Feldspars, feldspathoids, zeolites

Feldspar phase relations, exsolution, etc.

Finish materials left over from previous lecture.

• Feldspar structure movies
  • Albite (580K) (Si-dark blue, Al-light blue, Na-yellow)
  • Anorthite (350K) (Si-dark blue, Al-white, Ca-aqua)
  • K-feldspar (sanidine) (326K) (complete Al-Si disorder-both dark blue)
• Phase equilibria
• What happens in lower silica environments? (Figs. 13.137, 9.8b, 9.10, 9.14)

Feldspathoids

• anhydrous framework silicates
• composition reflects formation from low silica, high alkali melts
• structures related to feldspars but with 6-membered rings leading to more open structures and lower densities
• some feldspathoids contain unusual anions such as Cl^-, S⁼, SO₄^-2, and CO₃^-2

• Leucite (224K) KAlSi₂O₆ (K-purple, Al/Si-dark blue, O-red)

Nepheline: hexagonal; related to high tridymite structure

• Nepheline (322K) (Na,K)AlSiO₄ (Na-yellow, K-brown, Al-aqua, Si-blue, O-red)

Sodalite: cubic

• Sodalite (217K) Na₃Al₃Si₃O₁₂ (NaCl) (Na-yellow, Al-aqua, Si-blue, O-red, Cl-green)

Scapolite Series

• metamorphic minerals
• complete solid solution between Marialite (3NaAlSi₃O₈^.NaCl) and Meionite (3CaAl₂Si₂O₈^.CaCO₃ or CaSO₄)
• this is a good way to remember the formula but not the structures
• Na for Ca solid solution compensated for by Si for Al solid solution
• minerals are tetragonal
• imperfect but distinct cleavage, more fibrous in appearance than feldspar cleavage

Zeolites

• hydrous framework aluminosilicates
• large voids in structure containing Na⁺, Ca⁺², K⁺, and H₂O
• filling cavities and veins in basic igneous rocks
• alterations of volcanic tuffs and glasses
• low grade metamorphic rocks - "zeolite facies"
• soft, low density
• some fibrous (chains); some platy (sheets); some equant (frameworks)
• used as molecular seives and cation exchange resins

• Analcime (243K) NaAlSi₂O₆^.H₂O (Na-yellow, Al/Si-blue, O-red, H₂O-purple)

• Natrolite (499K) Na₂Al₂Si₃O₁₀^.2H₂O (Na-yellow, Al-aqua, Si-blue, O-red, H₂O-brown)

• Chabazite (642K) CaAlSi₂O₆^.3H₂O (Ca-gray, Al/Si-blue, O-red)