Osumilite-type phases, which have high temperature stability and low linear thermal expansion coefficients, were crystallized from modified basalt glass composition. To satisfy the compositional requirements of osumilite molecules, a deliberate amount of kaolin, sand, magnesite and TiO2 should be added to the basalt. The crystallization process was investigated by DTA, X-ray diffractometry and SEM. The effect of compositional changes on bulk crystallization and growth morphology was also studied. The linear thermal expansion coefficient was measured to characterize the produced glass-ceramics

Abdel-Hameed, Salwa A. M.

El-Shennawe, Ahmed W. A.

English

Repositorium für Naturwissenschaften und Technik

Original PaperCrystall ization of osumil i te f rom modif ied basalt ic glassesA h m e d W. A. E l - S h e n n a w e a n d Sa lwa A. M. A b d e l - H a m e e dGlass Research Dept., National Reseach Genter, Cairo (Egypt)Osumilite-type phases, which have high temperature stabihty and low linear thermal expansion coefTicients, were crystallized frommodified basalt glass composition. To satisfy the compositional requirements of osumilite molecules, a deliberate amount of kaolin,sand, magnesite and Ti02 should be added to the basalt. The crystallization process was investigated by DTA, X-ray diffractometryand SEM. The effect of compositional changes on bulk crystallization and growth morphology was also studied. The linear thermalexpansion coefficient was measured to characterize the produced glass-ceramics.1 . I n t r o d u c t i o nThe structure of osumilite-type minerals [1] with the generalformula C B 2 A 2 T 2 3 T I 2 O 3 0 is characterized by six-membereddouble rings of (Si, Α 1)04 tetrahedra which are stackedalong the  c-axis. These double rings are interconnected byT2 tetrahedra and octahedra (A-site) thus forming Channelsalong the  c axis which may be occupied by large alkali- oralkaline-earth cafions (C-site at 0,0,1/4). The B-site is lo-cated between two octahedra parallel to the  c-axis. The crys-tal chemistry and occurrence of natural double-ring Silicateshave recently been reviewed [2].In the past few years some synthetic osumilite-typephases suitable for technical application as the crystallinebasis in glass ceramic materials have attracted the interestof ceramists and engineers. This is mainly due to the hightemperature stability and low linear thermal expansion coef-ficients of some of these phases. Mixture of osumilite-typecomposition can be melted easily and homogeneous glassesare obtained after quenching to room temperature [1].MAS osumihte (Mg2Al4Siii03o) is an interesfing osum-ilite type phase which was first discovered [30] as a meta-stable accessory Compound in crystallization products ofglasses with composiüons close to the Joint cordierite-Si02.Addition of a small amount of BaO to an M g O - A l 2 0 3 -Si02 glass composition yields a product with M A S osumil-ite as the main phase [4 and 5]. Though metastable, M A Sosumilite can endure temperatures up to 1150 C for a longtime. At higher temperatures it decomposes into cordieriteand cristobalite. The most interesting property of this osum-ilite phase is its linear thermal expansion behaviour whichresembles that of cordierite: while the ö axis expansion islow but positive, the  c-axis shows a negative expansion with 2.6  X 10-^/K and  - 1 . 9 χ 10-^/K (25 to 600 C)Received 27 February, revised manuscript 31 May 2002.yielding an overall linear thermal expansion coefficient of1.1  X 10~^/K which is nearly as low as that of cordierite.Crystal-structure refinement data [1] are still lacking formost synthetic osumilite-type phases. Structure refinementsexist only for alkali-containing roedderite-type phases.Khan et al. [6] presented data for K2Mg5Sii2O30 based onsingle-crystal measurements. Nguyen et al. [7] did extensivework on several sodium, potassium and lithium containingroedderite-type phases using X-ray powder techniques.Those authors stated that, depending on the kind of alkalications present, the occupation of the B  and C-sites is dif-ferent: while the smaller N a ^ favours the nine-coordinatedB-site, the larger K^ prefers the twelve-coordinated C-site.However, they obtained M A S osumilite as a major phasebesides much anorthite, y^-quartz ss, cristobalite and tracesof magnesium titanate phases and thus were not able todetermine its exact composition.Hence, in this paper the authors report on the crystalli-zation behaviour of three modified basalt glass compo-sitions based on osumilite and the linear thermal expansionof the resulting glass-ceramics.To formulate osumilite-based glass-ceramic batch composition, a chemico-mineral molecular norm (CIPW) (8)was adopted to recalculate the chemical composition of basalt into osumilite and anorthite molecules. In this methodof calculation, and to achieve the highest possible amountof osumilite from such polycomponents basalt composition,Fe^^ ions of basalt may be totally oxidized into Fe^^ ionsby adding a deliberate amount of NH4NO3. Almost mostof the oxidized basalt composition, forming components,even Ca^^, up to certain limits, can find place in the osumil-ite structure as K2Mg2Al3Si9Al3O30, Na2Mg2Al3Si9Al3O30and CaMg2 Fe3''^Si9Al3O30. Some of the Ca^+ ions werealloted to the latter osumilite derivative and the rest wasalloted to an anorthite phase.--------° -=  = ° Table 1. Chemical compositions in wt% of the parent glass BO*oxide S i 0 2 AI2O3 Fe203 CaO MgO N a 2 0 K 2 O T i 0 2wt% 53.54 19.32 10.55 4.69 4.21 2.14 0.62 4.93* 2 and 4 % of M g O were added to this composition to yield B O M ,and BOM2 respectivelyTo satisfy the compositional requirements of osumilitemolecules, the norm calculations reveal that —16 kao-lin, ^0 .89 sand, 3.1 magnesite and 5.77 T i 0 2 (in wt%)should be added to the basalt. This base compositionmay yield 80.97 wt% osumilite. In order to facilitate themelting of the base composition and to catalyze the for-mation of osumilite glass-ceramic,  a deliberate amount ofT i 0 2 and F , such as MgF2, was added. The fluorinewas added with the view that it may replace one atomof oxygen in CaMg2Fe3^^Si9Al303o molecule to giveCaMg2Fe3^^Si9Al3029F2.Consequently, three glasses were prepared and denotedby BO, BOMi and B O M 2 , where the letters BO indicate thebasalt-osumilite composition, and Mi and M 2 indicate theaddition of M g F 2 in different quantities.2 . E x p e r i m e n t a lThe starting glass composition (table 1) was prepared from(in wt%) 74% basalt rock as a main reagent and 1 6 %kaolin, 0 . 9 % sand, 3.1 % magnesite, and 5 .77% T i 0 2 asmodifying agent. 3 g N H 4 N O 3 as oxidizing agent wasadded. M g F 2 was added by 2 or 4 wt% as nucleating agentto the batch. This composition corresponds to a mixtureof 80 % osumihte and the rest were anorthite, y^-quartz ssand titanium bearing phase. The composition was melted at1500 C for 4 h in a platinum crucible. The melt was thencast into hot steel moulds in the form of discs (1 cm thick)and rods (1x1x10) cm^ which were transferred to a mufflefurnace at 650 C for annealing.It is Worth mentioning that lower viscosities and goodworking traits were noticed especially in compositions con-taining  a high content of M g F 2 from which  a longer glassfibre could be drawn over relatively longer durations ac-cording to the adopted qualitative viscosity test [8].Differential thermal analyses of the prepared glass werecarried out under N 2 atmosphere in a Perkin-Elmer 7-seriesDTA S y s t e m . The analyses were carried out using 70 mgof the powdered samples (90 to 250 μm), the heating ratewas maintained at 10 Κ and constant flow rate of N 2 waskept at 50 cm^/min. Corundum was used as a reference material.The effect of heat treatment on crystallization was studied in glass samples isothermally cured at temperaturesranging from 950 to 1200°C at - 5 0 Κ intervals for 3 h.Some samples were heated for 24 and 30 h to study thethermal stability of the developed crystalline phases.X-ray powder diffraction analyses of the heat-treatedglass were carried out using  a Philips PW/390 X-ray dif-fractometer with nickel-filtered CuK« radiation. The instru-3.53.02.5c 2.01.5·1.00.50.0-0.5827.35XΛ 1056.7X B O M 2 746.3x1 \ Α654.2°C 1040.37°c\y884.58°C /VB O M 1 803.68°c/V.N,^  ̂V.,̂^̂^895.82X, BOΛ 1089.62°C786.37°C J \ f\ 679.3rC ^""""^500 600 700 800 900 1000 1100 1200Temperature in C • Figure 1. DTA traces of BO, BOM, and B O M 2 .ment settings were maintained for all the analyses by usingan external Standard SILICON disc.The mineralogical composition and microstructure ofthe glass-ceramic were examined by scanning electronmicroscope, Jeol JSM-T20, on fresh-fractured surfaces ofsome samples.Linear thermal expansion coefTicients of some sampleswere measured using Linseis L74/1250 dilatometer.3 . R e s u l t sThe DTA traces (figure 1) exhibit an endothermic peak inthe 654 to 679 C temperature ränge and three exothermicpeaks in the 746 to 803 C, 827 to 895 C and 1040 to1089°C temperature ranges, which correspond to crystalli-zation of ^-quartz ss, anorthite and cristobalite, and osumil-ite, respectively. As was mentioned in a previous paper [9],the endothermic reaction marks the glass transition (Tg)and softening (7^) temperatures are believed to have beencaused by an increase in heat capacity due to transform-ation of glass structure [10]. The exothermic reaction ofcrystallization effects, which is accompanied by releasingheat due to its lower free energy and reaches its maximumrate near the sharpness of the exothermic peak, the higherthe propensity, velocity and bulk crystallization is.Generally the behaviour of DTA curves shows shift tolower temperature by adding M g F 2 from 2 to 4 % to thebatch composition.X-ray diffraction patterns of base composition (BO)(figure 2) heat-treated at 950°C/3  h show the crystallizationof y^-quartz ss as major phase besides pyroxene. Anorthi tewas developed and distinguished when the samples wereheat-treated at 1050°C/3h. Indefinite osumihte compo-sition and minor cristobalite appeared on samples heat-treated at 1050°C/30h. All phases were stable when heat-treated tili 1100°C/24 h (figure 2).Adding 2 % M g F 2 (BOMj) to the batch composition ledto the formation of metastable (MgTi03) geikielite phase atan early stage by heating the sample to 950°C/3 h (figure 3).-'-~ ° —° —° ° ° ° — --Crystallization of osunnilite from modified basaltic glasses0 Osumilite• AnorthiteX Β Quartz (ss)+ Cristobalite1100°C/24hS SSCOΟ ΛXINCSJ1 •00>CC1050°C/30h II1050°C/3h950°C/3hFigure 2. X-ray diffractionpatterns of BO glass heat-treated at 950°C/3 h,1050°C/3h, 1050°C/30hand 1100°C/24h (i/-valuesare given in A°).0)1200°C/24h0 Osumilite• AnorthiteX Β Quartz (ss)+ Cristobalite50° 40° 30°201100°C/24h20° 10^Figure 3. X-ray diffraction patterns of BOM] glass heat-treatedat 950°C/3h, 1050°C/3h, 1050°C/30h, 1100°C/24h and1200°C/24 h. The MgTiO. reflections are Sit d ^ 3.79, 3.69 and3.58 A .came sharper and more distinguished by increasing tem-perature and time. All phases were still stable tili 1200°C/24 h.It was noticed that the amount of anorthite crystallizedin BOM  1 was larger than that in BO with respect to crystal-lization of ^-quartz ss.Increasing the amount of M G F 2 added to 4 % in B O M 2glass composition did not much affect the crystallizationbehaviour compared to that of B O M , , only more Separationof X-ray peaks occured which revealed more developmentof the crystallized phases (figure 4). MgTi03 did not crys-tallize here.The microstructure of the resultant crystalline productis dependent on the fluorine content in the original glass.The increase of fluorine content led to the formation ofvolume, homogeneous and fine- to ultrafine-grained micro-structures (figures 5a and b).Thermal expansion was measured for glass-ceramicsamples heat-treated at 1050°C/30 h over different tempera-ture intervals (table 2).It was generally observed that the linear thermal expan-sion of all glass-ceramic compositions increased slowly tili500 C, then began to decrease.Adding M G F 2 to the glass composition led to a slightincrease of the thermal expansion and this increase was di-rectly proport ional to the amount of M G F 2 added.This phase disappeared completely by heating the sample to1050°C/3 h where anorthite y^-quartz ss and minor pyroxenewere crystallized. Increasing the time to 30 h at the sametemperature led to appearance of osumilite. Cristobalite wasseparated and distinguished on a sample heat-treated at1100°C/24h. The intensity of all peaks increased and be4. DiscussionBorukhove et al. [11] determined the linear thermal expan-sion coefficient, a, of basaltic rocks in the glassy State as« 2 5 TO 500  15.94  X 10 -6 /K to « 7 5 0 TO 800  75  X lO-^/K. ° °- = =cΒΈωφa° s 1200°C/24h1100°C/24h0 Osumilite• AnorthiteX β Quartz (ss)+ Cristobalite 1050°C/3h950°C/3h50° 40° 30°2020° 10°Figure 4. X-ray diffraction patterns of BOM2 glass heat-treatedat 950°C/3h, 1050°C/3h, 1050°C/30 h, Ί 100°C/24 h and1200°C/24h.Table 2. Linear thermal expansion coefficient (o: in 10 )̂of BO, BOM, and BOM2 heat-treated at 1050°C/30hsample «20 to 300 oi2o to 4oo «̂ 20 to 500 0̂ 20 to 600 «20 to 700BO 5.3BOM, 6.4BOM. 6.65.66.46.75.86.46.66.06.16.65.95.96.3The fluorine ion, with a radius of 0.136 nm, is very close insize to the oxygen ion (radius 0.14 nm, so that a fluorineion could replace an oxygen ion in the glass network with-out causing a great disturbance in the arrangement of otherions. Since the fluorine ion is monovalent, however, theoverall replacement within the glass must be two fluorineions for each oxygen ion in order to ensure electrical neutrality. Α general effect of the fluorine would be the replace-ment of the strong = S i - 0 - S i = linkages by pairs of theweaker ^S i—F linkages with the result that the glass net-work structure would be weakened [12].In other words fluorine Substitution for bridging oxygenions leads to weakening the glass network due to the forma-tion of [Si03F]^~^ groups with a general formula [SiO(4_x)Ff""] which may increase as the fluorine content increases[13]. Due to the weakening of the glass structure, fluorinealso modifies the kinetic barriers for nucleation and growthof crystals, which can take place at lower temperatures. Vogel and Höland [14] found that what took place in a fluor-ine-free M g O - A l 2 0 3 - S i 0 2 glass at temperature as high as1130 °C could be achieved at 960 °C by the addition of fluor-ine. This effect is reflected in decreasing Tg, T^ and Tc, inthe tendency for the linear thermal expansion coefficient ofb) 10 μηηFigures 5a and b. SEM of a) BOM, heat-treated at 1050°C/30 h and b) BOM2 heat-treated at 1050°C/30 h.the glass to be increased in the sample containing MgF2,and it is increased by increasing the fluorine content.The absence of fluorite phase in all samples heat-treatedat different temperatures indicates that the F  ion in theglassy phase may replace one atom of oxygen inCaMg2Fe^+Si9Al3O30 to give CaMg2Fe^+Si9Al3029F2.The relatively low viscosity and high volume crystalliz-ability exhibited by the present titania- and fluoride-con-taining glasses may be also related to the weakening effectof either t i tanium or fluorine ions on the network structureof the glass [15 to 19]. According to those authors, Ti"^^ions can act as network former and exit in Silicon equivalentfourfold coordinated sites. However, Ti"^^ has a fieldstrength of 1.04 considerably above that of the fourfold-coordinated Al"^^ (0.96), which is known to enter the net-~--Crystallization of osumilite from modified basaltic glasseswork structure, but below that of the major glass network-forming Si^^ (1.56). Therefore, introducing weaker T i - Obonds will weaken the network structure, which, in turn, isreflected in lowering the viscosity of melts or glasses andconsequently leads to more mobilization of the glass-form-ing elements, i.e., to crystallization.It was observed that Ti02 can segregate forming smallcrystallines of MgTiOs, which act as heterogeneous nucleation sites for the crystallization of the glass matrix incompositions containing MgF2.5. References[1] Winter, W.; Armbruster, T ; Lengauer, C : Crystal structurerefinement of synthetic osumilite-type phases: Ba Mg2Al6-SigOso, SrMg2Al6Si903o and Mg2Al4Sii , 0 3 0 . Eur. J. Min-eral.  7 (1995) pp. 277-286.[2] Hawthorne, F. C ; Kimata, M.; Cerny, R: The crystalchemistry of the milarite-group minerals. Am. Mineral. 76(1991) pp. 1836-1856.[3] Schreyer, W.; Schairer, J. F.: Metastable osumilite and pet-alite-type phases in the System MgO -Al203-Si09. Am.Mineral. 47 (1962) pp. 90-104.[4] Winter, W; Bogdanov, C ; Müller, G. et al.: Crystallizationsequence of barium osumilite and MAS osumilite glass-ceramics with low thermal expansion. Glastech. Ber. 66(1993) pp. 109-117.[5] Winter, W; Müller, G ; Pannhorst, W: Osumilite typeglass-ceramics from glass powders. In: Duran, R; Fernan-dez, J. F (eds.): Third Euro-ceramics, 1993. Vol. 2, pp.1109-1114.[6] Khan, A. Α.; Baur, W. H.; Forbes, W. C : Synthetic mag-nesian merrinueite, dipotassium pentamagnesium dodeca-silicate: Α tetrahedral magnesiosolicate framework crystalstructure. Acta Cryst. B28 (1972) pp. 267-272.[7] Nguyen, N.; Choisnet, J.; Raveau, B.: Silicates synthetiquesa structure milarite. J. Solid State Chem. 34 (1980) pp. 1 9.[8] Abdel-Hameed, S. A. M.: Controlled crystallization ofsome Egyptian basalt-based glasses. Zagazig Univ., M. Sc.thesis 1995.[9] El-Shennawi, A. W. Α.; Morsi, M. M.; Khater, G. A. etal.: Thermodynamic investigation of crystallization behav-iour of pyroxenic basalt-based glasses. J. Thermal Analysis151 (1998) pp. 553-560.[10] Watanabe, K.; Giess, E. Α.: Crystallization kinetics ofhigh-cordierite glass. J. Non-Cryst. Solids 169 (1993) pp.306-310.[11] Borukhove, M. Yu.; Keithn, L. G.; Akilova, Τ Yu.: Effectof thermal vacuum treatment of some rocks on their ther-mal expansion. Uzb.Geol.Zh. 17 (1973) no. 3, pp. 13-15 .[12] Omar, A. Α.; Mahmoud, M. Y; Salman, S. M.: Crystalli-zation of some fluorine-containing glasses. Trans. J. Br.Ceram. Soc. 78 1978 no. 4, pp. 81 -86 .[13] Rabinovich, E. M.: On the structural role of fiuorine inSILICATE glasses. Phys. Chem. Glasses 24 (1983) pp. 54-56 .[14] Vogel, W; Höland, W: Nucleation and crystallization kinetics of an MgO-Al203-Si02 base glass with various dopants. In: Simmons, J. H.; Uhlmann, D. R.; Beall, G. H.(eds.): Nucleation and crystallization in glasses. Columbus,OH: Am. Ceram. Soc , 1982, pp. 125-145. (Advances inceramics. Vol. 4.)[15] Simmons, J. H.; Uhlmann, D. R.; Beall, G. H. (eds.): Nucleation and crystallization in glasses. Columbus, OH: Am.Ceram. Soc , 1982. (Advances in ceramics. Vol. 4.)[16] Weyl, W Α.: Coloured glasses. Sheffield: Soc Glass Tech-nol., 1951.[17] Zidaniewski, W: DTA and X-ray analysis study of nu-cleation and crystallization of MgO-Al203-Si02 glassescontaining Zr02, Ti02 and CeO.. J. Am. Ceram. Soc. 58(1975) no. 5 6 , pp. 163-169.[18] Barry, L. G.; Mason, B.; Dietrich, R. V: Mineralogy: con-cepts, descriptions and determinations. San Fransisco, CA:Freeman, 1983. Ρ 561.[19] Dingwell, D. B.; Scarfe, C. M.; Cronin: The effect of fiuor-ine on viscosities in the SYSTEM Na20-Al203-Si02: Impli-cations for phonolites, trachytes and rhyolites. Am. Min.70 (1985) pp. 80 -87 .Contact:Dr. Salwa A. M. Abdel-HameedGlass Research DepartmentNational Research CenterPCode 12622Dokki, CairoEgyptE-mail: Salwa_NRC@hotmail.com------

Crystallization of osumilite from modified basaltic glasses

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Crystallization of osumilite from modified basaltic glasses

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