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12
3 Mineralogical and thermal characterization of borate minerals
4 from Rio Grande deposit, Uyuni (Bolivia)
5 M. Garcia-Valles1 • P. Alfonso2 • J. R. H. Arancibia3 • S. Martı´nez1 •
6 D. Parcerisa2
7 Received: 21 July 2015 / Accepted: 12 November 2015
8  Akade´miai Kiado´, Budapest, Hungary 2015
9 Abstract Large volumes of borate resources exist in
10 Bolivia, with the most important being the Rio Grande
11 deposit, located close to the Salar of Uyuni. Here, borates
12 occur in beds and lenses of variable thickness. A miner-
13 alogical and thermal characterization of borates from the
14 Rio Grande was made using XRD, FTIR, SEM and DTA–
15 TG. The deposit is mainly composed of B2O3, CaO and
16 Na2O, with minor contents of MgO and K2O. Some out-
17 crops are constituted by pure ulexite aggregates (NaCaB5-
18 O6(OH)65H2O) of fibrous morphology; in other cases,
19 gypsum, calcite and halite also are present. The thermal
20 decomposition of ulexite begins at 70 C and proceeds up
21 to *550 C; this decomposition is attributed to dehydra-
22 tion and dehydroxylation processes in three steps: at 115,
23 150–300 and 300–550 C. The last weight loss of 1–5 % at
24 800 C is due to the removal of Cl2 from the decomposi-
25 tion of halite. DTA shows two endothermic events related
26 to the removal of water; in the first, NaCaB5O6(OH)65H2O
27 evolved from NaCaB5O6(OH)63H2O, at 108–116 C; in
28 the second, NaCaB5O6(OH)6 is formed at 180–185 C and
29 NaCaB5O9 (amorphous) is formed at 300–550 C. The
30 exothermic peak (658–720 C) is related to the crystal-
31 lization of NaCaB5O9. A small endothermic peak appears
32due to the halite melting. Later, another endothermic event
33(821–877 C) appears, which is related to the decomposi-
34tion of NaCaB5O9 into a crystalline phase of CaB2O4 and
35amorphous NaB3O5. The XRD pattern evidences that, at
361050 C, CaB2O4 still remains in the crystalline state. 7
38Keywords Borate minerals  Ulexite  Thermal
39evolution  TA–TG  XRD  FTIR
40Introduction
41Bolivia has large volumes of borate resources, the most
42important being the playa -lake type deposit of the Rio
43Grande, which has total reserves estimated at approxi-
44mately 1.6 Mt of boron [1, 2]. This deposit comprises an
45area of approximately 50 km2 located close to the southern
46part of the Salar of Uyuni in the contact between fluvio-
47deltaic and lacustrine sediments in the Rı´o Grande de Lı´pez
48delta. The deposit is being exploited, and borates are
49commercialized after a natural dehydration process, calci-
50nation and grinding.
51Borates are classified as critical materials by the Euro-
52pean Union [3]. Borate minerals are the main source of
53boron and have a multitude of industrial applications [4]. In
54addition to classical applications of boron in glass,
55ceramics, fertilizers, special alloys, aeronautics, nuclear,
56military vehicles, fuels, electronics and communications,
57new uses appear daily, such as for polymeric materials [5]
58and for imparting halogen-free flame-retardant properties
59to cellulose-based materials [6]. Ca-, Na-borate minerals
60are mainly applied for making fiberglass, but are also used
61for ceramics and ceramic glazes [7, 8]. Other applications
62reported for Ca-rich borates include nuclear technology [9]
63and the refractory industry.
A1 & M. Garcia-Valles
A2 maitegarciavalles@ub.edu
A3 1 Dept. Cristallografia, Mineralogia i Dip. Minerals, Fac.
A4 Geologia, Universitat de Barcelona, c/ Martı´ i Franque`s,
A5 s/n, 08028 Barcelona, Spain
A6 2 Dept. de Enginyeria Minera i Recursos Naturals, Universitat
A7 Polite`cnica de Catalunya, Avd. de les Bases de Manresa.
A8 61-73, 08242 Manresa, Spain
A9 3 Dept. de Ingenierı´a Quı´mica, Universidad Te´cnica de Oruro,
A10 C/ 6 de Octubre-Cochabamba, Oruro, Bolivia
AQ1
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DOI 10.1007/s10973-015-5161-4
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64 The industrial uses of borate minerals greatly depend
65 upon its thermal properties. Therefore, to recommend the
66 optimal application of borate minerals, knowing the ther-
67 mal properties is important.
68 Numerous studies on the thermal properties of borate
69 minerals have been reported; some of them use mixtures of
70 different borate minerals [10, 11] or a specific mineral,
71 such as ulexite [12–14].
72 In this paper, we present a mineralogical and thermal
73 characterization of borates from the Rio Grande deposit in
74 Bolivia with special emphasis on the mineralogy of the
75 different events during thermal treatment.
76 Geological setting
77 The Bolivian Altiplano is a major basin filled with thick
78 sequences of continental sediments of Cretaceous to Ter-
79 tiary age [15]. In the quaternary, endorheic basins of the
80 Altiplano were occupied by large lakes, which progres-
81 sively reduced its size due to intense evaporation and low
82 precipitation that occurred in the region during the last
83 10,000 years, giving rise to the salt lakes and salars, such
84 as Poopo´, Uyuni and Coipasa, in the Central Altiplano.
85 The western and southern areas of the Altiplano were
86 strongly affected by an intense volcanic activity from the
87 Oligocene to the Quaternary. Volcanic rocks range from
88 andesites to rhyodacites with abundant ignimbrites [2],
89 which are considered to be the source of lithium and boron
90 of the salars and nearby evaporitic deposits [16].
91 More than 40 borate deposits occur in the Andean belt
92 related to salars [17]. The Rio Grande borate deposit occurs
93 in the southern region of the salar of Uyuni, the largest on
94Earth, and is composed of deltaic-lacustrine sediments in
95contact with the salt crust (Fig. 1) [18]. Borates precipi-
96tated by capillary rise and subsequent evaporation of the
97groundwater. Silty sediments occur in contact with the
98water of the salar; groundwater rises due to porosity and
99drops evaporate when they reach the surface and the dis-
100solved components precipitate when they come into con-
101tact with the water layer. The borate deposit is not in the
102area of higher concentrations of Li, K and B, but rather
103further to the south. Ulexite precipitation is controlled by
104the concentrations of Ca, Na and B in the brine [2].
105In this deposit, borates occur in beds and lenses of
106variable thickness, from 0.5 to 5 m. In the western region
107of the deposit, the lenses outcrop in small reliefs of several
108cm. In the eastern area of the deposit, the bed of borates is
109present at a depth up to 2 m below the clay level. Borate
110minerals form brittle nodules with a cotton-ball texture
111near the surface interbedded within the fluvial-deltaic
112sediment layers constituted by gypsum, clays and sands.
113The clays are mainly montmorillonite, illite and kaolinite
114[2].
115Materials and methods
116Seven samples of borate minerals were obtained from
117different outcrops in the Rio Grande deposit along 4.5 km,
118and all the samples were collected from the natural
119occurrence in the deposit (Fig. 1).
120The chemical composition was determined by induc-
121tively coupled plasma mass spectrometry (ICP-MS) using
122an Agilent 7500ce OPTIMA 3200RL ICP-MS spectrome-
123ter with a reaction cell.
124The mineralogy of the natural and thermally treated
125samples was determined by X-ray diffraction (XRD). The
126spectra were obtained from powdered samples (particles
127under 45 lm) in a Bragg–Brentano PANAnalytical X’Pert
128Diffractometer system (graphite monochromator, automatic
129gap, Ka radiation of Cu at k = 1.54061 A˚, powered at
13045 kV, 40 mA, scanning range 4–100with a 0.017 2h step
131scan and a 50-s measuring time). The identification and
132semiquantitative evaluation of phases were conducted using
133a PANanalytical X’Pert HighScore software. Chemical
134bonds in the borate structure were also characterized by
135Fourier transform infrared spectroscopy (FTIR). Vibra-
136tional spectra were obtained in the 400–4000 cm-1 range
137using a Perkin Elmer Frontier FTIR spectrophotometer.
138Original borates textures were observed by scanning elec-
139tron microscopy (SEM) using a Quanta 200 FEI, XTE
140325/D8395 environmental scanning electron microscope.
141Thermal evolution of each mineral phase and the nature
142and mechanisms of thermal decomposition were obtained
143by differential thermal analysis and thermogravimetry
Road
Rio Grande
deposit Rio Grande
Bolivia
0 25km
Salar of
Uyuni
Fig. 1 Location of the Rio Grande borate deposit
AQ2
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144 (DTA–TG) using a Netzsch equipment (STA 409C model).
145 Analyses were conducted under N2 inert atmosphere at
146 80 ml min-1 constant flow ratio, using Pt crucible, tem-
147 perature range 25–1200 C with a linear rate of tempera-
148 ture gradient set to 10 C min-1. According to DTA–TG
149 results, to determine the mineral evolution with tempera-
150 ture, the heat treatment temperatures were established with
151 a setting time of half an hour, and subsequent analysis by
152 XRD was performed. The heat treatment ranged from 550
153 to 1050 C.
154 Results and discussion
155 Chemical composition
156 The chemical composition of the Rio Grande borate
157 deposit is presented in Table 1. The main components are
158 B2O3, between 36.21 and 42.60 wt%, CaO, between 12.70
159 and 13.74 wt%, and Na2O, from 7.61 to 13.04 wt%, which
160 suggests that borate minerals constitute the main mineral.
161 In some outcrops, the chemical composition is close to that
162 of pure ulexite (NaCaB55H2O) with 42.95 % B2O3. In
163 other cases, Na is relatively high. Other elements occur in
164 minor amounts; for example, MgO is up to 1.5 wt%, and
165 K2O up to 0.67 wt%. Fe, Al, Sr and other elements occur in
166 trace amounts.
167 Mineralogy
168 In accordance with the data obtained by the chemical
169 analyses, the XRD patterns show that ulexite (NaCaB5
170 O6(OH)65H2O) is the main mineral phase in the borate
171 deposit. Among the wide range of over 160 species of
172 borate minerals, ulexite is one of the most economically
173important [19]. In some cases, it is the only mineral pre-
174sent. However, in most outcrops, other evaporite minerals,
175mainly halite (NaCl) and gypsum (CaSO42H2O), also
176occur. Figure 2 shows a representative XRD pattern from
177the Rio Grande deposit.
178Ulexite occurs as crystalline aggregates with morphol-
179ogy of elongated fibers oriented parallel to each other along
180[001], greater than 100 microns in length (Fig. 3a). Equant
181sodium chloride crystals giving rise to dissolution phan-
182tasms are located among ulexite fibers (Fig. 3b). Gypsum
183also forms euhedral crystals up to several cm in size
184(Fig. 3c).
185Infrared spectroscopy, FTIR (Fig. 4), was used to con-
186firm the mineral phases determined by XRD. Ulexite from
187Rio Grande shows peaks in different spectral ranges. In
188general, the first bands correspond to water stretching
189vibrations and the other bands are simply defined as
190rhombohedral and tetrahedral borate bending modes. A
191broad band with several overlapping peaks is displayed
192between the 3600 and 3150 cm-1 region assigned to the
193stretching vibration mode of the O–H group. The bands at
1941667 and 1632 cm-1 are assigned to the bending mode of
195H–O–H and free water, respectively. The asymmetric
196stretching of three-coordinate boron (BO3) was observed in
197the range of 1479–1240 cm-1. The bands between 1240
198and 1155 cm-1 correspond to B–O–H in plane bending
199modes. An asymmetric stretching mode of B–O in BO4
200was observed between 1027 and 959 cm-1. The bands at
201887–839 and 756 cm-1 are assigned to the asymmetric and
202symmetric stretching of B-O in BO4, respectively. The
203band at 663 cm-1 is the bending to the symmetric
204stretching mode of three-coordinate boron. The final peaks,
205located at 561–546 cm-1, are assigned to the bending
206modes of BO4 groups [14].
207Thermal evolution
208Thermal decomposition of borates is a complex mechanism
209which involves dehydration, polymorphic transition and
Table 1 Chemical composition of the borates from the Rio Grande
deposit
wt% RG-1a RG-1b RG-2 RG-3 RG-4 RG-5 RG-7
CaO 13.38 13.74 11.82 13.83 12.90 13.59 12.70
MgO 0.67 0.42 1.46 0.13 0.69 0.94 1.08
Na2O 9.71 8.36 13.04 7.61 9.95 9.33 10.90
K2O 0.22 0.16 0.67 0.02 0.25 0.28 0.41
B2O3 41.13 40.99 36.21 42.60 39.34 41.65 38.91
Traces/ppm
Sr 151 786 230 247 441 140 149
Mn 1.51 16.6 2.23 5.90 8.74 3.17 2.40
Al 16.6 676 10.6 200 87.6 12.1 17.0
Fe 9.39 376 6.75 105 72.7 18.7 18.3
Ti – 28.1 – 9.52 – – –
As 1.90 8.20 2.00 4.30 9.30 3.40 1.60
5 10 15 20 25 30 35 40 45 50 55 60 65 70
2θ /°
In
te
ns
ity
/a
.u
.
Ulexite
Halite
RG–2
RG–3
Fig. 2 XRD patterns of two representative samples. RG-2 is the
halite rich sample, and RG-3 is nearly pure ulexite
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210 solid phase transformation [20]. In the case of ulexite, the
211 decomposition process develops in the different stages
212 shown in the DTA–TG curves (Figs. 5, 6). Decomposition
213begins at approximately 70 C and proceeds up to
214*550 C; these temperatures can be attributed to dehy-
215dration and dehydroxylation processes [21–23].
216The TG curves indicate that these processes present a
217weight loss in three steps (Table 2); 3.4–5.7 wt% of mass
218loss is attributed to the release of two molecules of crystal
219water at approximately 115 C (1). This loss is lower than
220the expected for the loss of two molecules of water, which
221is likely due to sample manipulation. The second loss,
22211–14.9 wt%, between 150 and 300 C, is attributed to the
223removal of the three molecules of crystal water (2), and the
224last, 11–16.2 wt% of mass loss is due the release of another
225three molecules of crystal water, between 300 and 550 C,
226that corresponds to dehydroxylation of ulexite (3).
227Sener et al. [20] indicate that in the first stage of dehy-
228dration 1.5 of water molecules is released at approximately
229118 C. During the second stage, between 118 and 260 C,
2300.5 in 2.5 water molecules is lost in two endothermic
231events. Later, the OH groups are liberated as three water
232molecules. However, Seyhun et al. [22] attributed the loss
233of three water molecules to the first event and two mole-
234cules to the second event.
235Figure 5 shows the TG curve of the pure ulexite sample;
236in this case, the global weight loss of 33.5 % corresponds
237to eight water molecules, similar to the results obtained in
238other studies of ulexite [14, 22].
239All samples, with the exception of RG-3, exhibit a final
240weight loss of 1–5 % at 800 C due to the removal of
241chlorine from the decomposition of the sodium chloride
242present in the samples.
243The transformations were controlled by XRD analysis at
244temperature intervals (Fig. 5). In the DTA analysis, four
245endothermic and one exothermic events are observed. The
246first two are related to the removal of the crystal water
247shown in the TG measurements at the interval of
248108–116 C (1) and 180–185 C (2) according to the fol-
249lowing reactions:
Fig. 3 SEM images of borates from Rio Grande, a fibrous ulexite,
b ulexite with deliquescent halite, c ulexite accompanied with
euhedral gypsum crystals
3600 3200 2800 2400 2000 1600 1200 800 400
Wave number/cm–1
0.4
0.6
0.8
1
1.2
1.4
Ab
so
rv
a
n
ce
/%
3600
3445
3150 1667
1632
1479
1240 1155
1027
959
839
887
756 561
546
663
Fig. 4 FTIR spectrum of ulexite from Rio Grande salar
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NaCaB5O6 OHð Þ65H2O !
108116 C
NaCaB5O6 OHð Þ63H2O
þ 2H2O
ð1Þ
251 NaCaB5O6 OHð Þ63H2O !
180185 C
NaCaB5O6 OHð Þ6þ3H2O
ð2Þ
253 The next reaction, corresponding to the loss of OH
254 groups, did not explicitly show an endothermic phe-
255 nomenon due to the slow process (3):
NaCaB5O6 OHð Þ6!
300550 C
NaCaB5O9 amorphousð Þ
þ 3H2O ð3Þ
257Ulexite releases eight mol of water in the range of
25870–550 C, and it changes to the CaNaB5O9 form (amor-
259phous borate phase). The XRD patterns (Fig. 2) show the
260amorphous structure of the borates at 550 C. At this
261temperature, the only crystalline phase present is halite.
262The exothermic peak at the interval of 658–720 C is
263related to the crystallization process of the amorphous
264NaCaB5O9 given as follows (4):
NaCaB5O9 amorphousð Þ !
658720 C
NaCaB5O9 crystallineð Þ
ð4Þ
266When the sample is rich in halite, a new endothermic
267peak appears near 803 C due to halite melting (Fig. 6).
268This temperature increases with the halite content.
269The endothermic peak at the interval of 821–877 C
270(Fig. 5) is related to the decomposition of NaCaB5O9 (5).
0 200 400 600 800 1000 1200
60
70
80
90
100
110
–1.2
–0.8
–0.4
0.4
0.8
1.2
0
Temperature/°C
TG
/%
D
TA
/µ
V/
m
g
183 °C
113 °C
1011 °C
877 °C
Exo
658 °C
10 20 30 40 50 60
2θ /°
1050 °C
900 °C
750 °C
658 °C
550 °C HI
HI
HI
a
b
Fig. 5 Pure ulexite (RG-3) a DTA–TG curves and b sequential XRD
patterns showing the new phases formed at each treatment
temperature
0 200 400 600 800 1000 1200
Temperature/°C
60
70
80
90
100
110
–0.4
0.4
0.8
1.2
0
TG
/%
D
TA
/µ
V/
m
g
Exo187 °C
116 °C
803 °C
821 °C
930 °C
720 °C
Fig. 6 DTA–TG curves corresponding to a borate sample rich in
halite (RG-7)
Table 2 Mass loss (wt%) for different ranges of temperature of the
Rio Grande borate deposit
Sample Range of temperature/C
70–115 150–300 300–550
RG-1a 3.4 12.7 13.6
RG-1b 3.4 14.1 13.4
RG-2 3.4 11.0 11.0
RG-3 4.0 14.4 14.8
RG-4 3.7 12.1 12.8
RG-5 3.6 14.1 14.7
RG-7 5.7 14.9 16.1
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271 Similar values have been reported for pure ulexite samples
272 [14, 20]. The temperature of this endothermic peak
273 decreases when the halite content increases in the borate
274 sample, up to 821 C in a sample with 23 wt% of halite
275 (Fig. 6).
NaCaB5O9 crystallineð Þ !
821877 C
CaB2O4 crystallineð Þ
þ NaB3O5 amorphousð Þ ð5Þ
277 The thermal evolution of the Rio Grande borates pro-
278 ceeds as mentioned for other ulexite occurrences [20]. On
279 the contrary, other authors, for example Stoch and
280 Waclawska [21], attributed an endothermic peak at 854 C
281 to the melting temperature of CaB2O4. The crystallization
282 of amorphous NaCaB5O9 directly to CaB2O4 and amor-
283 phous NaB3O5 was also previously suggested.
284 The melting of the previous phases was reported at
285 862 C [14]; Gazualla [24] determined that NaB3O5 melted
286 at 873 C and CaB2O4 melted at 1014 C. Nevertheless, in
287 the present work the experimental temperature reached
288 1050 C, and the XRD patterns indicated the presence of
289 crystalline CaB2O4 at least up to 1050 C (Fig. 5). In
290 addition, in the DTA diagram, obtained up to 1200 C, any
291 endothermic peak characteristic of melting is observed.
292 The endothermic temperature related to the formation of
293 CaB2O4 depends on the alkalis and boron contents. The
294 temperature of NaCaB5O9 decomposition increases with an
295 increase in the boron ratio in tetrahedral coordination to
296 achieve a maximum value, and then, it decreases again
297 (Fig. 7). This point coincides with the boric anomaly,
298which occurs at approximately 30 % molar in Na2O. This
299reaction is produced without mass loss (Fig. 5).
300The crystal structure of NaCaB5O6(OH)65H2O has
301three borate tetrahedra and two borate triangles [25]. When
302the structure is heated, water is released and NaCaB5O9 is
303formed, and the new structure is composed of two borate
304tetrahedra and three borate triangles [26]. Finally, when the
305structure decomposes into CaB2O4 ? NaB3O5, in the Ca-
306borate, crystalline, all the boron groups are in a plane tri-
307angular coordination [27].
308The borate crystal structure was destroyed by dehydra-
309tion, and an amorphous structure was formed; however,
310under heat, the crystalline structure is formed again [28].
311Then, with the temperature increase, ulexite dehydration
312causes changes in the stable phases and, thus, in the ratio of
313tetrahedral and triangular boron coordination.
314Conclusions
315The Rio Grande borate deposit is mainly comprised of
316ulexite, as well as occasional other evaporite minerals, such
317as halite and minor gypsum.
318The thermal evolution of ulexite from the studied
319deposit is shown in the next sequence (6):
NaCaB5O6 OHð Þ65H2O
!
108116 C
NaCaB5O6 OHð Þ63H2Oþ 2H2O
!
180185 C
NaCaB5O6 OHð Þ6
!
300550 C
NaCaB5O9 amorphousð Þ þ 3H2O
!
658720 C
NaCaB5O9 crystallineð Þ
!
821877 C
CaB2O4 crystallineð Þ þ NaB3O5 amorphousð Þ
ð6Þ
321Regarding the heat treatment of borates, this study has
322shown that, at 1050 C, the CaB2O4 crystalline phase is
323still present; therefore, the melting temperature will be
324above this temperature.
325The decomposition temperature of NaCaB5O9 is influ-
326enced by the halite content. This temperature decreases
327when the halite content increases in the sample; however,
328the halite melting temperature rises with the halite content.
329Halite should be removed from borates of the Rio
330Grande deposit to improve the industrial treatment of
331borates.
332Acknowledgements This research was financed by the Project
333AECID: A3/042750/11 and the Consolidated Group for Research of
334Mineral Resources, 2014 SGR-1661) and by the Bosch i Gimpera
335Fundation Project 307466. The authors would like to thank the staff of
336the Centres Cientı´fics i Tecnolo`gics of the University of Barcelona
337(CCiTUB) for their technical support.
0.24 0.28 0.32 0.36 0.4 0.44
Fraction of boron atoms in tetrahedral coordination
Na2O /(1 – Na2O)
820
830
840
850
860
870
Te
m
pe
ra
tu
re
/°C
En
do
th
er
m
ic
 b
re
ak
in
g 
Ca
Na
B 5
O
9
Fig. 7 Representation of the temperature of NaCaB5O9 decomposi-
tion as a function of the fraction of boron atoms in tetrahedral
coordination
AQ3
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338 References
339 1. Risacher F. Origine des concentrations extremes en bore et en
340 lithium dans les saumures de I’Altiplano bolivien. CR Acad Sci
341 Paris Se´r. 1984;2(299):701–6.
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AQ4
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