
samples have Mn contents below the detection limits. In general, transition metals (e.g., Fe and Ni)
have extremely low concentrations in all danburite samples. The three danburite origins can also be
separated from each other using the mass fractions of Y (Figure 2E). Beryllium is found to be highest in
the Tanzanian danburite (up to 178
µ
g
·
g
−1
) and lowest in the Mexican samples (3
µ
g
·
g
−1
). In addition,
low concentrations of less than 1, 9, and 2
µ
g
·
g
−1
of the elements Ba, Mg, and Cu, respectively, are
identified in all samples regardless of their origin. The positive correlation between Be and
∑
REE is
exceptionally strong for all three deposits (Figure 3A), and due to varying mass fractions of all shown
elements, this plot enables excellent discrimination among the three deposits. Univalent elements such
as Li, Na, and, K fall below the detection limits.
Minerals 2018, 8, x FOR PEER REVIEW 5 of 15
and Vietnam, while the samples from Mexico are mostly below detectability (Figure 1). Europium
shows a negative anomaly of the same order for samples from Tanzania and Vietnam, while no other
strong anomaly is observed (e.g., Ce). Total lanthanide content of Tanzanian samples is up to 1900
µg∙g−1, hence the mass fractions of LREE exceed those of HREE by a 500-fold enrichment. The
Vietnamese samples are different, with a total REE content of around 1000 µg∙g−1, hence the mass
fractions of LREE exceed those of HREE by a 200-fold enrichment. The Mexican danburites appear to
be REE-poor, with total REE contents below 1 µg∙g−1 for La and Ce, while the remaining REEs from
Nd to Lu are below the detection limits. Possible quadrivalent trace elements such as Ti, Hf, and Zr
were also below the detection limits. Thorium and Pb show low mass fractions of 0.1 and 11 µg∙g−1 in
the Mexican danburites, and 0.6 and 8 µg∙g−1 in the Vietnamese and Tanzanian danburites,
respectively. The trivalent element Al shows highly varying concentrations among the different
deposits and is anti-correlated with ∑REE, thus also with Be (see Figure 2A,B), while no correlation
was observed between Al and Sr (Figure 2C). The highest mass fractions of Al are found in the
Mexican samples (325 µg∙g−1) and the lowest in the Tanzanian samples (84 µg∙g−1) (Table 3). Strontium
is highest (1611 µg∙g−1) in the Mexican samples and lowest (66 µg∙g−1) in the Vietnamese samples
(Figure 2D). Manganese yields up to 18 µg∙g−1 in the Vietnamese danburites, while the Mexican and
Tanzanian samples have Mn contents below the detection limits. In general, transition metals (e.g.,
Fe and Ni) have extremely low concentrations in all danburite samples. The three danburite origins
can also be separated from each other using the mass fractions of Y (Figure 2E). Beryllium is found
to be highest in the Tanzanian danburite (up to 178 µg∙g−1) and lowest in the Mexican samples (3
µg∙g−1). In addition, low concentrations of less than 1, 9, and 2 µg∙g−1 of the elements Ba, Mg, and Cu,
respectively, are identified in all samples regardless of their origin. The positive correlation between
Be and ∑REE is exceptionally strong for all three deposits (Figure 3A), and due to varying mass
fractions of all shown elements, this plot enables excellent discrimination among the three deposits.
Univalent elements such as Li, Na, and, K fall below the detection limits.
Figure 1. Rare earth element (REE) mass fractions in danburite from Mexico, Tanzania, and Vietnam.
All values are presented as averages and normalized to C1-chondrite (data from [20]). Samples from
Tanzania and Vietnam have a high abundance of REEs, especially LREEs, while samples from Mexico
are largely devoid of these elements (bdl, below detection limit, from Pr to Lu).
The contents of the light volatile elements H, C, N, and S are generally low for both unpowdered
and powdered samples (see Table 4). Samples from both Tanzania and Mexico exhibit mass fractions
of H, C, N, and S of <10, <200, <100, and <15 µ g g−1, respectively. The samples from Vietnam show
significantly higher mass fractions for H, C, N, and S in unpowdered specimens, reaching values of
up to 300, 5600, 1000, and 15 µ g g−1, respectively. However, most elements are present in lower
concentrations in the powdered sample set (H, C, and N of 40, 3000, and 520 µ g g−1, respectively).
Higher values for powdered samples from Mexico and Tanzania are likely attributed to the
significantly increased surface-to-volume ratio facilitating higher adhesion of atmospheric gases.
Figure 1.
Rare earth element (REE) mass fractions in danburite from Mexico, Tanzania, and Vietnam.
All values are presented as averages and normalized to C1-chondrite (data from [
20
]). Samples from
Tanzania and Vietnam have a high abundance of REEs, especially LREEs, while samples from Mexico
are largely devoid of these elements (bdl, below detection limit, from Pr to Lu).
The contents of the light volatile elements H, C, N, and S are generally low for both unpowdered
and powdered samples (see Table 4). Samples from both Tanzania and Mexico exhibit mass
fractions of H, C, N, and S of <10, <200, <100, and <15
µ
g
·
g
−1
, respectively. The samples from
Vietnam show significantly higher mass fractions for H, C, N, and S in unpowdered specimens,
reaching values of up to 300, 5600, 1000, and 15
µ
g
·
g
−1
, respectively. However, most elements are
present in lower concentrations in the powdered sample set (H, C, and N of 40, 3000, and 520
µ
g
·
g
−1
,
respectively). Higher values for powdered samples from Mexico and Tanzania are likely attributed to
the significantly increased surface-to-volume ratio facilitating higher adhesion of atmospheric gases.
Significantly higher values for H and C in all Vietnamese samples agree well with a previous study
by Huong et al. [
15
], who characterized a high abundance of primary CO
2
-bearing fluid inclusions
in these samples, which likely accounts for the elevated concentrations of both elements, while fluid
inclusion is not present in the Mexican or Tanzanian specimens. High N mass fractions are in the
expected range of metamorphosed sediments, which are involved in danburite formation and are
known to contain ~200–3000
µ
g
·
g
−1
N for a typical metamorphic gradient of 500–700
◦
C (e.g., [
21
]).
Overall lower values of H, C, and N in the powdered Vietnamese sample set support the observation

Minerals 2018,8, 234 6 of 15
that these elements are derived from fluid inclusions and were lost during crushing in the agate mortar.
Nevertheless, powdered samples were still found to contain up to 40
µ
g
·
g
−1
structurally bound H
(equivalent to 0.036 wt % H
2
O+), which is close to the value of 0.04 wt % H
2
O+ determined by IR
spectroscopy as published in [4].
Minerals 2018, 8, x FOR PEER REVIEW 6 of 15
Significantly higher values for H and C in all Vietnamese samples agree well with a previous study
by Huong et al. [15], who characterized a high abundance of primary CO2-bearing fluid inclusions in
these samples, which likely accounts for the elevated concentrations of both elements, while fluid
inclusion is not present in the Mexican or Tanzanian specimens. High N mass fractions are in the
expected range of metamorphosed sediments, which are involved in danburite formation and are
known to contain ~200–3000 µ g g−1 N for a typical metamorphic gradient of 500–700 °C (e.g., [21]).
Overall lower values of H, C, and N in the powdered Vietnamese sample set support the observation
that these elements are derived from fluid inclusions and were lost during crushing in the agate
mortar. Nevertheless, powdered samples were still found to contain up to 40 µg∙g−1 structurally
bound H (equivalent to 0.036 wt % H2O+), which is close to the value of 0.04 wt % H2O+ determined
by IR spectroscopy as published in [4].
Figure 2. Molar abundance of Al versus (A) ∑REE, (B) Be, and (C) Sr, and Sr versus (D) Be and (E) Y.
Aluminum is negatively correlated with (A) REEs, (B) Be, and (D) partly Sr, while Sr correlates
negatively with (D) Be and (E) Y.
Figure 2.
Molar abundance of Al versus (
A
)
∑
REE, (
B
) Be, and (
C
) Sr, and Sr versus (
D
) Be and (
E
)
Y. Aluminum is negatively correlated with (
A
) REEs, (
B
) Be, and (
D
) partly Sr, while Sr correlates
negatively with (D) Be and (E) Y.

Minerals 2018,8, 234 7 of 15
Minerals 2018, 8, x FOR PEER REVIEW 7 of 15
Figure 3. (A) Molar abundance of Be shows a linear correlation with ∑REE and illustrates that
incorporation of REEs in the danburite structure is accompanied by Be. The Be/∑REE ratio in
Tanzanian samples is approximately 1.5:1, while it varies in Vietnamese samples from ca. 1:1 to ca.
1.5:1. Correlations between Be and ∑REE are ideally suited to distinguish danburite sampling
locations. However, the simple substitution equation Ca2+ + B3+ = REE3+ + Be2+, where the Be/∑REE
ratio is 1:1, is not sufficient to explain the varying ratios of Be and ∑REE. (B) Molar abundance of Be
and Ce shows a linear correlation, implying that incorporation of Ce into the danburite structure is
accompanied by Be. The Be/Ce ratio in Tanzanian samples is approx. 3:1, while in Vietnamese samples
it varies from ca. 2:1 to ca. 3:1. The equation 2 Ca2+ + 3 B3+ = Ce4+ + REE3+ + 3 Be2+, where the Be/Ce ratio
is 3:1, explains the Tanzanian and most of the Vietnamese cases very well. Therefore, we argue here
that Ce occurs not only as Ce3+, but also as Ce4+ in Tanzanian and Vietnamese danburite and the
substitution mechanism 2 Ca2+ + 3 B3+ = Ce4+ + REE3+ + 3 Be2+ takes places in these samples. (C)
Be/(∑REE − Ce) ratios show similar behavior to Be/Ce ratios. As the Be/(∑REE − Ce) ratios vary from
2:1 to 3:1, both substitution mechanisms should take place: Ca2+ + B3+ = REE3+ + Be2+, 2 Ca2+ + 3 B3+ =
Ce4+ + REE3+ + 3 Be2+.
Figure 3.
(
A
) Molar abundance of Be shows a linear correlation with
∑
REE and illustrates that
incorporation of REEs in the danburite structure is accompanied by Be. The Be/
∑
REE ratio in
Tanzanian samples is approximately 1.5:1, while it varies in Vietnamese samples from ca. 1:1 to ca.
1.5:1. Correlations between Be and
∑
REE are ideally suited to distinguish danburite sampling locations.
However, the simple substitution equation Ca
2+
+ B
3+
= REE
3+
+ Be
2+
, where the Be/
∑
REE ratio is 1:1,
is not sufficient to explain the varying ratios of Be and
∑
REE. (
B
) Molar abundance of Be and Ce shows a
linear correlation, implying that incorporation of Ce into the danburite structure is accompanied by Be.
The Be/Ce ratio in Tanzanian samples is approx. 3:1, while in Vietnamese samples it varies from ca. 2:1
to ca. 3:1. The equation 2 Ca
2+
+ 3 B
3+
= Ce
4+
+ REE
3+
+ 3 Be
2+
, where the Be/Ce ratio is 3:1, explains the
Tanzanian and most of the Vietnamese cases very well. Therefore, we argue here that Ce occurs not
only as Ce
3+
, but also as Ce
4+
in Tanzanian and Vietnamese danburite and the substitution mechanism
2 Ca2+ +3B3+ = Ce4+ + REE3+ + 3 Be2+ takes places in these samples. (C) Be/(∑REE −Ce) ratios show
similar behavior to Be/Ce ratios. As the Be/(
∑
REE
−
Ce) ratios vary from 2:1 to 3:1, both substitution
mechanisms should take place: Ca2+ + B3+ = REE3+ + Be2+, 2 Ca2+ + 3 B3+ = Ce4+ + REE3+ + 3 Be2+.

Minerals 2018,8, 234 8 of 15
Table 3.
Average chemical composition (in
µ
g
·
g
−1
and
µ
mol
·
g
−1
) and relative standard deviation (RSD, %) obtained from danburite of three different occurrences.
All concentrations were obtained by fs LA-ICP-MS (n= 12 line scans), except CaO and SiO
2
, which were evaluated with Electron Probe Microanalyzer (EPMA)
(averaged from n= 20 spot analyses per sample).
Element
Isotope Used
L.O.D.
Charcas, San Luis Potosi, Mexico Morogoro, Tanzania Luc Yen, Vietnam
Mex-1 Mex-2 Tanz-1 Tanz-2 Viet-1 Viet-2
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
CaO* – – 22.47 0.04 0.49 22.43 – 0.71 22.63 0.04 0.60 22.48 0.04 0.57 22.32 0.03 0.51 22.40 0.03 0.37
B 11 10 86218 7975 6.67 90471 8368 4.05 86280 7981 1.56 87270 8072 2.69 85698 7927 3.13 89138 8245 3.19
SiO2* – – 48.31 0.8 0.40 48.81 0.8 0.33 48.80 0.8 0.36 48.88 0.8 0.38 48.81 0.8 0.43 48.43 0.8 0.37
La 139
0.01
0.15 0.0011 63.5 0.01 0.0001 124.0 675 4.9 35.3 807 5.8 4.7 368 2.7 42.3 400 2.9 22.0
Ce 140
0.01
0.20 0.0014 48.1 0.05 0.0004 10.00 827 5.9 34.7 964 6.9 3.4 508 3.6 36.9 525 3.8 19.6
Pr 141
0.03
<0.03 – – <0.03 – – 51.4 0.37 35.1 59.5 0.42 4.0 37.4 0.27 35.3 36.5 0.26 17.5
Nd 143
0.08
<0.08 – – <0.08 – – 91.1 0.63 34.5 106 0.74 4.9 80.9 0.56 32.2 72.8 0.51 16.2
Sm 147
0.05
<0.05 – – <0.05 – – 4.65 0.03 28.7 5.68 0.04 9.6 5.94 0.04 27.1 5.23 0.03 18.7
Eu 151
0.05
<0.05 – – <0.05 – – 0.28 0.002 37.7 0.31 0.002 14.1 0.34 0.002 28.5 0.41 0.003 38.2
Gd 157
0.04
<0.04 – – <0.04 – – 2.05 0.013 28.1 2.38 0.015 10.9 2.46 0.016 37.1 2.13 0.014 28.3
Tb 159
0.01
<0.01 – – <0.01 – – 0.15 0.001 22.4 0.16 0.001 15.5 0.18 0.001 29.5 0.20 0.001 26.1
Dy 163
0.05
<0.05 – – <0.05 – – 0.58 0.004 35.7 0.68 0.004 7.80 0.85 0.005 30.5 0.92 0.006 33.7
Ho 165
0.01
<0.01 – – <0.01 – – 0.10 0.001 35.9 0.11 0.001 11.3 0.13 0.001 40.3 0.20 0.001 31.1
Er 167
0.04
<0.04 – – <0.04 – – 0.25 0.001 30.0 0.28 0.002 38.6 0.46 0.003 41.0 0.61 0.004 33.3
Tm 169
0.02
<0.02 – – <0.02 – – 0.03 0.0002 40.5 0.04 0.0002 49.3 0.09 0.0005 31.4 0.12 0.0007 40.1
Yb 173
0.04
<0.04 – – <0.04 – – 0.23 0.0013 44.4 0.26 0.0015 32.5 0.64 0.0037 42.1 0.90 0.0052 22.4
Lu 175
0.02
<0.02 – – <0.02 – – 0.02 0.0001 50.3 0.03 0.0002 43.5 0.12 0.0007 23.7 0.13 0.0007 32.2
Al 27 10 325 12.0 14.9 171 6.4 40.5 84.3 3.1 36.2 101 3.7 7.58 288 10.7 12.8 257 9.5 6.41
As 69 1 46.6 0.6 8.74 7.05 0.1 19.0 <1 0.01 56.5 <1 0.01 67.8 1.13 0.02 103 1.10 0.01 46.0
Ba
135,
137 0.1 0.50 0.004 54.1 0.19 0.001 36.60 0.29 0.002 25.06 0.22 0.002 14.77 0.48 0.003 89.66 0.63 0.005 27.03
Be 9 3 3.06 0.3 12.5 4.94 0.5 26.0 156 17.4 32.8 178 19.8 2.42 71.7 8.0 14.09 67.0 7.4 12.2
Cr 53 5 <5 – – <5 – – <5 – – <5 – – 6.44 0.12 43.8 <5 – –
Cu 65 1 1.56 0.02 65.3 <1 – – <1 – – <1 – – 1.10 0.02 78.2 2.00 0.03 56.4
Fe 57 20 <20 – – <20 – – <20 – – <20 – – <20 – – <20 – –
K 39 7 <7 – – <7 – – <7 – – <7 – – <7 – – <7 – –
Mg 25 9 <9 – – <9 – – <9 – – <9 – – <9 – – <9 – –
Mn 55 1 <1 – – <1 – – <1 – – <1 – – 18.4 0.34 8.93 14.57 0.27 14.3
Na 23 50 <50 – – <50 – – <50 – – <50 – – <50 – – <50 – –
Ni 62 18 <18 – – <18 – – <18 – – <18 – – <18 – – <18 – –
Pb
207,
208,
209 0.1 0.27 0.0013 49.2 0.23 0.0011 40.0 7.46 0.0360 33.8 8.05 0.0389 8.94 11.1 0.0534 5.05 10.6 0.0514 5.60

Minerals 2018,8, 234 9 of 15
Table 3. Cont.
Element
Isotope Used
L.O.D.
Charcas, San Luis Potosi, Mexico Morogoro, Tanzania Luc Yen, Vietnam
Mex-1 Mex-2 Tanz-1 Tanz-2 Viet-1 Viet-2
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Ø
(µg·g−1)
Ø
(µmol·g−1)
RSD
(%)
Sb
121,
123 1 13.0 0.1 81.9 35.8 0.3 75.6 <1 – – <1 – – <1 – – <1 – –
Sr 88 0.1 767 8.8 12.09 1611 14.8 40.6 387 4.4 1.44 381 4.4 1.02 66.5 0.8 2.81 66.1 0.8 5.08
Th 232
0.01
<0.01 – – <0.01 – – 0.47 0.0020 41.3 0.66 0.0028 8.45 0.11 0.0005 51.7 0.11 0.0005 44.6
Ti 49 3 <3 – – <3 – – <3 – – <3 – – <3 – – <3 – –
V 51 0.5 0.71 0.01 36.54 0.98 0.02 19.1 0.84 0.02 10.8 0.83 0.02 11.3 0.73 0.01 33.8 0.95 0.02 22.5
Y 89 0.1 <0.1 – – <0.1 – – 4.21 0.05 35.1 5.23 0.1 6.19 10.6 0.1 21.2 14.1 0.2 27.9
Zn 67 10 59.6 0.9 34.6 27.5 0.4 53.1 15.8 0.2 32.5 15.5 0.2 41.7 63.2 1.0 39.3 89.9 1.4 49.0
* (wt %), excluded due to concentrations below limits of detection (L.O.D.) in
µ
g
·
g
−1
in all samples: Li (<5), Sc (<1), Ga (<0.5), Se (<10), Rb (<0.5), Zr (<0.1), Nb (<0.01), Ag (<0.1), Cd (<1),
Sn (<1), Cs (<0.1), Hf (<0.01), Ta (<0.01), W (<0.01), Ir (<0.01), Pt (<0.01), Au (<0.1), Tl (<0.1), Bi (<0.01), and U (<0.01).

Minerals 2018,8, 234 10 of 15
Table 4.
Light volatile elements in unpowdered and powdered danburite samples provided as
µ
g
·
g
−1
;
calculated H2O+ values are given in wt %.
Sample Location
Unpowdered Samples Powdered Samples
Heq. H2O+
(wt %) C N S H eq. H2O+
(wt %) C N S
Mexico <10 <0.01 200 ±5 50 ±10 13 ±1 31 ±10 0.028 130 ±50 230 ±130 25 ±7
Tanzania <10 <0.01 120 ±40 100 ±50 8 ±1 14 ±1 0.012 70 ±40 100 ±50 22 ±3
Vietnam 300 ±50 0.244 5600 ±20 1000 ±10 15 ±3 40 ±10 0.036 3000 ±40 520 ±20 19 ±11
4. Discussion
4.1. Substitution Mechanisms of REEs, Be, and Sr in Danburite Structure
Referring to the similarity in ionic size and charge, eightfold-coordinated Ca
2+
(1.12 Å) can be
replaced to a certain extent by Sr
2+
(1.26 Å). This substitution commonly takes place in danburite
from all deposits and is mostly observed in the Mexican samples, where the concentration of Sr
reaches 1611
µ
g
·
g
−1
, followed by the Tanzanian and Vietnamese samples, with Sr concentrations up to
387 µg·g−1and 66 µg·g−1, respectively (Table 3).
Ca2+ = Sr2+ (1)
Another substitution in danburite is the replacement of Ca
2+
by REE
3+
(here we presume that
all REEs are trivalent, with the exception of Ce, which can be quadrivalent under strongly oxidizing
conditions). It is obvious that danburite from all deposits prefer to incorporate LREE over HREE by
a 200- to 500-fold enrichment. The REE
3+
have decreasing radii with respect to increasing atomic
number, i.e., from La (1.16 Å) to Ce (1.15 Å) to Lu (0.98 Å). Moreover, LREE radii are more compatible
with the eightfold-coordinated Ca
2+
lattice site. This explains why LREEs, especially La and Ce,
are preferentially incorporated in the danburite lattice. The negative Eu anomaly observed in the
Vietnamese and Tanzanian danburite is in accordance with a general depletion of Eu in highly oxidized
magma, such as granites and pegmatites [22].
The substitution of Ca
2+
by a REE
3+
requires charge compensation and is therefore coupled with
the substitution of B
3+
(0.11 Å) by Be
2+
(0.27 Å) and/or Si
4+
(0.26 Å) by Al
3+
(0.39 Å). These coupled
substitutions theoretically allow all sites to be filled and charges to be balanced accordingly:
Ca2+ + B3+ = REE3+ + Be2+ (2)
Ca2+ + Si4+ = REE3+ + Al3+ (3)
An omission-style substitution of Ca
2+
by a trivalent REE
3+
is also suitable to gain charge balance:
3Ca2+ = 2REE3+ +(vacancy) (4)
However, the positive correlation of molar abundance of REE and Be suggests that Equation (2) is
the dominating process of REE incorporation into the danburite lattice (Figure 3A). The remaining
substitution Equations (3) and (4) are theoretically possible, but are not supported by the datasets,
which show, e.g., a negatively correlated relationship of Al with
∑
REE (Figure 2A). In the Vietnamese
samples, the Be/REE ratio is at an approximate 1:1 trend. In the Tanzanian samples, all the values
are approximately equal to 1.5:1 (Figure 3A). A Be/REE ratio that is equal to or higher than 1:1
indicates that REEs are fully coupled with Be; subsequently, the two other forms of substitution,
Ca
2+
+ Si
4+
= REE
3+
+ Al
3+
(3) and 3Ca
2+
= 2REE
3+
+

(4), are subordinate mechanisms. However,
Equation (2) implies a Be/REE ratio equal to 1:1, rather than the 1.5:1 ratio measured in the
Tanzanian samples. Hence, the excessive molar abundance of Be over REE in the Tanzanian samples
needs to be explained by another substitution process with different ratios for Be and REEs or by
an REE-independent substitution mechanism. This first hypothesis leads to the suggestion that

Minerals 2018,8, 234 11 of 15
Ce occurs not only as Ce
3+
, but also Ce
4+
in the samples (with ionic sizes of 1.15 Å and 0.97 Å,
respectively). The existence of Ce
4+
in geological materials has been observed in various studies [
23
–
26
].
Hence, we extend substitution mechanism (2) to account for the probable presence of quadrivalent Ce:
2Ca2+ + 3B3+ = Ce4+ + REE3+ + 3Be2+ (5)
Equation (5) is an example of a substitution mechanism where the ratio of
∑
REEs
(all REE
3+
and Ce
4+
) to Be is equal to 3:2 (or 1.5:1). According to Equation (5), the ratios
Be/Ce and Be/(
∑
REEs
−
Ce) are both 3:1. Our chemical data (Figure 3B,C) show that the
Be/Ce and Be/(
∑
REEs
−
Ce) ratios in the Tanzanian samples are approximately 3:1 and 1.5:1.
Therefore, we assume that mechanisms (2) and (5) take place predominantly in the Vietnamese
and Tanzanian samples, respectively, with Ce likely present as Ce
3+
and Ce
4+
, respectively. This might
suggest that REE uptake into Tanzanian danburite occurs at elevated oxygen fugacity compared to
Mexican and Vietnamese danburite.
4.2. Substitution Mechanisms Involving OH and Al in the Danburite Lattice
The presence of Be may also be the result of a REE-independent substitution of B
3+
by Be
2+
coupled with the substitution of O2−by OH−:
B3+ + O2−= Be2+ + OH−(6)
The presence of OH
−
species in the danburite lattice was indicated in [
4
,
7
] by means of FTIR
spectroscopy. The bridging oxygen O5 in the B
2
O
7
group is an ideal candidate for partial OH
−
replacement, which allows the presence of low amounts of OH
−
in danburite. However, a coupled
incorporation of Be
2+
and OH
−
was not observed in our study (Figure 4A). Beran [
4
] proposed a
coupled 1:1 substitution of Si
4+
and O
2−
by Al
3+
and OH
−
to charge balance OH incorporation.
However, a direct substitution (1:1) was not confirmed by either dataset in the present study.
Instead, we observed a positive correlation of Al
3+
with OH
−
(Figure 4B) in a 1:3 ratio, which suggests
a coupled incorporation of Al3+ and OH−, substituting for B3+ and O2−, respectively:
2B3+ + 3O2−= Al3+ + 3OH−+(vacancy) (7)
Regarding the possibility of Al incorporation into danburite, it should be noted that the
geochemical behavior of B
3+
and Al
3+
is very similar; however, they differ in radius size, with 0.11 Å for
B
3+
and 0.39 Å for Al
3+
when in a tetrahedral coordination environment. Hence, a simple substitution
mechanism such as B
3+
= Al
3+
would not be possible. Although a substitution of Al
3+
with B
3+
has been observed in the system albite NaAlSi
3
O
8
- NaBSi
3
O
8
reedmergnerite [
27
], as well as in a
synthetic phlogopite KMg
3
(BSi
3
)O
10
(OH)
2
[
28
], this process seems not to be valid for the danburite
datasets (Figure 4A,B). Substitution mechanism (7) explains the Mexican samples as well, where the Al
concentration is high and both REE and Be concentrations are low.
In general, the four main substitution mechanisms discussed above take place with different
priority in the three studied locations. The substitutions of Ca
2+
by Sr
2+
and 2B
3+
by Al
3+
are
more common in the Mexican samples, while the substitutions of Ca
2+
by REEs in the forms
Ca
2+
+ B
3+
= REE
3+
+ Be
2+
and 2Ca
2+
+ 3B
3+
= Ce
4+
+ REE
3+
+ 3Be
2+
are more common in the
Vietnamese and Tanzanian samples, respectively.

Minerals 2018,8, 234 12 of 15
Minerals 2018, 8, x FOR PEER REVIEW 12 of 15
Figure 4. (A) Beryllium and OH correlate negatively, which suggests that incorporation into the
danburite structure is not coupled. (B) However, Al shows a positive correlation with OH at a 1:3
ratio for all sampling locations, making a coupled incorporation of Al with OH likely.
4.3. Constraints on the Geochemical Formation Environment of Danburite
Since danburite samples from Tanzania and Vietnam show a similar strong enrichment in LREE,
the reported low values in the Mexican samples must therefore mean either a deficit of REE in the
source material or REEs were already sequestered in datolite, which co-occurs with Mexican
danburite. However, the depletion of REEs in the Mexican samples is accompanied by exceedingly
high amounts of Sr (ca. 1000 µg∙g−1 on average), which is an independent indicator of a different
source composition with a higher component of biogenic calcareous sediments in the source material
of this location. Biogenic limestone is known to contain high Sr values by being virtually free of REE
[29,30]. In comparison, samples from Vietnam and Tanzania exhibit high REE, Y, and Be coupled
with low Sr, thereby representing a composition that results from the involvement of highly
differentiated late-stage silicic magmas (i.e., pegmatites). This is in agreement with the observed
negative Eu anomalies in these samples, which are characteristic for late magmas, where the
depletion in Eu is driven by fractional crystallization of plagioclase, which is commonly found to
incorporate high amounts of Eu2+ [22]. This is in agreement with the nature of the outcrop and mineral
assemblage in which the danburite was found. The high compatibility of LREE in the danburite lattice
must therefore only be limited by the availability from the source material (i.e., highest in Tanzanian
and lowest in Mexican samples). High contents of REE are in accordance with the significantly low
amounts of N in the Tanzanian samples, due to the incompatibility of N in highly fractionated
magmatic rocks, indicating a strong pegmatite component in the Tanzanian samples. Charge
compensation and a Be/∑REEs ratio of 1.5:1 indicate the presence of Ce4+ (Equation (5)), additionally
supporting a highly oxidized pegmatitic source for the Tanzanian samples. Nitrogen mass fractions
Figure 4.
(
A
) Beryllium and OH correlate negatively, which suggests that incorporation into the
danburite structure is not coupled. (
B
) However, Al shows a positive correlation with OH at a 1:3 ratio
for all sampling locations, making a coupled incorporation of Al with OH likely.
4.3. Constraints on the Geochemical Formation Environment of Danburite
Since danburite samples from Tanzania and Vietnam show a similar strong enrichment in LREE,
the reported low values in the Mexican samples must therefore mean either a deficit of REE in
the source material or REEs were already sequestered in datolite, which co-occurs with Mexican
danburite. However, the depletion of REEs in the Mexican samples is accompanied by exceedingly
high amounts of Sr (ca. 1000
µ
g
·
g
−1
on average), which is an independent indicator of a different source
composition with a higher component of biogenic calcareous sediments in the source material of this
location. Biogenic limestone is known to contain high Sr values by being virtually free of REE [
29
,
30
].
In comparison, samples from Vietnam and Tanzania exhibit high REE, Y, and Be coupled with low Sr,
thereby representing a composition that results from the involvement of highly differentiated late-stage
silicic magmas (i.e., pegmatites). This is in agreement with the observed negative Eu anomalies in
these samples, which are characteristic for late magmas, where the depletion in Eu is driven by
fractional crystallization of plagioclase, which is commonly found to incorporate high amounts of
Eu
2+
[
22
]. This is in agreement with the nature of the outcrop and mineral assemblage in which
the danburite was found. The high compatibility of LREE in the danburite lattice must therefore
only be limited by the availability from the source material (i.e., highest in Tanzanian and lowest in
Mexican samples). High contents of REE are in accordance with the significantly low amounts of
N in the Tanzanian samples, due to the incompatibility of N in highly fractionated magmatic rocks,
indicating a strong pegmatite component in the Tanzanian samples. Charge compensation and a

Minerals 2018,8, 234 13 of 15
Be/
∑
REEs ratio of 1.5:1 indicate the presence of Ce
4+
(Equation (5)), additionally supporting a highly
oxidized pegmatitic source for the Tanzanian samples. Nitrogen mass fractions generally increase with
decreasing temperature and increasing involvement of metamorphic rocks (e.g., [
21
]), which suggests
that the Vietnamese and Mexican samples were either formed at greater distance from the pegmatite
or sourced from a higher proportion of recycled metamorphic rocks. Hence, we conclude that even
though danburite always forms in a transition zone of metacarbonates and pegmatites, there are
significant geochemical differences, i.e.,
∑
REE, Be, Sr, Al, and OH in danburite, that directly reflect the
different proportions and compositions of these source materials. Thus, our results suggest that trace
element concentrations are suitable for determining the origins and locations of danburite crystals
(i.e., for gem-testing laboratories).
5. Conclusions
In this study, we investigated trace element variations of danburite from three different locations
in Mexico, Tanzania, and Vietnam. The most important trace elements in danburite that reflect their
provenance include REEs, Sr, Al, Be, and, to a lesser extent, Mn, Zn, and Y. Mexican samples are
fairly devoid of REEs, while Tanzanian samples contain up to 1900
µ
g
·
g
−1
and Vietnamese samples
have intermediate total values of around 1100
µ
g
·
g
−1
. LREEs are more abundant than HREEs in
all danburite samples, showing a 200- to 500-fold relative enrichment. Strontium and Al are more
enriched in Mexican danburite than in Tanzanian and Vietnamese danburite, with mass fractions up to
1611 and 325 µg·g−1, respectively.
Based on fs-LA-ICP-MS and CHNS analysis, we identified four mechanisms of trace element
substitution in danburite: Two replacements of Ca
2+
by Sr
2+
(Ca
2+
= Sr
2+
) and of B
3+
by Al
3+
, which are
coupled with an incorporation of OH
−
for O
2−
: 2B
3+
+ 3O
2−
= Al
3+
+ 3 OH
−
+

, are dominant in
the Mexican samples. The incorporation of REE
3+
for Ca
2+
coupled with a simultaneous replacement
of B
3+
by Be
2+
is present in the Vietnamese and Tanzanian samples. Different valance states of
Ce are present in the Vietnamese and Tanzanian samples, leading to two different substitutions:
Ca2+ + B3+ = REE3+ + Be2+ and 2Ca2+ + 3B3+ = Ce4+ + REE3+ + 3Be2+, respectively.
The observed significant differences in trace element abundance not only suggest a high potential
for provenance discrimination, but also provide information on the contrasting source compositions of
the three deposits. The formation of danburite generally involves both metacarbonates and pegmatites
as source materials. Different proportions of these two source components were involved in the
formation of danburite at the three locations, and likely explain the observed trace element variations.
Low REE and Be coupled with high Sr, Al, N, and OH in the Mexican samples indicate a dominant
biogenic metacarbonate component, while the Vietnamese and Tanzanian samples show high REE and
Be coupled with low Sr, Al, N, and OH, characteristic of a predominantly pegmatitic source. The 200-
to 500-fold enrichment of LREE over HREE in the Tanzanian and Vietnamese samples results from the
preferential replacement of Ca ions by similarly sized LREE ions. The negative Eu anomaly, which is
characteristic of highly fractionated igneous rocks, is characteristic of Vietnamese and Tanzanian
danburite and supports the predominance of the pegmatitic source at these locations.
Author Contributions:
L.T.-T.H., L.M.O., K.P.J., C.A.H., and K.K. designed and coordinated the study. K.P.J.
and C.A.H. supervised the project. L.T.-T.H., L.M.O., and K.K. collected and prepared the samples. M.W.F.
prepared epoxy mounts and performed electron probe microanalyses. L.M.O., D.S.M., B.S., and U.W. collected
and evaluated trace element concentrations. M.W.F. and O.A. collected and evaluated light volatile element
concentrations and calibrated the CHNS analyzer. L.T.-T.H., L.M.O., and M.W.F. wrote the first drafts of the
manuscript. K.P.J., C.A.H., K.K., and O.A. carefully edited the final version. All authors contributed to the final
version and gave their approval for submission.
Funding:
Vietnamese danburite samples were collected during a field trip funded by a NAFOSTED Project,
grant number 1055.99-2013.13. L.T.-T.H is grateful to the ASEAN-European Academic University Network,
the Austrian Federal Ministry of Science, Research and Economy, and the Austrian Agency for International
Cooperation in Education and Research for financial support.
Acknowledgments:
Lauren Gorojovsky is acknowledged for preparation and analyses of reference materials for
the CHNS elemental analyzer, and we kindly acknowledge the insightful comments by two anonymous reviewers.

Minerals 2018,8, 234 14 of 15
Conflicts of Interest: The authors declare no conflict of interest.
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