A natural mixture of feldspar and quartz, pegmatite is used as such.
Ref : PEGM
PEGMATITE PA45 - CORNWALL STONE
0.3Na²O 0.7K²O 1.1Al²O3 9.3SiO²
Natural granites with a high percentage of feldspar.
These are the most commonly used minerals in their natural state for high and medium temperatures.
They are associated with many pastes and glazes and are indispensable in the manufacture of porcelain.
It behaves similarly to feldspar but a little less fusible.