All Minerals

Plagioclase

Formula: NaAlSi₃O8 to CaAl2Si2O8

Plagioclase feldspar is not one mineral but a series between two: Albite - NaAlSi₃O₈ (high sodium) and Anorthite - CaAl₂Si₂O₈ (high calcium). Thus, every plagioclase is some mixture of those two. The more sodium (Na₂O), the closer to albite; the more calcium (CaO), the closer to anorthite.

Plagioclase is a primary flux source in many clays, a natural contributor to vitrification in stoneware and the backbone of porcelain vitrification. It is also a primary flux source in many glazes. It often crystallizes as anorthite in high-calcium bodies and glazes.

Albite is lower melting, has higher thermal expansion, and stronger fluxing action. Is produces more glassy, less crystalline glasses/glazes.

Anorthite is higher melting, lower thermal expansion and promotes crystal formation (anorthite). It tends to producet ougher, more stable melts.

Most stoneware and porcelain bodies contain plagioclase as ground feldspar. Some clays also contain natural feldspar. During firing, feldspar begins to melt and form a glassy phase that dissolves quartz and reacts with kaolin. In calcium-rich systems, anorthite crystals can form, these increase strength, reduce pyroplastic deformation and help thermal shock resistance.

Orthoclase (microcline) feldspars are a different group. They melt slightly higher, have less crystallizing tendency and lower thermal expansion.

Related Information

Links

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk