Thermal and XRD analysis of synthesis of fluoro-topaz

Article abstract of DOI:10.1016/j.tca.2012.03.006

The present work deals with thermal and XRD analysis of fluoro-topaz synthesis by sintering of aluminium and silicon oxides (in the form of corundum and quartz) with ammonium fluoride and carried out in open Pt crucibles (for XRD studies) and a closed furnace of simultaneous DTA/TG thermoanalytical apparatus called MOM Derivatograph. The furnaces are connected to a gas suction device to remove gas decomposition products. The study shows the endothermic peak at 125-155°C, representing the formation of ammonium aluminium hexafluoride and cryptohalite. The endothermic peak at 220-240°C represents the beginning of dissociation of ammonium aluminium hexafluoride to the more stable ammonium aluminium tetrafluoride. The endothermic peak at 340-350°C represents the dissociation of cryptohalite and ammonium aluminium tetrafluoride with the formation of aluminium fluoride. The formation of fluoro-topaz is represented by the wide endothermic peak at 770-800°C. The medium wide or sharp endothermic peaks at 950°C and 1000°C represent the subsequent dissociation of fluoro-topaz to corundum or mullite, respectively.

Full text of DOI:10.1016/j.tca.2012.03.006

Thermochimica Acta 538 (2012) 29–35
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Thermal and XRD analysis of synthesis of fluoro-topaz
∗
A.M. Abdel-Rehim
Alexandria University, 69 Sultan Hussein Str, Shallalat, Alexandria, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2011
Received in revised form 11 March 2012
Accepted 12 March 2012
Available online 21 March 2012
The present work deals with thermal and XRD analysis of fluoro-topaz synthesis by sintering of alu-
minium and silicon oxides (in the form of corundum and quartz) with ammonium fluoride and carried
out in open Pt crucibles (for XRD studies) and a closed furnace of simultaneous DTA/TG thermoanalytical
apparatus called MOM Derivatograph. The furnaces are connected to a gas suction device to remove gas
decomposition products.
◦
The study shows the endothermic peak at 125–155 C, representing the formation of ammonium alu-
Keywords:
Fluoro-topaz synthesis
Derivatograph
◦
minium hexafluoride and cryptohalite. The endothermic peak at 220–240 C represents the beginning of
dissociation of ammonium aluminium hexafluoride to the more stable ammonium aluminium tetraflu-
◦
oride. The endothermic peak at 340–350 C represents the dissociation of cryptohalite and ammonium
Corundum–quartz mix
aluminium tetrafluoride with the formation of aluminium fluoride.
◦
The formation of fluoro-topaz is represented by the wide endothermic peak at 770–800 C. The medium
◦
◦
wide or sharp endothermic peaks at 950 C and 1000 C represent the subsequent dissociation of fluoro-
topaz to corundum or mullite, respectively.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
The thermal behaviour of the starting materials (quartz, corun-
dum and ammonium fluoride) is well known [4,13–21]. Quartz is
the stable form of silica at ordinary temperature up to 870 C. It
◦
Fluoro-topaz Al (SiO )F is a fully fluorinated analogue of topaz
2
4
2
Al (SiO )[F0.9, (OH)0.1]2 which is a very popular gemstone, occurs
has two enantiotropic modifications, alpha and beta forms. The
inversion of alpha-quartz to beta-form is represented by a very
small sharp endothermic peak on the DTA curve at 573 C. Corun-
dum or alpha-aluminium oxide is thermally the most stable form
of aluminium oxide.
2
4
as large, beautifully shaped and coloured crystals. It is a charac-
teristic mineral formed in pneumatolytic or hypothermal stages of
igneous activity. It occurs chiefly in acid igneous rocks, such as gran-
ites, granite pegmatites and rhyolites. It is a common constituent
of greisen, formed by fluorine – metasomatism. Minerals associ-
ated with topaz may include quartz, fluorite, tourmaline, beryl,
cassiterite, muscovite and Li-minerals [1–6].
◦
The thermal behaviour of ammonium fluoride was reported
in literature [14,17,18]. The DTA curve of ammonium fluoride
[14] shows two large and sharp endothermic peaks. The first one
◦
The synthesis of fluoro-topaz is achieved by different methods.
The methods are thermal hydrolysis of aluminium fluoride and
at 170 C corresponds to the formation of ammonium bifluoride
◦
and liberation of ammonia. The second, at 240 C represents the
◦
silica between 750 and 850 C, or heating a mixture of sodium sil-
dissociation of the resulted ammonium bifluoride into hydrogen
fluoride and ammonia vapours. These processes are accompanied
by remarkable decrease in mass (TG curve) in two distinct steps.
The first is due to the liberation of ammonia and the second is due
to the volatilization of ammonia and hydrogen fluoride from the
dissociation of ammonium bifluoride.
◦
icon fluoride, aluminium oxide and water to 500 C at pressure of
4
000 bar, or sintering of kaolinite either with ammonium fluoride
◦
or aluminium fluoride at 750 C [1–4,7–12].
The present work illustrates a differential thermal analysis of
synthesis of fluoro-topaz by sintering of aluminium and silicon
oxides (in the form of corundum and quartz) with ammonium flu-
oride. Also, it includes the study of the influence of ammonium
fluoride on the thermal behaviour of corundum and quartz under
different conditions together with a study of the products of sin-
tering, namely, fluoro-topaz, mullite and corundum.
The fluorinating action of fluorine, fluorides, hydro-
gen fluoride and silicon tetrafluoride are well known
[1–3,7–10,13,14,16,19–22]. Ammonium fluoride is considered
as an important fluorinating agent. The reaction of corundum
or aluminium oxide with ammonium fluoride was reported to
◦
take place at 110–180 C with the formation of ammonium alu-
◦
minium hexafluoride, which dissociates at 300 C to ammonium
◦
∗ Tel.: +20 3 486 1356.
E-mail address: aabdelrehim@gmail.com
aluminium tetrafluoride. The latter one decomposes at 360 C
with the formation of aluminium fluoride [2,13]. The reaction of
0
040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2012.03.006

3
0
A.M. Abdel-Rehim / Thermochimica Acta 538 (2012) 29–35
quartz or silicon dioxide with ammonium fluoride takes place
at 125–155 C by an endothermic reaction with the formation
The thermal analysis of synthesis of fluoro-topaz by sintering
of corundum–quartz mixes with ammonium fluoride was con-
ducted with DTA apparatus (MOM Derivatograph, system; Paulik
et al.) [24]. This apparatus records simultaneously four curves:
the change of temperature of the sample (T), differential thermal
analysis (DTA), thermogravimetric analysis (TG) quantitatively in
milligrams, and the derivative thermogravimetric curve (DTG) on
a single sample under controlled conditions.
◦
of cryptohalite (ammonium silicon hexafluoride). Cryptohalite
◦
is unstable and dissociates at 320–335 C as represented by the
sharp and large endothermic peak on the DTA curve, with the
liberation of ammonia, silicon tetrafluoride and hydrogen fluoride
◦
[
10,14]. At 200 C and excess of ammonium fluoride, the product of
the reaction is composed of cryptohalite and a double compound
(
NH ) SiF ·NH F.
The chamber of the closed derivatograph furnace is made of
high alumina refractory material. There are porcelain suction tubes
beside the tubes holding the thermocouples of the sample and
inert material, reaching into the furnace chamber to remove the
gas decompositions products. The tubes are connected to a gas
adjustable suction device. The latter is composed of a gas washer
cylinder, a pump and a needle valve. Gas products are sucked off
through this device in order to prevent corrosion and entry into the
furnace and balance during the measurement.
4
2
6
4
Alumina or mullite reacts at 1 atm silicon tetrafluoride below
◦
6
60 ± 7 C to form aluminium fluoride and silica. At higher temper-
atures, the product is fluoro-topaz. Mixtures of fluoro-topaz and
aluminium fluoride decompose in 1 atm of silicon tetrafluoride at
9
◦
73 ± 8 C and form tabular alpha-aluminium oxide. Fluoro-topaz
◦
itself decomposes at 1056 ± 5 C in 1 atm silicon tetrafluoride to
give acicular mullite (2Al O ·1.07SiO ). Mullite is stable in presence
2
3
2
◦
of 1 atm silicon tetrafluoride above 1056 C [19,20].
Acicular mullite is also produced by controlled decomposition
of fluoro-topaz at 1000–1200 C in presence of silicon tetrafluoride.
Acicular mullite grains interlock forming an efficient filter used as
a diesel particulate filter (DPF) [23].
The parameters during the test were as follows: platinum cru-
cible: medium size; inert material, aluminium oxide, mass of mix
500 mg, temperature range, ambient up to 1200 C, sensitivity of
◦
◦
DTA circuit, 1/5; sensitivity of DTG circuit, 1/10; mass used in TG
◦
Mixtures of alumina and silica react with silicon tetrafluoride
curve 500 mg; heating rate 10 C/min. The DTA and temperature
◦
above 600 C to form fluoro-topaz. Pyrolysis of fluoro-topaz gives
measuring thermocouples were Pt–Pt/Rh wires. The atmosphere
was air, and the volatile silicon tetrafluoride and other gases were
removed as formed.
whiskers of mullite and silicon tetrafluoride [21,22].
2
. Experimental techniques
2.3. Phase identification
2.1. Starting materials
X-ray procedure: The phases of the products of the reaction
of sintering of corundum–quartz mix with ammonium fluoride
were identified microscopically and by X-ray diffraction. The fine
ground sintered material was mixed with sodium chloride as stan-
This research was carried out with aluminium oxide and silicon
dioxide in the form corundum and quartz, respectively. The chem-
ical composition of corundum used: 99.3% Al O ; 0.3% SiO ; 0.1%
2
3
2
◦
◦
dard. Its peaks at 2ꢀ = 31.38 and 45.44 were used for corrections.
Fe O ; 0.06% MgO; and 0.1% CaO. The chemically processed quartz
2
3
Nickel-filtered copper radiation was used. The sensitivity of the
is composed of 99.99% SiO2.
4
experiment was 4 × 10 imp/min and the statistical error was 1.5%.
In thin sections, corundum is pale bluish white, has high relief,
¯
high birefringence, no cleavage and lamellar twinning (1011). It
3. Results and discussion
crystallizes in trigonal system and is optically negative. Quartz is
colourless, visible twinning, low relief and weak birefringence. It
crystallizes in trigonal system and is optically positive.
The X-ray diffraction data of the processed corundum and quartz
are consistent with the corresponding ASTM data of these minerals.
The patterns show only the characteristic corundum and quartz
peaks, which are sharp and intense, suggesting good crystallinity.
No peaks of any mineral impurity were detected.
Ammonium fluoride used in experiments is a white crystalline
material having chemical composition: ammonium fluoride 98%,
ammonium bifluoride 1%, residue after ignition 0.01%, hexafluo-
rosilicate 0.1%, chloride measure 0.001%, sulphate measure 0.05%,
iron measure 0.005% and heavy metal (as lead) 0.001%.
For studying the synthesis of fluoro-topaz by sintering of
corundum–quartz mix with ammonium fluoride, DTA experiments
were carried out using different amounts of ammonium fluoride.
Mixes of corundum–quartz and ammonium fluoride of molar ratios
1
:1:12.5, 1:1:10 and 1:1:5 are used. The DTA curves obtained were
evaluated on the basis of literature data [1–4,8,9,12–23,25,26] to
explain the reactions, which may be connected to certain peaks on
the DTA curves.
3.1. Using mixes of corundum–quartz and ammonium fluoride of
molar ratio 1:1:12.5
Starting materials were mixtures of corundum and quartz with
ammonium fluoride in particular amounts. Mixes were processed
by repeated grinding in an automated agate mortar, followed by
sieving until all the powder passed through a 200 mesh sieve.
Finally the mixtures were then ground with a pestle and mortar
for 1 h to achieve homogeneity.
The DTA, TG and DTG curves of corundum–quartz mix with
ammonium fluoride of molar ratio1:1:12.5 is shown in Fig. 1. The
first wide and large endothermic peak at 155 C represents the
◦
formation of ammonium silicon hexafluoride (cryptohalite) and
ammonium aluminium hexafluoride and the liberation of ammo-
◦
nia and water vapours. The very small endothermic peak at 240 C
may represent the dissociation of the resulted ammonium bifluo-
ride and to some extent decomposition of the unstable ammonium
aluminium hexafluoride, yielding the more stable ammonium alu-
minium tetrafluoride. The large and sharp endothermic peak at
345 C represents the intensive dissociation of cryptohalite and
ammonium aluminium tetrafluoride with the formation of alu-
minium fluoride.
2
.2. Apparatus
Heating experiments were carried out using platinum crucibles
◦
heated in a muffle furnace. The furnace has abrasion resistance
ceramic annealing chamber and operates up to 1100 C. The evolved
silicon tetrafluoride and other gases are removed using a gas suc-
tion device connected to the furnace chamber. The temperature
was regulated automatically with an accuracy ± 5 C.
◦
The results obtained are consistent with literature data
[2,7,8,10,13,14] as quartz reacts with ammonium fluoride at
◦

A.M. Abdel-Rehim / Thermochimica Acta 538 (2012) 29–35
31
Fig. 1. DTA, TG and DTG curves of synthesis of fluoro-topaz using
corundum–quartz–ammonium fluoride mix of molar ratio 1:1:12.5. Mass of
◦
sample 500 mg. Heating rate 10 C/min.
◦
1
25–155 C with the formation of ammonium silicon hexafluo-
ride (cryptohalite), which dissociates by an endothermic reaction
◦
at 330–335 C. Corundum or alpha-aluminium oxide reacts with
◦
ammonium fluoride at 180 C with the formation of ammonium
aluminium hexafluoride, which dissociates at 300 C to ammonium
aluminium tetrafluoride. The last one dissociates at 360 C with the
◦
◦
formation of aluminium fluoride.
◦
The wide endothermic peak at 800 C indicates the reaction
between the resulted aluminium fluoride and quartz with the
formation of fluoro-topaz. It was previously reported that the for-
mation of fluoro-topaz from sintering of quartz with aluminium
◦
fluoride takes place at 780 C [2,7–19].
◦
The endothermic peak at 950 C represents the dissociation
of fluoro-topaz. In presence of excess of aluminium fluoride, the
dissociation of fluoro-topaz takes place simultaneously with its
desilication, with the formation of corundum. The results obtained
are in good agreement with literature data [1–4,7–9], as fluoro-
Fig. 2. X-ray diffraction patterns of the products of synthesis of fluoro-topaz using
corundum–quartz–ammonium fluoride mix of molar ratio 1:1:12.5. (A) 155 C, (B)
◦
◦
◦
◦
◦
240 C, (C) 345 C, (D) 800 C and (E) 950 C. A: NH4AlF4 (JCPDS File No. 20-77);
B: (NH4)2SiF6 (cryptohalite) (JCPDS File No. 7-13); C: (NH4)3AlF6 (JCPDS File No.
22-1036); F: ␥-AlF3 (JCPDS File No. 20-6); T: topaz (JCPDS File No. 12-765); Cr:
corundum (JCPDS File No. 10-173); Q: quartz (JCPDS File No. 33-1161); and AlF3
◦
topaz loses silicon tetrafluoride above 827 C and its decomposition
◦
to corundum takes place at 950 C.
(
JCPDS File No. 9-138).
The thermogravimetric curve (TG) shows the remarkable
decrease in mass in five distinct steps:
and to some extent ammonium aluminium hexafluoride. That
yields the more stable ammonium aluminium tetrafluoride.
1
. The first step is the mass loss (about 13%), representing the
volatilization of ammonia and water vapours from the reaction
of corundum–quartz mix with ammonium fluoride at 155 C and
(Fig. 2B).
◦
3. The third step is the large and sharp decrease of mass (about
◦
5
8.5%) at 345 C. This decrease is due to the volatilization of
its dissociation.
ammonia, silicon tetrafluoride and hydrogen fluoride from the
dissociation of cryptohalite and ammonium aluminium tetraflu-
oride with the formation of aluminium fluoride. (Fig. 2C).
2
. The second step is the decrease of mass (about 10%), due to the
volatilization of ammonia, hydrogen fluoride and water vapours
at 240 C from the dissociation of ammonium bifluoride resulted
◦

3
2
A.M. Abdel-Rehim / Thermochimica Acta 538 (2012) 29–35
4
. The fourth step is the mass loss (about 7.5%), representing the
volatilization of silicon tetrafluoride from the reaction of alu-
◦
minium fluoride and quartz at 800 C with the formation of
fluoro-topaz (Fig. 2D).
5
. The fifth step is the small decrease of mass (about 2%) due to
the dissociation of fluoro-topaz at 950 C and liberation of sili-
◦
con tetrafluoride and some sublimation of aluminium fluoride.
Remarkable sublimation of aluminium fluoride is observed at
◦
◦
1
000 C and its intensive sublimation at 1256 C without melting
[
1].
The total mass loss is 91% (stoichiometric loss 89.2%) and the
◦
residue remain at 1000 C is about 9% of the initial mass of sample
mix and is composed of corundum (Fig. 2E).
◦
The products of the runs at 155, 240, 345, 800 and 950 C were
identified both microscopically and by X-ray diffraction. By micro-
scopic examination of thin sections of these products, it is observed
that few quartz grains appear in the products of the runs at 155 and
◦
2
40 C, indicating fluorination of these mixes with ammonium flu-
oride and the formation of cryptohalite and ammonium aluminium
hexafluoride.
◦
At 240 C, the product consists of cryptohalite and ammonium
◦
aluminium tetrafluoride with few relict quartz grains. At 345 C,
aluminium fluoride constitutes the main composition of this prod-
◦
uct with unreacted relict quartz grains. At 800 C, the product
is composed mainly of fluoro-topaz and aluminium fluoride. At
◦
9
50 C, corundum constitutes the main composition of the end
product with aluminium fluoride and few undissociated fluoro-
topaz grains.
The X-ray diffraction patterns of these products are shown in
◦
Fig. 2A–E at 155, 240, 345, 800 and 950 C, respectively. The product
◦
at 155 C (Fig. 2A) consists of ammonium aluminium hexafluoride
(
NH ) AlF and cryptohalite (NH ) SiF with small peaks of quartz.
4 3 6 4 3 6
Fig. 3. DTA, TG and DTG curves of synthesis of fluoro-topaz using
◦
The product of the run at 240 C (Fig. 2B) consists mainly of ammo-
nium aluminium tetrafluoride NH AlF4 and cryptohalite. At 345 C
corundum–quartz–ammonium fluoride mix of molar ratio 1:1:10. Mass of
◦
◦
4
sample 500 mg. Heating rate 10 C/min.
(
Fig. 2C), aluminium fluoride constitutes the total composition of
this product with quartz. Fluoro-topaz constitutes the total com-
◦
position of the product at 800 C (Fig. 2D) with aluminium fluoride.
ratio 1:1:10 is 79% and residue 21%, whereas the stoichiometric
mass loss is 77.7%. The total mass loss on ignition of the other mix
of molar ratio 1:1:5 is about 60.5% and the residue 39.5%, whereas
the stoichiometric mass loss is 58.9%. The residue obtained after
sintering of mix of molar ratio 1:1:10 is composed of mullite and
corundum, while that obtained from mix of molar ratio 1:1:5 is
composed of mullite and unreacted corundum and quartz.
Fluoro-topaz peaks are sharp and intense, suggesting good crys-
tallinity. Corundum constitutes the total composition of the end
product at 950 C (Fig. 2E) with aluminium fluoride.
The microscopic study of the products of sintering of
corundum–quartz and ammonium fluoride mix of molar ratio
:1:12.5 is consistent with their X-ray diffraction patterns.
◦
1
◦
The products of the runs at 770 and 1000 C using
3
.2. Using corundum–quartz–ammonium fluoride mixes of molar
corundum–quartz–ammonium fluoride mix of molar ratio
1:1:5 were examined microscopically. At 770 C, the product of
◦
ratios 1:1:10 and 1:1:5
sintering consists of fluoro-topaz and unreacted corundum and
quartz due to the deficiency of fluoride ions. The end product at
1000 C consists mainly of mullite with unreacted corundum and
quartz grains. No fluoro-topaz grains are detected in this product,
indicating complete dissociation of fluoro-topaz in absence of
aluminium fluoride and the formation of mullite.
The DTA, TG and DTG curves of corundum–quartz and ammo-
nium fluoride mixes of molar ratios 1:1:10 and 1:1:5 are shown
in Figs. 3 and 4, respectively. They show the first three endother-
mic peaks at relatively close temperatures and the same reaction
products similar to those obtained for DTA curve of mix of molar
◦
◦
ratio 1:1:12.5. The very small endothermic peak at 560 C may
The X-ray diffraction patterns of the products of sintering of mix
◦
represent phase transformation of quartz. The exceptions here
are firstly, fluoro-topaz formation takes place at a slightly lower
temperature than the previously recorded for mix 1:1:12.5, as rep-
of molar ratio 1:1:5 at 770 and 1000 C are shown in Fig. 5A and B,
◦
respectively. The product at 770 C (Fig. 5A) is composed mostly of
fluoro-topaz with unreacted corundum and quartz. Fluoro-topaz
peaks are well defined, sharp and intense. Aluminium fluoride
peaks disappear completely in this run, indicating its absence. This
means that all aluminium fluoride is consumed in the formation of
fluoro-topaz.
◦
resented by the endothermic peaks at 790 and 770 C. This process
is accompanied by gradual decrease in mass (TG curve) due to the
volatilization of silicon tetrafluoride. The second exception is the
◦
sharp endothermic peak at 1000 C, which represents the dissoci-
ation of fluoro-topaz to mullite due to the deficiency of aluminium
fluoride produced.
The X-ray diffraction pattern of the product of sintering of
mix of molar ratio 1:1:5 at 1000 C (Fig. 5B) shows the intense,
◦
The TG curves of both mixes of molar ratios 1:1:10 and 1:1:5
show decrease of mass in five distinct steps similar to that for mix of
molar ratio 1:1:12.5. The total mass loss on ignition for mix of molar
sharp and well defined peaks of mullite with small peaks of
unreacted corundum and quartz. The fluoro-topaz peaks disappear
completely in this run, indicating the complete dissociation of

A.M. Abdel-Rehim / Thermochimica Acta 538 (2012) 29–35
33
Fig. 5. X-ray diffraction patterns of the products of synthesis of fluoro-topaz using
corundum–quartz–ammonium fluoride mix of molar ratio 1:1:5 at temperature (A)
Fig. 4. DTA, TG and DTG curves of synthesis of fluoro-topaz using
770 ◦C and (B) 1000 ◦C. T: topaz (JCPDS File No. 12-765); M: mullite (JCPDS File
corundum–quartz–ammonium fluoride mix of molar ratio 1:1:5. Mass of
No. 15-776); Cr: corundum (JCPDS File No. 10-173) and Q: quartz (JCPDS File No.
33-1161).
◦
sample 500 mg. Heating rate 10 C/min.
fluoro-topaz produced to mullite. This means that the dissociation
of fluoro-topaz in absence of aluminium fluoride takes place with
the formation of mullite. This is in good agreement with the data in
fluoride are different, depending upon the temperature of sintering
and the amount of ammonium fluoride used.
◦
literature, as fluoro-topaz liberates fluorine when heated to 900 C
3.5. Mechanism of reaction
and mullite is produced [2–4,7–9].
In general, the X-ray peaks of the products of sintering of differ-
ent mixes of corundum–quartz and ammonium fluoride, namely
fluoro-topaz, aluminium fluoride, mullite and corundum are nar-
row, sharp and intense, suggesting good crystallinity. The X-ray
diffraction data of these synthetic minerals is consistent with the
corresponding ASTM values of natural minerals.
The mechanism of the reaction of fluoro-topaz synthesis by
sintering of corundum–quartz mix with ammonium fluoride is a
complicated one and can be considered as the following:
◦
At 125–155 C: The reaction of corundum and quartz with
ammonium fluoride for all mixes takes place with the formation
of ammonium aluminium hexafluoride and cryptohalite (Fig. 2A),
according to:
3.3. General characteristics of the synthesized fluoro-topaz
Al O + SiO2 + 18NH F → 2(NH ) AlF + (NH ) SiF
Cryptohalite
2
3
4
4
3
6
4
2
6
Corundum
Quartz
Mineralogy: The synthesized fluoro-topaz is colourless in thin
sections and is characterized by high relief, perfect basal cleavage
0 0 1), weak birefringence and it is optically positive. It crystal-
+
10NH + 5H O
(1)
3
2
(
lizes in an orthorhombic system in the form of elongated prismatic
crystals.
◦
At 220–240 C: Ammonium aluminium hexafluoride is unstable
and begins to decompose at such temperature, yielding the more
stable ammonium aluminium tetrafluoride, according to:
3.4. X-ray diffraction study
(
NH ) AlF → NH AlF + 2NH + 2HF
(2)
4
3
6
4
4
3
The X-ray diffraction data of the produced fluoro-topaz is consis-
or the reaction of corundum and quartz with ammonium fluoride
for all mixes takes place at 220–240 C and the phases resulted
are ammonium aluminium tetrafluoride and cryptohalite (Fig. 2B)
according to:
tent with those of the corresponding values of the natural one. The
unit cell dimensions and constants of the synthetic fluoro-topaz are
given in Table 1. It is observed that the calculated cell dimensions,
constants and optic axial angles of the synthesized fluoro-topaz are
consistent with the corresponding data of the natural mineral.
It is concluded from the results obtained that the products of
the reaction of sintering of corundum–quartz mix with ammonium
◦
Al O + SiO2 + 14NH F → 2NH AlF + (NH ) SiF + 10NH
2
3
4
4
4
4
2
6
3
Corundum
Quartz
Cryptohalite
+ 5H O
(3)
2

3
4
A.M. Abdel-Rehim / Thermochimica Acta 538 (2012) 29–35
Table 1
Unit cell dimensions and axial angles of fluoro-topaz.
◦
◦
◦
V (Å3)
Mineral
a (Å)
b (Å)
c (Å)
˛ ( , min)
ˇ ( , min)
ꢁ ( , min)
Topaz
Synthetic
Standard
Orthorhombic
4.643
4.646
8.774
8.792
8.381
8.394
90.00
90
90.00
90
90.00
90
341.423
343.096
In absence of aluminium fluoride, fluoro-topaz dissociates at
1000 C with the formation of mullite as end product (Fig. 5B) for
◦
◦
At 340–350 C: Ammonium aluminium tetrafluoride and cryp-
tohalite are unstable and dissociate according to:
mix of molar ratio 1:1:5.
The studied conditions of formation of fluoro-topaz and its ther-
mal stability give good evidence about the association of topaz with
late-stage pneumatolytic action, its existence as a common con-
stituent of greisens and its occurrence in high temperature quartz
veins, pegmatites, granites and contact zone.
NH AlF → AlF + NH + HF
(4)
(5)
4
4
3
3
(
NH ) SiF → SiF + 2NH + 2HF
4
2
6
4
3
or the reaction of corundum and quartz with ammonium fluoride
for all mixes takes place at 340–350 C with the formation of alu-
◦
minium fluoride (Fig. 2C) according to:
4. Conclusions
Al O + SiO + 10NH F → 2AlF + SiF + 10NH + 5H O
(6)
2
3
2
4
3
4
3
2
Corundum
Quartz
The thermal analysis of synthesis of fluoro-topaz by sintering
of corundum–quartz mixes with ammonium fluoride shows that
the reaction mechanism is a complicated one. Different products of
sintering are obtained, depending on the temperature and amount
of ammonium fluoride.
3
.6. Fluoro-topaz formation
Fluoro-topaz may be formed in different ways for all mixes at
◦
temperature range 770–800 C:
The DTA curves, microscopic and X-ray diffraction study of the
products of sintering show that the wide endothermic peak at
•
•
Quartz may react with aluminium fluoride produced with the
formation of fluoro-topaz (Figs. 2D and 5A), according to the fol-
lowing equation:
◦
1
25–155 C represents the formation of ammonium aluminium
◦
hexafluoride and cryptohalite. The endothermic peak at 220–240 C
represents the dissociation of ammonium bifluoride resulted and
the unstable ammonium aluminium hexafluoride, yielding the
more stable ammonium aluminium tetrafluoride. The large and
2
SiO + 2AlF → Al SiO F + SiF
(7)
2
3
2
4
2
4
Quartz
Fluoro-topaz
◦
sharp endothermic peak at 340–350 C represents the intensive
Corundum and quartz may react with aluminium fluoride with
the formation of fluoro-topaz (Figs. 2D and 5A), according to:
dissociation of cryptohalite and ammonium aluminium tetraflu-
oride with the formation of aluminium fluoride. These processes
are connected with a sharp decrease in mass due to the volatiliza-
tion of silicon tetrafluoride and liberation of ammonia and water
vapour.
2
Al O + 3SiO + 2AlF → 3Al (SiO )F
(8)
2
3
2
3
2
4
2
Corundum
Quartz
Fluoro-topaz
The DTA curves show the beginning of the reaction between
aluminium fluoride produced and quartz at 600 C with the for-
The beginning of the reaction of fluoro-topaz formation is
◦
◦
observed to take place at 600 C. The intensive formation of fluoro-
topaz by sintering of corundum–quartz mix with ammonium
mation of fluoro-topaz. The intensive formation of fluoro-topaz
◦
◦
takes place at 770–800 C, as represented by the wide endothermic
fluoride (for all molar ratios) takes place at 770–800 C
peaks at such temperatures. The medium wide or sharp endother-
◦
mic peaks at 950 and 1000 C represent the subsequent dissociation
3
.7. Fluoro-topaz dissociation
of fluoro-topaz to corundum or mullite, respectively, depending on
the amount of ammonium fluoride in the initial mix.
◦
Fluoro-topaz is unstable at temperature higher than 850 C. It
◦
has the property of losing silicon tetrafluoride at 827 C as repre-
sented by the small endothermic peak at such temperature [2–4,7].
1
. Using corundum–quartz and ammonium fluoride mix of molar
ratio 1:1:12.5, fluoro-topaz constitutes the main composition of
◦
Fluoro-topaz liberates fluorine on heating to 850–900 C and mul-
◦
the product at 800 C with aluminium fluoride.
lite is produced.
2
. Using mixes of molar ratios 1:1:10 and 1:1:5, the product at
In the present work, the products of fluoro-topaz dissocia-
tion are either mullite or corundum depending on the amount of
ammonium fluoride in the initial mix, temperature and aluminium
fluoride produced.
◦
7
70–790 C is composed of fluoro-topaz with unreacted corun-
dum and quartz and no aluminium fluoride is detected. All
aluminium fluoride resulted, is consumed in the formation of
fluoro-topaz.
In presence of excess of aluminium fluoride produced from mix
of molar ratio 1:1:12.5, dissociation of fluoro-topaz together with
◦
its desilication take place at 950 C with the formation of corundum
The produced synthetic fluoro-topaz is colourless in thin sec-
(
Fig. 2E), according to:
tions and crystallizes in orthorhombic system, in the form of
elongated crystals, with perfect basal (0 0 1) cleavage and it is opti-
cally positive.
6
Al (SiO )F + 4AlF → 8Al O + 6SiF4
(9)
2
4
2
3
2
3
Fluoro-topaz
Corundum
In presence of deficient amount of aluminium fluoride produced
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◦
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