Clay Minerals (1998) 33,443-452

An approach to the chemistry of pyroxenes formed during the firing of Ca-rich silicate

ceramics

M. D O N D I , G. E R C O L A N I , B. F A B B R I AND M. M A R S I G L I

CNR-IRTEC, via Granarolo 64, 48018 Faenza, Italy

(Received 29 November 1996; revised 11 September 1997)

A B S TRACT: Carbonate-bearing ceramic bodies are frequently used in the manufacture of bricks, roofing tiles, wall and floor tiles, pottery and tableware. During the firing of these bodies, clinopyroxene is usually formed in very small crystals, 1-5 gm in diameter or less. In the literature this phase is generally referred to as diopside, but no quantitative data are available. In order to chemically characterize these 'ceramic' pyroxenes, nine industrial products were analysed by XRF and XRD (bulk sample) and SEM-EDS (fracture surface). Quantitative ZAF analyses of pyroxene crystals showed a certain chemical variability: SiO2 35-50%, A1203 9-20%, Fe203 1-15%, MgO 3-14%, and CaO 16-25%. Sodium, K and Ti are always <1%, while ferrous iron is always <0.2% in the bulk sample. Overall, 'ceramic' clinopyroxenes present wide chemical analogies with 'fassaite', e.g. the abundance of aluminium and ferric iron, and the excess ofwollastonite molecules with respect to the diopside-hedenbergite series.

The formation of a pyroxene-type phase occurs frequently during the firing of Ca-rich ceramic bodies. This phase represents - - together with anorthite, wollastonite and melilite - - the product of the solid-state reactions between carbonates and clay minerals (Peters & Iberg, 1978; Lach, 1978; Shoval, 1988; Echallier & Mery, 1992).

Pyroxene is found under industrial firing condi- tions starting from -900~ to >1200~ This wide range of thermal stability explains its widespread presence as a newly-formed component of terracotta (bricks, roofing tiles, etc.), majolica and earth- enware (Lach, 1978; Shoval, 1988; Echallier & Mery, 1992; Dondi et al., 1996).

X-ray diffraction (XRD) analysis of these ceramic materials normally reveals the principal lines of a generic clinopyroxene; in addition, XRD patterns are often so poorly defined that a more specific attribution is not reliable. This occurs mainly as the result of the small size of pyroxene crystals, usually -1 I.tm or less, and rarely reaching 5-10 ~tm (Shoval, 1988; Echallier & Mery, 1992; Dondi et al., 1996).

'Ceramic' pyroxene is generally mentioned in the literature as diopside (Peters & Iberg, 1978; Lach, 1978; Shoval, 1988; Echallier & Mery, 1992) despite the above-mentioned uncertainty concerning diffraction data and the lack of quantitative chemical data. For example, Peters & Iberg (1978) claimed the following composition: diopside >80%, hedenbergite <20% and some acmite in solid-solution. As a matter of fact, no direct determination was performed because of the difficulty caused by the small crystal size and the irregularity of the sample surface, due to the porosity of the ceramic artifact.

The aim of this paper is to characterize chemically pyroxene formed during the firing of different ceramic materials. For this purpose, scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) techniques were used.

M A T E R I A L S A N D M E T H O D S

To appraise the chemical variability of the pyroxene in silicate ceramics, nine industrially manufactured

@ 1998 The Mineralogical Society

444 M. Dondi et hi.

products were considered. These materials were obtained with more or less Ca-rich bodies but they were produced with different shaping techniques (extrusion, pressing, slip casting, jolleying) and firing condit ions (maximum temperature: 900-1140~ time: 1-60 h cold-to-cold).

The selected samples represent bricks (samples 42C, 260C and 263C), roofing tiles (342N), paving bricks (TC), wall tiles (BR and MP), earthenware (TR) and majolica pottery (MJ). Samples BR and MP were fired in a laboratory roller kiln at different maximum temperatures.

The experimental procedure is represented in Fig. 1. All the products were analysed from the chemical and mineralogical point of view, by means of X-ray fluorescence spectrometry (Philips | PW 1480) on powder pellets, and XRD (Rigaku | Miniflex) of randomly oriented powders. Their bulk chemical composition is reported in Table 1 together with some notes on shaping technique and firing conditions. Ferrous iron was determined by Pratt's method (Ports, 1987).

Chemical analysis of pyroxene was performed by means of EDS with electron microprobe (Link

Analytical ~ eXLI) applied to a scanning electron microscope (Leica | Stereoscan 360). Samples were carbon coated and analysed on the fracture surface, without any polishing. Scanning electron micro- scope observations and preliminary determinations on several microcrystals were carried out in order to distinguish pyroxene from other phases.

The experimental conditions were: accelerating voltage 20 kV; microprobe current 700 pA; focused beam ~2 ~tm in diameter; counting time 60 s. The EDS counting (three analyses on each point) was converted into oxide percentage by means of a routine ZAF system provided by Link Analytical. Normalization to 100% was often necessary; small sulphur contributions were neglected.

Microanalysis precision was estimated in terms of relative standard deviation of measurements. It was <1% for Si and Ca and <5% for AI, Fe and Mg. Values around 10% are registered for Ti, Na and K, which are, however, always in low concentrations. The accuracy was evaluated by analysing some in-house reference materials (natural wollastonite and fassaite; synthetic gehle- nite and anorthite). Synthetic samples were

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prepared by blending CaCO3, SiO2 and A1203 in 0.8 the appropriate molar ratios followed by firing for 0.r an extended period at 1200~ The difference 0.6 between reference and measured values for the 0.5 main elements (Si, AI and Ca) was always <5% ~*

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The pyroxenes formed in the ceramic products considered were recognized by SEM as very small crystals, ranging approximately from 1-5 gin. These microcrystals generally appear to be poorly defined with rounded terminations. Well-formed crystals were only found occasionally (Fig. 2).

Twenty-eight pyroxene compositions, together with the number of ions calculated on the basis of six oxygens and the percentage of end-members, are detailed in Table 2. Iron was considered to be entirely trivalent, due to the negligible amount of FeO in bulk samples.

Overall, the chemical composition of 'ceramic' pyroxene varies widely even in the same product. Silica ranges from 35-50% with most values between 40 and 45%, CaO from 16-25%, with many values close to 23-25%, while MgO fluctuates from 3 - 1 4 % (mainly 6 -12%) . Trivalent ions are abundant: 9-20% A1203 and 1-15% Fe203, but most pyroxenes showed values in the 12-17% range for alumina and 6-12% range for iron oxide. Titanium, Na and K are minor components, usually <1%.

The crystallochemical composition of pyroxenes was reconstructed following the recommendations of the IMA Sub-committee (Morimoto, 1988). From this point of view, clinopyroxene is characterized by two tetrahedral positions (T) and two octahedral sites (M1 regular and M2 distorted) per formula.

In the crystals considered, Si occupies 1.4 to 1.8 T sites (most data are in the 1.5-1.7 range). As a consequence, AI in tetrahedral coordination ranges from 0.2 to 0.6, with the maximum frequency around 0.3-0.5 per formula unit (p.f.u.).

Position M1 is mainly occupied by Mg, AI, Fe and traces of Ti. Magnesium is present in amounts from 0.2 to 0.8 p.f.u., with the maximum frequency of data between 0.3 and 0.6 p.f.u. Both Al and Fe

FASSAITE

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FIG. 3. Binary diagram of 'ceramic' pyroxenes crystal- chemistry: trivalent ions in octahedral sites ([6]R3+) vs.

tetrahedrally-coordinated AI ([4]A1).

in octahedral coordination range from 0.1 to 0.4 each.

Site M2 is mostly taken up by Ca (0.6-1.0) and by traces of Na (up to 0.13) and K (up to 0.08). Since potassium is incompatible with the clino- pyroxene structure, its content was presumably slightly overestimated. In sample BR, the M2 positions are only partially occupied by Ca (0.6-0.8 per formula) and alkalis, so that a limited contribution from Mg has to be taken into account,

The chemical characteristics of pyroxenes formed in ceramics are not fully consistent with those of the isomorphous series diopside-hedenbergite (Table 3). In contrast, they appear to be closer to those of fassaite (Minguzzi et al., 1976-77; Deer et al. 1978; Prewitt, 1980). In particular, the following features can be stressed: (1) abundance of both tetrahedrally- and octahedrally-coordinated AI: (2) abundance of Fe 3§ and virtual absence of Fe2+; (3) predominance of the wollastonite mole- cule (CaSiO3) over enstatite (MgSiO3) and ferrosi- lite (FeSiO3).

The abundance of trivalent ions at the T and M1 sites is represented graphically in Fig. 3, where [4]AI is plotted vs. the sum of [61(AI+Fe3+). Analogous indications are provided from the ternary diagram [4IAI-16]R 3+ [61R2+ (Fig. 4). As a

matter of fact, in both diagrams 'ceramic' pyroxenes fall within, or in proximity to, the fassaite field and far from the diopside composition.

FIc~. 2. (Opposite) SEM micrographs of 'ceramic' pyroxenes from: (,A) wail tile (BR-1 lO0~ (B) roofing tile (324N-960~ (C) floor tile (TC-1000'~C); (D) brick (260C-930"C). Scale bar is 10 ~tm (A) and 5 lam for B-C-D

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TABLE 3. Comparison of the average chemical composition of the diopside-hedenbergite end-members, natural fassaites (after Minguzzi et al., 1976-77) and 'ceramic' pyroxene (this work). Average of n compositions.

End-members Natural fassaite (n = 55) 'Ceramic' pyroxene (n = 28) wt% Diopside Hedenbergite min max average _ cy min max average _ cr

SiO2 55.5 48.4 37.5 51.1 44.2 -t- 2.4 35.8 48.5 43.1 _+ 3.3 TiO2 - - 0.1 5.7 0.8 _+ 0.4 0.1 1.2 0.6 ___ 0.2 A1203 - - 6.4 15.8 9.9 + 2.0 9.0 19.9 15.0 -F 2.5 F e 2 0 3 - - - 0.6 23.0 5.6 4- 2.6 0.7 14.6 8.6 _ 3.4 FeO - 29.0 0.2 15.6 2.l __+ 2.0 - - traces MgO 18.6 - 3.6 17.2 12.6 4- 2.4 3.4 17.4 9.2 + 3.6 CaO 25.9 22.6 18.5 26.6 24.5 _ 0.7 16.3 24.8 21.7 • 2.9 Na20 - - 0.0 2.5 0.3 + 0.3 traces 1.9 0.6 -I- 0.5 K20 - - 0.0 0.6 0.1 ___ 0.1 traces 1.7 0.6 ___ 0.4

It can be stressed that many points appear to be richer in trivalent ions than the natural fassaites. This seems to be related to the composition of ceramic bodies, which is clearly poorer in MgO and FeO with respect to the fassaite-bearing meta- morphic rocks. Moreover, the firing process tends to oxidize Fe 2+ to Fe 3+ (He, 1994).

In any event, the high AI contents seem to be consistent with the clinopyroxene structure, though they may sometimes be close to the maximum value of At203 that is possible in solid solution (Kirkpatrick, 1974).

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t,l R3+ [elR2. FIG. 4. Ternary diagram of 'ceramic' pyroxenes crystal-chemistry: vertices represent t r i v a l e n t ([6]R3+) and bivalent ions ([6]R2+) in the octahedral position

and A1 in tetrahedral sites ([4]A1).

The abundance of A1203 influences the propor- tions among the end-terms (wollastonite, enstatite, ferrosilite) calculated considering Fe to be bivalent. Therefore, 'ceramic ' pyroxenes are particularly rich in the wollastonite molecule ( 4 1 - 6 4 % , mostly 5 0 - 6 0 % ) which is in excess with respect to the diopside-hedenbergite solid solution (50%). This situation is exemplified in the ternary diagram Wo- En-Fs (Fig. 5) where ' ceramic ' pyroxenes are distributed within or near the fassaite field.

Furthermore, there was a fair chemical variability for the pyroxenes found in the same product. This can be explained by assuming the presence of local equilibria which may be affected by both the firing condit ions and the chemical gradients in the ceramic material.

For this purpose, some preliminary considerations can be expressed taking into account that the number of analyses is not sufficiently representative to draw any general conclusion. Some compositional differences can be noticed among the pyroxenes belonging to three distinct groups of samples. (a) Cl inopyroxenes in bui ld ing clay products (samples 42C, 260C, 263C and 324N) seem to be the richest in Ca and AI, and poorest in Mg, with >53% wollastonite molecules usually present. These features could be due to the high CaO and the relatively low MgO contents of ceramic bodies. In fact, their CaO/A1203 ratios range from ~1.0-2.0 , wh i l e the i r M g O / C a O ra t ios are all - 0 .2 . Furthermore, a certain role could be played also by the slow firing cycles and the not particularly h i g h t e m p e r a t u r e s ( 9 0 0 - 9 6 0 ~ 2 4 - 3 6 h). (b) Pyroxenes found in wall tiles (samples BR and MP) are the poorest in Ca and relatively rich in Mg,

Chemistry of 'ceramic" pyroxenes 451

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Fro. 5. Ternary diagram of wollastonite (Wo)-enstatite (En)-ferrosilite (Fs). Fassaite field is after Minguzzi et al. (1976-77).

with not particularly large amounts of AI. Also, the chemistry of pyroxenes could actually be influenced by the bulk composition of ceramic bodies: CaO/ A1203 ratios tend to be low (0.4-0.7), while MgO/ CaO ratios are proportionally high (0.4). Moreover, in such products, fired at various maximum temperatures with fast cycles (1 h), there appears to be a certain trend concerning the Si-A1 partition in tetrahedral sites: the higher the firing temperature the lower the A1/Si ratio. (c) In earthenware and majolica (samples TR and MJ) pyroxenes are characterized by large amounts of Ca, A1 and Mg, as well as a low Fe content. This is conspicuous for sample TR, which is especially rich in A1203 and MgO, and very low in Fe203. On the other hand, the chemistry of pyroxenes in sample MJ is closer to the field of building clay products. All of these bodies have chemical compositions and firing conditions quite similar to each other.

C O N C L U S I O N S

The chemical composition of 'ceramic' pyroxene was determined by means of SEM observations and EDS analyses of various Ca-rich industrial products.

Although the analyses of these pyroxenes show a fair variability from one sample to another and sometimes even in the same product, a general convergence toward a fassaitic composition is evident. An indicative mean composition is:

3+ (Cao.gNao.05)(Mgo.sAlo.zsFeo.25)(S11.6A10.4)O6

neglecting traces of Ti and K. Therefore, according to the mineralogical nomenclature, 'ceramic'

pyroxenes should be defined as peraluminous diopside, rich in ferric iron.

Some differences may be found among pyrox- enes from building clay products, fast-fired wall t i les , and e a r t h e n w a r e - m a j o l i c a pot tery. Furthermore, in the samples fired at different maximum temperatures, there may be a relationship between the chemical composition and the intensity of thermal treatment. In any case, further analyses are necessary to assess these observations.

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