Abstract

Two spinel dunite xenoliths (Fo89.8–91.2 in olivine) from La Palma contain minor amounts (<1%) of a pale-blue sodalite-group mineral with haüyne/lazurite chemistry. Selected-area electron diffraction (SAED) patterns of this phase indicate a cubic unit cell with dimensions 9.12 ± 0.02 Å, and space group P4̅3n. Superstructure spots along three <110> directions are common, implying commensurate or incommensurate modulations along <110> directions. Raman spectra show peaks typical of both lazurite and haüyne. It is concluded that the mineral has a structure intermediate between those of pure lazurite and pure haüyne, and it is here referred to as haüyness. The haüyness occurs together with strongly nepheline-normative glass in thin veinlets (<0.1 mm), in interstitial glass pockets, and as inclusions in olivine porphyroclasts. To our knowledge lazurite or haüyne has not previously been described in mantle rocks. The haüyness is strongly depleted in REE and most other highly lithophile elements relative to the coexisting glass, whereas Dmineral/glass for Sr is ≈1.0, and DEu higher than the other REE. The haüyness crystallized from a melt now present as phonolitic glass, probably in response to rapidly decreasing pressure during transport of the xenoliths to the surface. The coexistence of haüyness and FeS-rich sulfide globules in some samples suggests slightly more oxidizing conditions than for samples in which the glass contains sulfide globules alone.

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