Chemical models for lithium aluminosilicate stabilities in pegmatites and granites

The primary lithium aluminosilicates in pegmatites are spodumene (α-LiAlSi₂O₆) and petalite (LiAlSi₄O₁₀). These minerals are frequently replaced by fine-grained assemblages of eucryptite (α-LiAlSiO₄), albite, micas, and clay minerals as a result of subsolidus cation exchange reactions with residual pegmatitic fluids.

All three of the lithium aluminosilicates may be converted to albite in quartz-saturated, Na-rich environments, but in quartz-undersaturated environments (as within large single crystals of spodumene), the replacement assemblage eucryptite + albite is stable to high values of the exchange potential μ_NaLi−1. The stability of the primary pegmatite assemblage quartz + albite + spodumene (or petalite) + amblygonite-montebrasite + lithiophilite means that lithiophilite or amblygonite-montebrasite cannot be altered to natrophilite (NaMnPO₄) or lacroixite (NaAlPO₄F), respectively, until Na–Li exchange has locally converted all of the lithium aluminosilicates to albite. Of the three lithium aluminosilicates, eucryptite is the most susceptible to replacement by muscovite, but spodumene also may be converted directly to mica (“killinite”). In acidic fluids with a high capacity to hydrolyze solid phases and leach alkali cations, the lithium aluminosilicates may be altered to (or become unstable relative to) kaolinite, cookeite, muscovite, lepidolite, and topaz. The common late-stage assemblage topaz + lepidolite + quartz forms in KF- and HF-rich environments outside of the lithium aluminosilicate stability region. All of the lithium aluminosilicates are unstable relative to amblygonite (or montebrasite) + quartz in P- and F-rich environments, but spodumene is stable to higher activities of these acidic volatiles than are eucryptite and petalite.