Triaxial porcelains are complex ceramic materials consisting of glass and crystalline phases (mainly  mullite)  produced from a mixture of clay, feldspar and quartz, fired at about 1300ºC (1, 2). Potash feldspars (microcline and orthoclase) are common fluxes and usually appears associated with plagioclases.
This paper concerns the crystallite size and shape evolution of mullite by firing triaxial compositions at different temperature and  with different fluxing agent (K-feldspar  or Li bearing petalite).
Two compositions were studied: 14% quartz, 53% kaolin, 33% K-feldspar  (MGS) and 14% quartz, 53% kaolin, 33% Li-feldspar (MGC). Firing conditions were: heating ratio 2ºC/min , 180 min  at T max and different (T ºC max, rising  time in minutes) respectively: 1270, 815 (L1), 1300, 830 (L2), 1320, 840 (L3) and 1340, 860 (L4). 
X Ray diffraction patterns of powdered samples were obtained in a Bruker D5005 diffractometer in the range 10–110 º (2 theta) at scan step ( 0.02 º ,20 sec). Complementary SEM observations of fired samples, were made in an Hitachi S-4100 microscope.
Crystallite size and shape were obtained by the Kojdecki method (3,4,5) using LaB₆ (NIST SRM660a) as standard. The Table  includes the obtained  shape and size data for modelation of prism crystallite shape The parameter C/A, is almost constant in MGS samples but not  in MGC samples where the higher size the higher C/A value, so there is a preferred crystallite growth from 1300 to 1340ºC .
The difference between the evolution of crystallite growth in MGC respect MGS compositions is in agreement with the morphologies observed by SEM (greater development of mullite.

Sample       A (Å)           B(Å)             C (Å)               C/A        V ^1/3
MGCL1     442              450                762                1,724      533
MGCL2     387              395              1010                2,610      536
MGCL3     488              496              1180                2,418      659
MGCL4     690              704              2196                3,183    1022
MGSL1     441               450               913                2,070      566
MGSL2     428               436               836                1,953      538
MGSL3     503               512               772                1,535      584
MGSL4     414               422               753                1,819      508

In K-feldspar formulation (MGS) there is no size growth for mullite crystallites with increasing temperature, while in petalite formulations the higher temperature the bigger crystallite sizes. Moreover different features have been found for crystallite growth in this case at highest temperature respect to that found in simple sintering or reactive sintering, with clear decrease for C/A shown at 1300ºC (5) whereas slight increases are found for mullites obtained from 3:2 gels in the range 1300-1500ºC (6).

References
1 Norton, F.H. (1988) Fine Ceramics: Technology and Applications. McGraw-Hill Inc. New York.
2. Kingery, W.D. (1976) Introduction to Ceramics. Wiley. New York.
3. Kojdecki, M. A. (2001) Mater. Sci. Forum, 378–381, 12–17
4. Kojdecki, M. A (2004) Mater. Sci. Forum, 443-444, 107-110
5. Kojdecki, M. A., Bastida, J., Serrano, F. J. & Clausell, J. V. (2001) Mater. Sci. Forum, 378–381, 747–752
6. Kojdecki, M. A., Ruiz de Sola E, Serrano F.J; Delgado Pinar V, Raventós M, Esteve V.J (2007) J. Appl. Cryst. 40, 260–276

Acknowledgements. Generalitat Valenciana project: No GV02-527.

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1. Microstructural evolution and formation of titania-doped mullites from heated single-phase gels
2. Crystallite size of kaolinites as indicator of different geochemical types of bauxite in Maestrazgo area (NE Spain)