Synthesis and characterization of wollastonite-2M by using a
1
diatomite precursor
2
D.NOVEMBRE1*,C.PACE1&D.GIMENO2 3
1 Dipartimento di Ingegneria e Geologia, Università G. D’Annunzio, Chieti
4
Scalo, 66100, ITALY 5
2 Dept. Geoquimica, Petrologìa i Prospecció Geològica, Facultat de
6
Geologia, Universitat de Barcelona, 08028, SPAIN 7 (* Correspondence: dnovembre@unich.it) 8 9 Abstract 10
Solid phase reaction synthesis of wollastonite 2M by a natural rock 11
precursor as source of amorphous silica and CaCO3 was here achieved.
12
Chemical treatments were carried out on a diatomitic rock from Crotone 13
(Calabria, Italy) in order to measure its reactive silica and CaCO3 contents.
14
Four series of synthesis were performed at 1000º C at ambient pressure by 15
mixing, at different stoichiometry, the diatomitic rock with, a natural 16
limestone as source of additive CaCO3, and sodium carbonate (Na2CO3) as
17
triggering agent. 18
Wollastonite 2M was characterized by chemo-physical, crystallographical 19
and morphological-microtextural analyses. All these characterizations 20
together with infrared and nuclear magnetic resonance (29Si) responses 21
provide values comparable to literature data. Estimation of the amorphous 22
phase in the synthesis powders was performed through the quantitative 23
phase analysis using the combined Rietveld and reference intensity ratio 24
methods, resulting in a final product of 96.3% wollastonite 2M. 25
26
Keywords: diatomite, calcium silicates, wollastonite 2M. 27
28
Introduction
29
The calcium silicate wollastonite (CaSiO3) is a common mineral in
30
metamorphic and metasomatic (skarn) rocks (Deer et al., 1978) produced in 31
most of cases by the devolatization of an impure limestone precursor. 32
Typically, calcite reacts with silicic magma, even-laminated chert 33
intercalation in carbonate rocks or quartz detrital impurities in limestone in 34
the course of a prograding regional metamorphic (MacKinnon, 1990) or in a 35
thermal metamorphic context to form wollastonite and the resulting CO2
36
escapes from the rock system. In fact, CO2 escaping from the rocks (open
37
system) is critical in order to favor an efficient crystallization process in 38
order to reach a deposit of economic size of this mineral, since a low (or 39
alternatively, water-diluted) partial pressure of CO2 favors the
40
crystallization of wollastonite and a not easy reversibility of the reaction. 41
This need of a relatively natural open-system its explained in local geologic 42
terms (and associated mineral deposit prospecting criteria) in the way that a 43
major fault or structure close to the deposit is needed as a means of a 44
pathway of conduit to deplete the system of CO2 (Beisswenger, 1996).
45
Other natural (non-economic) occurrences of wollastonite have been 46
reported: in marble enclaves reacting with the melt sheet impact crater, a 47
very odd type (Rosa, 2004) and in a number of volcanic sites as 48
pyrometamorphic products of enclaves captured by erupting magmas close 49
to the earth surface, or directly on it (see i.e. Lacroix, 1893; Sabine and 50
Young, 1975; Barker and Black, 1980); and as product of mixing of silicate 51
and carbonate magmas (Dawson et al 1992). 52
Wollastonite synthesis is a fact of twofold interest: in petrogenesis of 53
metamorphic rocks, and in industry. In fact, most of metamorphic petrology 54
manuals contain P-T wollastonite synthesis diagrams, and in a very striking 55
way not always coincident, especially in the low temperature field. A 56
revision of bibliography shows that the attempts to build such a diagram 57
have more than a century, both in theoretical and empirical way, and that all 58
currently used references lie in the work by Harker and Tuttle (1956) (see 59
references therein). On other hand, industrial interest (focused on portland 60
cement clinker and ceramic industries) has led to the making of the lime-61
silica phase diagram (Rankin and Wright 1915, mainly improved by 62
redefinition of thermal limits of phase transition by Welch and Gutt, 1959). 63
Most of described cases involving magma-limestone enclaves reaction are 64
related to basaltic magma. In some cases, like the Somma-Vesuvius 65
volcanic system, reacted large limestone fragments occur as xenolithic 66
nuclei of large sized volcanic bombs, both massive or geodic-drusic 67
(Lacroix, 1893) holding centimeter-sized wollastonite crystals (and a large 68
number of other calc-silicatic minerals). The initial purpose of this 69
experimental work was to explore the mechanisms and timing of 70
wollastonite formation under these conditions (high temperature and low 71
pressure range of synthesis), in order to helping to interpret how fast might 72
grow these enclaves occurring within shallow-intruded of extruding magma 73
basic magma bodies. The well-known process of other calcosilicate 74
formation associated to wollastonite is not considered here. 75
In general, CaSiO3 present two structurally quite different forms (Hesse,
76
1984); the phase stable above ca. 1150°C called pseudowollastonite or α-77
wollastonite and the phase stable below 1150°C called β-wollastonite or 78
wollastonite. A number of polytypes of α-wollastonite have been recognized 79
and described (Yamanaka and Mori, 1981); regarding β-wollastonite, the 80
polytypes are distinguished by the symbols nT and nM, where T and M 81
stand for triclinic and monoclinic symmetry and n indicates the number of 82
subcells with d subcell (100)≈ 7.7Å. The polytype 2M is often called 83
parawollastonite. 84
Wollastonite is in general characterized by an acicular particle shape, 85
whiteness, fluxing properties; these properties lead to its applications in the 86
fields of tile factories, paint, paper and vinyl tile manufactures. Ceramics 87
based on wollastonite, for example, are characterized by several 88
advantages, e.g. low linear thermal expansion coefficient, high strength, low 89
thermal conductivity and nonwettability with aluminum melts (Pavlov et al., 90
2008); used in compositions of paints the needle-laminar shape of 91
wollastonite crystals improves the coverage and strength parameters of 92
coatings and allows for substantial saving of pigments (Nikonova et al., 93
2003); when wollastonite is used in lining in metallurgy, it facilitates fast 94
oxidation of aluminum and the formation of an aluminum oxide film 95
preventing further reaction between wollastonite ceramics and metal melt 96
(Demidenko and Konkina, 2003); wollastonite is also a candidate material 97
for high frequency insulator owing to its low dielectric loss at high 98
frequency (Lin et al., 2006); wollastonite is also used as a substitute of 99
asbestos, since it has not been found to cause any health risks (Nikonova et 100
al., 2003; Teir et al., 2005; Gladun et al., 2007), e.g., replacing potential 101
health-hazard asbestos in automotive brake pads (Justness, 2012). Recently, 102
wollastonite ceramics have been studied as bioactive materials for 103
orthopedic applications and used to improve the mechanical properties of 104
the biopolymers because of its good bioactivity and biocompatibility 105
(Hench, 1991; Lin et al., 2006). 106
The world production of wollastonite is around 600.000 Tm and is largely 107
dominated by China (300-350.000 tonnes), followed by India (100-130.000 108
Tm) and the USA (Brown et al., 2011), and minor productions in Mexico, 109
Finland, Spain. 110
As far as synthesis of wollastonite is concerned, experimental procedures 111
refer to the phase reaction method (Kurczyk and Wuhrer, 1971; 112
Kridelbaugh, 1973; Kurczyk, 1977; Kotsis and Balogh, 1989; Ibañez et al., 113
1990; Grigoryan et al. 2010), the hydrothermal process (Glasser & Taylor, 114
1978; Grigoryan et al., 2006, 2008, 2010), the glass ceramic method (Philip 115
& Binet, 1971), the sol-gel method (Villegas and Fernandez Navarro, 1988); 116
the molten salts technique (Trubnikov et al., 1989); the microwave method 117
(Vichaphund et al., 2011) and the hydrothermal microemulsion method (Lin 118
et al., 2006). 119
Kridelbaugh (1973) studied the kinetic of the reaction calcite(s) + quartz(s)=
120
wollastonite(s) + carbon dioxide(g) using powders of natural calcite and
121
quartz employing cold-seal pressure vessels. Temperature, pressure and 122
grain size were varied to determine their effects upon reaction rate 123
(temperature ranged from 800 to 950°C, pressure varied from 1000 to 124
25.000 psi and grain size ranged from 10 to 70 microns). As it is 125
predictable, higher temperatures and finer sizes favored reaction rates, while 126
variation in pressure produced erratic behavior. The author explains that in a 127
dry system nucleation of wollastonite occurs on the quartz grains, while in a 128
fluid phase of CO2 and H2O, wollastonite forms on calcite fragments. The
129
activation energy for the reaction CaCO3 + SiO2 = CaSiO3 + CO2 is between
130
25.5 and 29 Kcal/mole and the rate equation is of parabolic form X=ktn, 131
where X is the percent reaction, k the rate constant, t the time and n the 132
order of the reaction. 133
Regarding the solid state method, Bryden et al., (1992) and Ibañez and 134
Sandoval (1993) pointed out that, when mixing silica and lime raw materials 135
of natural and technogenic occurrences in a wide range of compositions, the 136
first phase identified is a β-calcic orthosilicate; it deals of a thermodynamic 137
instable phase that, for molar compositions CaO/SiO2= 1, transforms into
138
wollastonite according to the diagram of Welch and Gutt (1959). 139
In the past a lot of natural silica-containing materials have been applied in 140
the synthesis of wollastonite, i.e. SiO2 smokes (Kotsis and Balogh, 1989;
141
Nour et al., 2008), silica sources characterized by high specific surface, as 142
silica gel, or amorphous materials such as opale or diatomites (Grigoryan et 143
al. 2008) or gaizes (Grigoryan et al. 2010). In particular, Grigoryan et al. 144
(2008) report on the synthesis of calcium hydromonosilicate from diatomite 145
under hydrothermal conditions (150°C) and its transformation into 146
wollastonite (α + β) at 1150°C. Grigoryan et al. (2010) report on the 147
synthesis of β-wollastonite by using a natural source of amorphous silica 148
and CaO (a Lithuanian gaize) with different methods, i.e. the phase reaction 149
method, the hydrothermal one and the cogranulated technique. In particular, 150
with the phase reaction method the gaize was mixed to calcium carbonate 151
(providing a CaO/SiO2 molar ratio of 0.95) in the presence of Na2CO3
152
mineralizer; the mixture was placed in alumina crucibles and exposed for 153
two hours in a furnace at the temperature of 1000°C. The product 154
synthesized reveals an X-ray pattern characteristic of β-wollastonite 155
associated to a phase indicated as β-(CaO)2SiO (probably a
β-dicalcic-156
silicate). However, it can be noticed that a very poor characterization of the 157
synthesized products is provided, only establishing the density of the 158
resulting product in 2.7382 g/cm3 and not reporting the X-ray pattern but
159
only the d/n values. The synthesized powders are in effect polimineralic, 160
being the presence of wollastonite associated to that of the other phase β-161
(CaO)2 SiO; moreover the absence of unreacted phases and /or amporphous
162
components cannot be excluded. 163
The scope of this work is to synthesize monomineralic powders only 164
constituted by wollastonite 2M by the using a natural source of amorphous 165
silica and CaO never used in the past with the solid state method, i. e. a 166
diatomitic rock from Crotone (Italy). Systematic samplings during the 167
experimental run furnish the way to follow the progress in the crystallization 168
of the mineral and consent to fix the time at which the climax in the 169
crystallization is reached due to the absence of other phases. 170
Complementary characterizations (calculation of the density, the specific 171
surface, the Rietveld structure refinement, the morphological analysis, and 172
the infrared and nuclear magnetic resonance analyses) together with the 173
estimation of the amorphous phase and unreacted CaO in the synthesis 174
powders will represent a complete spectrum of characterization of the 175
synthesized mineral. Furthermore, a discussion on the geologic significance 176
of the achieved synthesis is provided, giving special attention to the 177
pyrometamorphic processes related to active volcanism. 178
Experimental
180
Methods 181
Diatomitic rock finely grinded (particle size below 60μm) and products of 182
synthesis were analysed by powder X-ray diffraction (XRPD); the 183
instrument was a Siemens D5000 operating with a Bragg-Brentano 184
geometry; CuKα=1.518 Å, 40 kV, 40mA, 2-45°, 2-90° 2theta scanning 185
interval, step size 0.020° 2theta). Identification of phases and relative peak 186
assignment were performed with reference to the following JCPDS codes: 187
00-043-1460 for wollastonite 2M; 00-043-1001 for CaO; 00-016-0152 for 188
cristobalite and 00-033-0302 for larnite. 189
Both the crystalline and amorphous phases in the synthesis powders were 190
estimated using quantitative phase analysis (QPA) applying the combined 191
Rietveld and reference intensity ratio (RIR) methods; corundum NIST 676a 192
was added to each sample, amounting to 10% (according to the strategy 193
proposed by Gualtieri 2000), and the powder mixtures were homogenized 194
by hand-grinding in an agate mortar. Data for the QPA refinement were 195
collected in the angular range 5-120° 2theta with steps of 0.02° and 10s step
-196
1, a divergence slit of 0.5° and a receiving slit of 0.1mm.
197
Data were processed with the GSAS software (Larson & Von Dreele, 1997) 198
and the graphical interface EXPGUI (Toby, 2001) starting with the 199
structural models proposed by Hesse (1984) for wollastonite 2M. The 200
following parameters were refined: background parameters, zero shift, cell 201
parameters and peak profiles. 202
Morphological analyses were obtained by means of scanning electron 203
microscopy (JEOL JSM-840 served by a LINK Microanalysis EDS system, 204
with operating conditions of 15kV and window conditions ranging from18 205
to 22 mm) (Ruggieri et al., 2011). 206
Induced coupled plasma optical emission spectroscopy technique (ICP-207
OES, Perkin Elmer Optima 3200 RL) was performed on synthesized 208
powders through previous fusion (Pt meltpot) in lithium meta-tetra borate 209
pearls and subsequent acid solubilisation and analytical determination 210
following the procedure as explained in Fernandez-Turiel et al. (2003). 211
Wollastonite 2M density was calculated by He-picnometry using an 212
AccuPyc 1330 pycnometer. The specific surface and porosity were obtained 213
by applying the BET (Brunauer-Emmett-Teller) method with N2 using a
214
Micromeritics ASAP2010 instrument (operating from 10 to 127 kPa), 215
following the procedure as explained in Ruggieri et al. (2010). 216
The infrared analysis was performed with a spectrometer FTLA2000, served 217
by a separator of KBr and a DTGS detector; the source of IR radiation was a 218
SiC (Globar) filament. Samples were treated according the method of 219
Novembre et al. (2011) using powder pressed pellets (KBr/sample ratio of 220
1/100, pressure undergone prior determination 15t/cm2); spectra were 221
processed with the program GRAMS-Al (GRAMS/AI TM Spectroscopy 222
Software, Thermo Scientific Company). 223
29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR)
224
spectra were recorded using a BRUKER AVANCE-SPECTROSPIN 225
spectrometer (7.05 Tesla). Spectra were processed by the program Bruker 226
XWINNMR. A 90 excitation pulse (8 µs) and a 5 s relaxation delay were 227
used to collect a variable number of scans (from 100 to 400) at 4000 Hz of 228
rotation speed in 7 mm zirconia rotor. The 29Si chemical shifts were 229
referenced to 3-(trimethylsilyl) -2,2,3,3- tetradeutero propionic acid (sodium 230
salt). 231
232
Siliceous raw material: the “Tripoli rock” 233
Diatomitic rock here utilized is a siliceous rock cropping out in the Crotone 234
Basin, in Southern Italy. According to Novembre et al. (2004, 2014) XRPD 235
analysis reveals a general mineralogical composition mainly made of 236
amorphous opaline silica, and secondarily of quartz, montmorillonite, 237
chlorite, kaolinite, K-micas and minor carbonates. A vertical and lateral 238
variability in the chemistry of the deposit is observed; calcimetric 239
(volumetric) analysis reveals for the sample here used a mean value of 10% 240
in CaCO3. Following the method proposed by Novembre et al. (2004), i.e.
241
acid solubilization (HCl 36-37%) and successive attack with NaOH (3M), 242
"Tripoli" final composition resulted in: 10 wt% carbonates; 22 wt% 243
insoluble fraction (clay minerals and quartz); 68 wt% opaline silica. 244
245
Synthesis 246
Three series of synthesis were performed characterized by different 247
stoichiometry, i.e. a fixed amount of Tripoli was mixed to variable amounts 248
of CaCO3 (sample L3 from Ruggieri et al., 2008) and sodium carbonate
249
(Na2CO3) as triggering agent (Table 1). A fourth experiment was performed
250
without sodium carbonate. The mixtures, finely grounded and powdered, 251
were heated inside alumina crucibles at 1000°C and ambient pressure for 252
two hours. 253
Synthesis products were sampled periodically and a loss of weight of the 254
open crucible was observed and attributed to CO2 diffusion through the
granular material (Novembre et al., 2010; Novembre et al., 2010; Pasculli 256
and Novembre, 2012). 257
258
Results and discussion
259
Mineralogical and chemical characterization of synthetic products 260
Results of XRPD analyses performed on the three synthesis runs are 261
illustrated in Figs 1, 2 and 3 while the mineralogical assemblages for each 262
synthesis run is reported in Table 1. 263
In synthesis run 1 (Fig. 1) the crystallization of Wollastonite 2M, formerly 264
associated to that of meta-cristobalite, unreacted calcium oxide and larnite, 265
is evident in the time interval 0.5-1.5 h; situation does not substantially 266
change with time, except for the disappearance of meta-cristobalite, as 267
visible in the spectrum at 2h. 268
In synthesis run 2 (Fig. 2) the crystallization of Wollastonite 2M is 269
associated with meta-cristobalite, unreacted calcium oxide, larnite and 270
quartz (contained in the starting Tripoli rock) in the time interval 0.5 -1h; 271
longer synthesis periods led to disappearance of larnite and CaO thus 272
resulting in parawollastonite plus meta-cristobalite in the time interval 1.5 - 273
2h. 274
In synthesis run 3 (Fig. 3) Wollastonite 2M is associated with calcium 275
oxide, larnite and quartz at 0.5h; appearance of meta-cristobalite is testified 276
at 1h; in the time interval 1.5-2h disappearance of quartz, meta-cristobalite 277
and calcium oxide is attested, thus resulting in sole Wollastonite 2M. 278
As specified in the above paragraph, “Tripoli rock” contains minor amounts 279
of clay minerals. Pimraksa and Chindaprasirt (2009) studied the 280
diatomaceous earth of the Lampand Province in the north of Thailand and 281
observed that is constituted not only by diatoms, but also by clay minerals 282
such as kaolinite, montmorillonite and illite. They used these diatomaceous 283
earth for the production of lightweight bricks and analysed their pozzolanic 284
behaviour. The authors observed that calcination temperature of 700°C led 285
to dehydroxylation of kaolinite (Le Chatelier, 1887; Jeffries, 1944;Frost and 286
Vassallo, 1996; Fernandez et al., 2011 etc…); it resulted the collapse of the 287
clay minerals structures as confirmed by the disappearance of clay minerals 288
peaks in the xrd spectra, and relative change of the coordination state of Si 289
and Al atoms as traced by NMR study (Sanz et al., 1988; Chandrasekhar, 290
1996; Gimeno et al., 2003; Drits et al., 2016). 291
Also in our case the calcination temperature of 1000°C provokes the 292
disruption of clay minerals structure and this is confirmed by the absence of 293
the peaks characteristic of clay minerals in the xrd spectra of Figgs. 1, 2 and 294
3, so we can state that the presence of clay minerals do not constitute an 295
obstacle in the synthesis of wollastonite. 296
As far as the presence of meta-cristobalite in the analyzed spectra is 297
concerned, Vakalova et al. (2009) report active crystallization of this phase 298
during heating of an amorphous-crystalline raw material (diatomite) at 299
1000°C; in particular, the authors explain that crystallization of meta-300
cristobalite can be caused both by transformation of quartz and by 301
crystallization of amorphous silica components. In fact, Tripoli raw 302
mineralogical assemblage is characterized by the presence of minor 303
quantities crystalline quartz. When analyzing the spectrum at 0.5h (synthesis 304
run 2) the presence of quartz is associated with that of meta-cristobalite; in 305
the successive spectrum registered at 1h, a reduction in the height of quartz 306
peaks is evident and a contemporary increase in counts relative to 307
cristobalite peaks is attested (see the zoomed area in Fig. 2). The same 308
considerations can be made for synthesis run 3 with respect to sample at 0.5 309
and 1h, respectively (zoomed area in Fig. 3). In particular, no evidence of 310
meta-cristobalite is revealed in the spectrum registered at 0.5 h of synthesis 311
run 3, thus proving that the rate of transformation of quartz in meta-312
cristobalite is higher in synthesis run 3 than in synthesis run 2. 313
In the run synthesis 1 no evidence of quartz is revealed, this signifying that 314
disruption of this mineral in favor of meta-cristobalite proceeds at a higher 315
rate than in the other synthesis runs. 316
From the analysis of the above spectra, it is clear that all the synthesis runs 317
attest crystallization of wollastonite 2M. 318
In particular, run syntheses 1 and 2 provide the crystallization of 319
wollastonite 2M always in association with other phases. The presence of 320
unreacted CaO is evidenced at the end of run synthesis 1, attesting non 321
optimal kinetic condition here achieved; moreover presence of the dicalcic-322
silicate larnite is evidenced also at the end of the run, thus proving a starting 323
mixture characterized by an excess in CaO with respect to the stoichiometry 324
of wollastonite formula. 325
On the other hand, no presence of unreacted CaO is revealed at the end of 326
synthesis run 2; it can be also stated that in this case larnite appears as a 327
metastable phase during the transient, fastly disappearing in the sample at 328
1h. On the contrary, meta-cristobalite rests at the end of the run in 329
association with wollastonite 2M, and this can be justified with a starting 330
mixture characterized by an excess of SiO2 with respect to the stoichiometry
331
of wollastonite formula. 332
Run synthesis 3 seems to be the best one as wollastonite rests as isolated for 333
a specific time interval (1.5-2h), i.e. with no presence of larnite and meta-334
cristobalite and no traces of residual CaO. 335
Once established that the best conditions in the synthesis of wollastonite 2M 336
are those of run synthesis 3, a fourth experiment (run synthesis 4) was 337
performed without the use of Na2CO3, in order to test the real utility of the
338
mineralizer (Table 1). The results of the XRD analyses are reported in Fig.4. 339
It results a slowdown in the kinetics of synthesis; Wollastonite 2M is in fact 340
associated with calcium oxide, larnite and quartz at 2h; appearance of meta-341
cristobalite is testified at 5h; in the time interval 7-10h disappearance of 342
quartz, meta-cristobalite and calcium oxide is attested, thus resulting in sole 343
Wollastonite 2M. These results are quite coherent with the ones obtained, at 344
lesser temperatures and much larger times in the classical experiences of 345
Harker and Tuttle (1956). 346
Results of run synthesis 4 confirmed the efficacy of the use of Na2CO3 as
347
triggering agent. 348
For these reasons, only synthesis run 3 was considered for more detailed 349
characterizations. 350
Rietveld structural refinement was conducted on run synthesis 3. In Fig. 5 351
the observed and calculated profiles and difference plot for wollastonite 2M 352
and Corundum NIST 676a with tick marks at the positions of the Bragg 353
peaks are reported for the sample at 2h of synthesis run 3. Table 2 shows the 354
results of the Rietveld refinement. The cell parameters, refined with a 355
monoclinic symmetry, space group P 21/a, are a0= 15.42264 (0.00054);
356
b0=7.31691 (0.00061) Å and the c0=7.05752 (0.00049) Å. The results of the
357
QPA analysis (Table 2) indicate that the calculated amorphous phase in the 358
sample at 2h of synthesis run 3 is 3.5(9) %, thus resulting in a final product 359
of 96.3(9) % wollastonite 2M. 360
Chemical analysis of wollastonite 2M (sample at 2h of synthesis run 3) is 361
reported in Table 3. The values of the SiO2/CaO ratio are in agreement with
362
bibliographic data (Tolliday, 1959). The chemical formula of wollastonite 363
2M was calculated to (Si5.91Al0.11) (Ca5.93Mg0.07K0.04) O18.
364 365
Morphological and physical characterization 366
Textural characterization of synthesis run 3 is illustrated in Fig. 6 a-b. Clear 367
crystals of parawollastonite are evidenced in sample at 2h. It results a 368
acicular-needled shaped morphology; the average size of the crystals is of 369
about 7 μm in length and of about 0.9 μm in width. 370
Specific surface of the mineral is reported in Table 4. It results a value of 371
1.4203 (0.0035) m2/g that is in good agreement with the range interval 372
reported by Teir et al. (2005). As far as density is concerned, value of 373
2.7818 (0.0290) g/cm3 was calculated for wollastonite 2M; the value is 374
comparable to that proposed by Grigoryan et al. (2008) for the same 375
mineral. 376
377
Infra-red spectroscopy (IR) and 29Si magic angle spinning nuclear magnetic 378
resonance (29Si MAS NMR). 379
Infra-red spectroscopy (IR) absorption spectra obtained from synthesis run 3 380
(samples at 1.5 and 2h) are shown in Fig.7. Values of absorption bands 381
found here are compiled in Table 5 together with known data for 382
parawollastonite (Ptacek et al., 2010). In particular, six bands are evidenced 383
in the region of the asymmetric stretch and located respectively at 1086, 384
1063, 1012, 962, 936 and 898 cm -1. On the other hand, a sole band is 385
relative to symmetric stretch and positioned at 683 cm -1. 386
The 29Si MAS-NMR spectrum of wollastonite 2M (sample at 2h, run 387
synthesis 3) is illustrated in Fig. 8; it reveals a peak at -92.95 ppm attributed 388
to Si (4Al) site in mineral structure, whose position is coherent with data by 389 Gillespie (2007). 390 391 Geological consequences 392
Occurrence of wollastonite in plutonic (i.e. granitic)-carbonatic host rock 393
contact is a relatively common and limited (in most cases some cm in 394
thickness) effect of metasomatism of silica. In these case evidence of 395
widespread assimilation of limestone by magma seems to be rare, and it is 396
difficult to note these effects on the magmatic side of the contact. By 397
opposite of this, in volcanic or shallow (basaltic) subvolcanic environment a 398
number of cases (Reverdatto 1970, Nichols 1971, Sabine and Young 1975, 399
Baker and Black 1980, Michaud 1995, Chadwick et al 2007) of 400
pyrometamorphism of limestone (and flint) xenoliths have been described, 401
as well as limited but very characteristic assimilation of lime by magma 402
(increase of total lime in a 30-60 %); available data suggest that the range of 403
temperature might be 1000-1100 ºC or higher. Under these circumstances, 404
wollastonite is a common phase in the paragenesis, both on the 405
metamorphic rock and as phenocrysts and microliths in the volcanic rock 406
(see i.e. Sabine and Young 1975, Baker and Black 1980). Precipitation of 407
wollastonite forming liquid has been also described in Ca-rich synthetic 408
medieval glass (see i.e. Gimeno & Pugès 2002) and industrial glass (Pirsson 409
1910). Also, late-stage hydrothermal metasomatic generation of wollastonite 410
(i.e., hydrogrossular) is not uncommon, together with the occurrence of 411
other minerals in the range of 850-650 ºC or lesser (Sabine and Young 1975, 412
Baker and Black 1980). 413
In most of these occurrences the calk-silicate paragenesis is complex, and 414
therefore somewhat it is difficult to interpret directly in terms of temperature 415
of formation (under low pressure conditions). Also, some of the 416
characteristic mineral phases of basaltic magma (typically clinopyroxene) 417
persists changing their composition, with a sharp increase of lime in the 418
final stoichiometry of resulting phase. Thus the approach in order to study 419
the types of mineral reactions involved, the phases present (stable and 420
metastable) in the paragenesis, the kinetics of the crystallization process, the 421
telescoping of paragenesis under successive retrograding conditions, the 422
trace element balance of the assimilation process, and even the true 423
significance and dimension of these assimilation remain obscure. A possible 424
way to increase our knowledge is to continue the work here developed, with 425
the design of specific experiments for the main single phases involved (i.e., 426
diopside, grossular, etc,) in order to clarify the range of temperature and the 427
minimum time required in order to achieve the synthesis of each phase. 428
A very interesting (and still poorly developed) way of pyrometamorphism 429
and carbonate assimilation processes is the study of radiogenic isotopic 430
mineral chemistry, a fact that has been explored studying the intracristalline 431
isotope variation in plagioclase phenocrysts in the case of Merapi volcano 432
(Chadwick et al. 2009), the whole rock (and groundwater) strontium 433
exchange in a subvolcanic plug (Sabine et al 1982), and, in a pyroclastic 434
context, by the separate study of the mineral radiogenic isotope (Sr) 435
chemistry (diopside, salite, feldspar, glass and whole rock) during the 436
evolution of a single eruptive (Mercato) event of Vesuvius (Aulinas et al., 437 2008). 438 439 Conclusions 440
This work has achieved synthesis of wollastonite 2M from a natural cheap 441
and abundant geologic resource by a simple chemical process. Among the 442
three synthesis runs proposed, the best results are evidenced for run 443
synthesis 3; in this case, in fact, the presence of parawollastonite is 444
evidenced as isolated after only 1,5h of the thermal treatment. When 445
replicating this experiment in the absence of the mineralizer a remarkable 446
dilation of synthesis rates is evidenced as the presence of sole wollastonite 447
is reached with 5.5h of delay. The use of diatomite as reactant suggests that 448
synthesis might be enhanced, or at least can produce intermediate products 449
not usually considered previously. 450
Efficacy of the experimental protocol here proposed is tested through 451
chemical-physical, crystallographic, morphological and spectroscopic 452
characterization of experimental products of run synthesis 3. 453
Results evidence values comparable to the reported ones in the literature for 454
the mineral. Estimation of the amorphous phase in the synthesis powders 455
gives a final product of 96.3% wollastonite 2M. These results seem to allow 456
transfer to an industrial production scale. 457
This experimental work can be also applied to the geological problem of 458
pyrometamorphism (sanidinite facies conditions) of limestone xenoliths by 459
basic magmas in shallow or earth surface environments. The results suggest 460
that synthesis can be easily achieved in extremely short periods of time 461
(hours) and therefore can be produced during an ongoing eruption, both in 462
thick, quickly deposited, proximal piles of agglomerate (made by bombs 463
with a xenolitic limestone core), in xenolithic limestone-bearing lava flows 464
or in a shallow intrusive environment at the top of a magmatic chamber 465
where the host rocks are constituted by limestone (i.e. Vesuvius). In earth 466
surface or in a shallow open-system media CO2 can be evacuated and this
467
greatly favours the formation of wollastonite, by contrast with the case of 468
closed-system regional or contact metamorphism. 469
Also, this study offers new questions to be explored in the future: how 470
comparable is the experimental work conducted with magma-limestone 471
interaction? How comparable are basic and silicic magmas respect to 472
wollastonite synthesis (influence of composition, lower temperature in the 473
case of the silicic one, kinetics of the reaction)? It seems also necessary to 474
introduce MnO (and FeO) in the reagents in order to reach best reproduction 475
of natural wollastonite. 476
On other hand, this study does not clarify the lesser viable temperature 477
conditions for wollastonite synthesis, a fact of twofold interest, industrial 478
and in terms of metamorphic petrology (i.e., wollastonite synthesis is 479
possible to develop as an exhalite sediment under high temperature 480
hydrothermal conditions?). 481
Last but not least, the use of amorfous silica (diatomite) in the synthesis 482
confirms that meta-cristobalite is synthetized and this shows that the well-483
stablished SiO2-CaO-CO2 system merits further experimental attention in
484
the low temperature field. 485
486
Acknowledgements
The analytical work was supported by the CGL2011-28022 of MICINN 488
Spanish research contract (responsible D.G.). The chemical treatment of raw 489
material, synthesis processes and XRD characterization of synthesized 490
zeolites were performed at the Dipartimento di Ingegneria e Geologia 491
dell’Università “D’Annunzio”, Chieti, Italy. All the other analyses were 492
conducted at the Centres Científics i Tecnològics of the Universitat de 493
Barcelona (CCiT-UB), Spain. The authors thank the technical staff at CCiT 494
(UB) for their great help during the work. 495
496
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735 736
Figure captions
737
Fig. 1 XRPD analysis of synthesis run (1). Woll: wollastonite 2M; Lar: 738
Larnite; Crist: cristobalite. 739
Fig. 2 XRPD analysis of synthesis run (2). Woll: wollastonite 2M; Lar: 740
Larnite; Crist: cristobalite; Qz: quartz. 741
Fig. 3 XRPD analysis of synthesis run (3). Woll: wollastonite 2M; Lar: 742
Larnite; Crist: cristobalite; Qz: quartz. 743
Fig. 4 XRPD analysis of synthesis run (4). Woll: wollastonite 2M; Lar: 744
Larnite; Crist: cristobalite; Qz: quartz. 745
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difference plot for Wollastonite 2M and Corundum NIST 676a with tick 747
marks at the positions of the Bragg peaks. From the Bottom: Wollastonite 748
2M, Corundum Nist 676a. 749
Fig. 6 a-b: SEM images of wollastonite 2M crystals synthesized at 2h (run 750
synthesis 3). 751
Fig. 7 Infrared spectra of synthesis run (3). 752
Fig. 8 29Si MAS-NMR spectrum: sample at 2h (synthesis run 3) of 753 wollastonite 2M. 754 755 756 757
Synthesis run starting mixture moles Mineralogical Assemblage
0.5-1.5h: Cristobalite + Wollastonite 2M + Calcium oxide + Larnite;
2h: Wollastonite 2M +Calcium oxide + Larnite
0.5h: Wollastonite 2M + Cristobalite + Calcium oxide + Larnite;
1-2h: Wollastonite 2M + Cristobalite 0.5h: Wollastonite 2M + Cristobalite +
Calcium oxide + Larnite;
1h: Wollastonite 2M +Calcium oxide + Cristobalite;
1.5-2h: Wollastonite 2M 2h: Wollastonite 2M + Cristobalite +
Calcium oxide + Larnite;
5h: Wollastonite 2M +Calcium oxide + Cristobalite; 7-10h: Wollastonite 2M
4 3.000gr Tripoli + 2.000gr CaCO3 0.03SiO2 - 0.03CaO 3 3.000gr Tripoli + 2.000gr CaCO3 + 0.096gr Na2CO3 0.03SiO2 - 0.03CaO 1 3.000gr Tripoli + 5.000gr CaCO3 + 0.128gr Na2CO3 0.03SiO2 - 0.05CaO 2 3.000gr Tripoli + 1.000gr CaCO3 + 0.076gr Na2CO3 0.03SiO2 - 0.01CaO
Wavelenght (Å) 1.5418 No. of observation 1597
Rwp 0.16
Rp 0.12
CHI2 2.35
Space group Wollastonite 2M P21/a
a (Å) 15.42264(0.00054)
b (Å) 7.31691(0.00061)
c (Å) 7.05752(0.00049)
% amorphous 3.5(9)
sample CaO [%] Si O2 [%] Al2O3[%] MgO[%] K2O [%] SiO2/CaO Time(h) XRD spectrum
ICP -2h 47.5 50.75 0.11 0.07 0.04 1.06 2h Wollastonite 2M
Asymmetric stretch Symmetric stretch
Si – O – Ca Si – O – Si
2h 1086-1063-1012-962-936-898 683
Woll (a) 1081-1060-1016-965-925-900 682 sample
