The composition,structure, and stability of guinier-preston zones in lunar and terrestrial orthopyroxene

Lunar and terrestrial orthopyroxenes (Mg,Fe,Ca)₂Si₂O₆ contain varying abundances of coherent, Ca-enriched Guinier-Preston (G.P.) zones. G.P. zones 5–6 unit cells thick have been found in one lunar sample whereas all other examples (lunar and terrestrial) are only one unit-cell-thick. Electron diffraction maxima from the larger lunar G.P. zones indicate that d ₁₀₀=18.52 Å whereas, d ₁₀₀=18.2 Å for the host. This increase in the a direction corresponds to an increase in calcium content in the G.P. zones over that of the host of ～25 mol% Ca₂Si₂O₆. Diffraction patterns of the hk0 net from an area containing G.P. zones show extra spots (h=2n+1) not observed in the host orthopyroxene (Pbca), that violate the a-glide of the host. The G.P. zones, therefore, have space group Pbc2₁ if it is assumed that the c-glide of pyroxene is retained and the space group of the G.P. zone is a subgroup of Pbca. The loss of the a-glide in the G.P. zones results in 4 distinct silica chains and 4 distinct cation sites M1A, M1B, M2A, M2B; by symmetry, equivalent M2A or M2B sites are clustered together in only one-half of the unit cell. As one-fourth of the divalent cations in the G.P. zones are calcium, ordering of Ca on M2A or M2B would produce a zone 9 Å thick extended parallel to (100) with the composition of Ca(Mg,Fe)Si₂O₆, but constrained by the host to the structure of orthopyroxene. This zone and the Ca-poor half-unit-cell then constitute an 18 Å thick G.P. zone. Heating experiments of varying duration indicate that the zones become unstable with respect to the host orthopyroxene at ～950°C for Wo_0.6 and ～1,050°C for Wo_2.5. The zones are interpreted in terms of the pyroxene subsolidus as a metastable phase having either a solvus relationship with orthopyroxene or originating as a distinct phase. The size, distribution, composition and structure of G.P. zones may be an important indicator of the low-temperature thermal history of orthopyroxene.