Cordierite ( 2 MgO · 2 Al 2 O 3 · 5 SiO 2 ) synthesis by unconventional methods

The cordierite was synthesized by two unconventional methods: 1) the method which uses hydrosilicate precursors and 2) the method which uses organic precursors. Comparison of these synthesis methods with the classical one is the main subject of the presented paper. The infl uence of Li2O on the cordierite phase formation was investigated too. The results have suggested that the method which uses hydrosilicate precursors provides the SiO2 linkage as magnesium metasilicate, but is not adequate for the cordierite synthesis. Only in the presence of Li2O considerably amount of the μ-cordierite modifi cation was formed, confi rming the mineralizing effect of this oxide. In addition, the results have clearly demonstrated that the method which uses organic precursors is suitable for the cordierite synthesis. Thus, even without mineralizer it can be used for the preparation of the almost pure α-cordierite at 1200°C. Because of that this method was used to obtain pigments with cordierite structure, in which Al3+ was partially substituted with Cr3+ (pink pale colour), and Mg2+ was partially substituted with Co2+ (blue colour).


I. Introduction
The cordierite is a magnesium/aluminium aluminosilicate [1] with the crystallo-chemichal formula Mg 2 [4] Al 3 [6] (Si 5 Al [4] O 18 ) and a complex structure having six tetrahedral units: five of [SiO 2 ] and one of [ AlO 4 ].Binding of the tetrahedral units is ensured by the [AlO 6 ] octahedral and [MgO 4 ] tetrahedral.The cordierite has several polymorphic modifications.A low temperature modification, β-Mg 2 Al 4 Si 5 O 18 , crystallizes in the orthorhombic system and a high temperature modification, α-Mg 2 Al 4 Si 5 O 18 named indialite, crystallizes in the hexagonal system [2].Existence of a metastable modification, μ-Mg 2 Al 4 Si 5 O 18 , was also mentioned in literature [2].The μ-Mg 2 Al 4 Si 5 O 18 modification crystallizes in the hexagonal system and could be obtained by crystallization of some glasses at 900°C for 200 hours.
The cordierite has important properties such are: low electrical conductivity, very low thermal expansion coeffi cient, relatively high hardness (7÷7.5 on Mohs scale) and high chemical inertness.Because of these properties, the cordierite represents a valuable mineralogical constituent in some electrotechnical porcelains, thermally stable ceramic masses or supports for catalysts.An interesting fi eld of application is also synthesis of thermoresistant pigments by including some chromophore ions (Co 2+ , Cr 3+ ) in the cordierite crystalline network [3][4][5].
One very attractive unconventional synthesis method (the method which uses hydrosilicate precursors) is based on the precipitation reaction taking place between a sodium silicate solution and solutions of some soluble salts of alkaline-earth metals [15][16][17][18].Silicates of the respective metals are formed by annealing of the obtained precipitates at temperatures over 800°C and represent a source of linked SiO 2 , being able to react further with the Al 2 O 3 , Cr 2 O 3 etc.Thus, the reduced reactivity of SiO 2 , introduced as quartz or even as silica gel (which transforms in cristobalite during annealing), can be in large measure avoided.This results in decrease of the formation temperature of some complex oxide compounds (CaO•Al 2 O 3 •2SiO 2 , 3CaO•Cr 2 O 3 •3SiO 2 , CaO•SnO 2 •SiO 2 ) for 200-300°C in respect to the classical method.
Another important unconventional synthesis method is based on the formation of a complex organic Al/ Mg precursor (the aluminium and magnesium glyoxylate) [18][19][20][21][22][23][24][25][26][27][28].The complex organic precursor is obtained by the oxidation of 1,2-ethanediol with aluminium and magnesium nitrates according to the following reactions: In order to obtain the cordierite phase, the complex organic Al/Mg precursor is synthesized in the presence of SiO 2 .The advantage of the method is again considerably lower synthesis temperature.
In this work, two unconventional methods were employed for the cordierite synthesis: 1) the method which uses hydrosilicate precursors and 2) the method which uses organic precursors.In addition, comparison of these synthesis methods with the classical one is also presented.Finally, the possibility of preparing some thermoresistant pigments with cordierite structure was investigated too.

II. Experimental
Three synthesis methods, one classical and two unconventional, were employed for synthesis of different cordierite samples (Tables 1-3).Due to diffi culties regarding the cordierite formation two set of experiments were done, one without and another with Li 2 O (introduced as Li 2 CO 3 ).

Method which uses hydrosilicate precursors
The cordierite (2MgO•2Al 2 O 3 •5SiO 2 ) was also synthesized by the precipitation reaction between the precursors containing Mg, Si and Al cations (Table 2).MgSO 4 ×7H 2 O (Reactivul Bucureşti, Romania) and Na 2 O•2.5SiO 2 granules (Loba Feinchemie) were used as source of MgO and SiO 2 , respectively, whereas Al(OH) 3 , Al-hydroxide (ALOR-ORADEA, Romania) or (NH 4 )Al(SO 4 ) 2 ×12H 2 O, alum (Reactivul Bucureşti, Romania) as source of Al 2 O 3 .Magnesium sulphate was dissolved in warm water (50°C) and mixed with aluminium hydroxide to form a suspension.The sodium silicate was also dissolved in warm water (50°C) and then slowly added to the previously prepared suspension.The precipitation reaction took place practically instantaneous: The resulted suspension was heated 30 minutes, and the prepared particles, with notation P.

Method which uses organic precursors
The aqueous solution of magnesium and aluminium nitrates and 1,2-ethandiol was heated in a water bath at approximately 80°C.During the exothermal reaction, elimination of NO 2 took place and the complex organic Al/Mg precursor (the aluminium and magnesium glyoxylate) was prepared.In order to obtain the cordierite, the reaction was preceded in the presence of SiO 2 .Aerosil (Degussa, Germany), as reactive form of SiO 2 , was dispersed in the solution containing 1,2-ethandiol, magnesium and aluminium nitrates.The obtained solid phase (the samples P.4 and P.4.1, see Table 3) was dried and then annealed at different temperatures in porcelains crucibles.

Preparation of thermoresistant pigments
In order to obtain pigments with cordierite structure, partial substitution of Al 3+ with Cr 3+ (the samples P.5 and P.5.1) and Mg 2+ with Co 2+ (the samples P.6 and P.6.1) was performed (Table 3).For this purpose sample preparation was done with the method which uses organic precursors, as it is suitable for synthesis of the cordierite having complex composition.

Characterization
The phase composition of samples annealed at different temperatures, between 800°C and 1200°C, was ascertained by X-ray diffraction, using a DRON 3 diffractometer with Cu Kα radiation.
The colourimetric characterization of the obtained pigments has been made by diffuse refl ectance spectrophotometry, using a SPEKOL 10 (Carl-Zeiss-Jena) spectrophotometer.

III. Results and Discussion
XRD patterns of the samples annealed at 1200°C are presented in Figs.1-4.It can be seen that the phase compositions of annealed samples is strongly infl uenced by synthesis method and presence of Li 2 O.
In the sample P.1, synthesized using the classical method without Li 2 O, even at 1200°C the cordierite is not formed.The only reaction product identifi ed on the XRD pattern is a small proportion of the spinel, MgO•Al 2 O 3 , phase (Fig. 1).On the other side, the sample P.1.1,prepared with the same method and 2 wt.%Li 2 O, consists of μ-cordierite as the main phase, and the spinel, quartz and Li 2 O•Al 2 O 3 •6SiO 2 phases (Fig. 1).This confi rms very strong infl uence of Li 2 O addition on the phase composition of annealed product.The similar ionic radius of Li + and Mg 2+ , but a pronounced fondant action of Li 2 O could justifi ed the mineralizing effect of this oxide on the μ-cordierite formation.The sample P.1.1 with Li 2 O showed higher degree of shrinkage and improved sinterability, most probably due to presence of a liquid phase at the annealing temperature.These conditions are favorable for the formation of the μ-cordierite modifi cation.XRD pattern of the sample P.2, prepared from hydrosilicate precursors and without Li 2 O (Fig. 2), confi rmed that the cordierite phase was not formed at 1200°C.The similar result was obtained for the sample P.3, when (NH 4 )Al(SO 4 ) 2 ×12H 2 O (as a source of Al 2 O 3 with increased reactivity) instead of Al(OH) 3 was used.The presence of the spinel MgO•Al 2 O 3 , enstatite (MgO•SiO 2 ), α-Al 2 O 3 and cristobalite phases indicate that the reaction between enstatite and Al 2 O 3 , with SiO 2 liberation and the spinel formation was performed at these conditions.The formation of magnesium metasilicate (enstatite), in the method which uses hydrosilicate precursors, was also confi rmed in literature [15][16][17][18].The above results suggest that this method, providing the SiO 2 linkage as magnesium metasilicate, is not adequate for the cordierite synthesis.This is in accordance with the thermodynamic data [6], affi rming the necessity to have the spinel MgO•Al 2 O 3 phase as the previous phase, which could subsequently react with SiO 2 and form the cordierite.XRD pattern of the sample P.2.1 (with 2 wt.%Li 2 O) annealed at 1200°C is presented in Fig. 2. It can be seen that in the presence of Li 2 O considerably amount of the μ-cordierite modifi cation was formed, confi rming again the mineralizing effect of this oxide.There is a high portion of the spinel and small amount of lithium aluminosilicate and cristobalite phases.
In the sample P.4, prepared from the organic precursor based on the aluminium and magnesium glyoxylate and Aerosil (SiO 2 ), almost pure cordierite phase was obtained at 1200°C, even when the mineralizer was not added (Fig. 3).This could be explained with the facts that the method which uses organic precursor enables the formation of the MgO•Al 2 O 3 phase already at temperatures lower than 800°C and favors reaction of the formed spinel with SiO 2 at somewhat higher temperature.The infl uence of Li 2 O addition (the sample P.4.1, Fig. 3) is not pronounced such in the P.1 and P.2 samples, but it enables preparation the sample with higher proportion of the α-cordierite, but also small amount of the μ-cordierite, lithium aluminosilicate and the untransformed spinel and cristobalite phases.The temperature infl uence on the phase evolution was studied too.Thus, XRD patterns of the sample P.4.1 annealed at 800 and 1000°C are presented in Fig. 4. It can be seen that already at 800°C the μ-cordierite is present as main phase and at 1000°C the α-cordierite and lithium aluminosilicate have appeared.
These results have clearly demonstrated that the method which uses organic precursors is suitable for the cordierite synthesis.Thus, even without mineralizer it can be used for the preparation of the almost pure α-cordierite at 1200°C.Because of that this method was used to obtain pigments with cordierite structure.In the samples P.5 and P.5.1 Al 3+ was partially substituted with the choromophore cation Cr 3+ , whereas in the samples P.6 and P.6.1 Mg 2+ was partially substituted with Co 2+ .The phase composition of these samples annealed at 1200°C is very similar to that observed for the samples P.4 and P.4.1.The diffuse refl ectance spectra of the samples P.5/P.5.1 and P.6/P.6.1 annealed at 1200°C are presented in Figs. 5 and  6, respectively.It can be seen that partial substitution of Al 3+ with Cr 3+ (pink-pale colour) produces the diffuse refl ectance curves specifi c for Cr 3+ hexacoordinated materials (with two absorption bands: at 400 nm and 550 nm).On the other side, partial substitution of Mg 2+ with Co 2+ (blue colour) produces the diffuse refl ectance curves specifi c for Co 2+ tetracoordinated materials (large absorption bands between 530 and 660 nm).This all confi rmed that the method which uses organic precursors can be successfully used for the preparation of different pigments with cordierite structure

IV. Conclusions
Using the classical method, only in the presence of 2 wt.%Li 2 O as mineralizer the μ-cordierite modifi cation was obtained.
The method which uses hydrosilicate precursors provides the SiO 2 linkage as magnesium metasilicate but was not adequate for the cordierite synthesis.Only in the presence of Li 2 O considerably amount of the μ-cordierite modifi cation was formed, confi rming the mineralizing effect of this oxide.The method which uses organic precursors in the presence of SiO 2 was suitable for the cordierite synthesis.Almost pure cordierite phase was obtained at 1200°C even without mineralizer.In the presence of 2 wt.%Li 2 O, μ-cordierite was formed already at 800°C.
The method which uses organic precursors can be used for the preparation of different pigments with cordierite structure.Pink pale colour pigments resulted by the partial substitution of Al 3+ with Cr 3+ and blue colour pigments resulted by the partial substitution of Mg 2+ with Co 2+ .
2 (without Li 2 O) and P.2.1 (with Li 2 O), were separated from a liquid phase by fi ltration.The cordierite samples P.3 (without Li 2 O) and P.3.1 (with Li 2 O) were prepared on the similar way, however, (NH 4 )Al(SO 4 ) 2 ×12H 2 O was used as Al 2 O 3 source instead of Al(OH) 3 .The obtained particles were washed, dried and fi nally annealed at different temperatures in porcelain crucibles.

Table 1 . Composition of the samples synthesized by classical methodTable 2 . Composition of the samples synthesized from hydrosilicate precursors
1