Microstructural evolution in a model triaxial porcelain was studied by X-ra
y diffractometry and electron microscopy of quenched samples after firing f
or 3 h at 600 degrees -1500 degreesC, The clay component dehydroxylated to
metakaolin at similar to 550 degreesC. Metastable sanidine formed from deco
mposition of the feldspar at >600 degreesC and dissolved at >900 degreesC,
Liquid formation at similar to 1000 degreesC was associated with melting of
feldspar and silica discarded from metakaolin formation via the K2O-Al2O3S
iO2 eutectic. Liquid content increased at 1000 degrees -1200 degreesC with
further feldspar melting and additionally at >1200 degreesC because of quar
tz dissolution. Small (less than or equal to7 nm) mullite and gamma -alumin
a crystals precipitated in pure clay relicts and larger (less than or equal
to 30 nm) mullite crystals in mixed clay-feldspar relicts at 1000 degreesC
. In the evolving microstructures, three regions were observed. These regio
ns were derived from pure clay relicts containing primary (type-I) mullite;
feldspar-penetrated clay relicts, also containing secondary (granular type
-II) mullite; and the matrix of fine clay, feldspar, and quartz, containing
secondary (granular type-II and elongated type-III) mullite, In addition t
o shape, the mullite size changed, increasing from regions containing type-
I to type-III mullite, because the increasingly fluid liquid enhanced cryst
al growth. Below 1300 degreesC, primary mullite was richer in Al2O3 than th
e secondary mullite, and the glass composition was inhomogeneous, with the
K2O and Al2O3 contents varying throughout the microstructure. Above 1400 de
greesC, mullite began to dissolve.