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SYNTHESIS OF PIGEONITES FOR SPECTROSCOPIC AND SPACE WEATHERING STUDIES

Author: 
Donald Lindsley
Topic: 
Geochemistry/Petrology
Delivered As: 
Poster
Abstract Text: 

Natural pigeonites (essentially clinopyroxenes of composition near Ca0.1(Mg1-x Fex)0.9)SiO3 are difficult to obtain: pigeonite is fairly common in some terrestrial basalts but is fine-grained and difficult to separate. Pigeonites that formed in plutonic rocks may have had large grain size, but upon slow cooling below their minimum stability temperatures have almost invariably “inverted”: decomposed to a distinctive intergrowth of the more stable assemblage orthopyroxene + augite, and thus cannot be recovered as true pigeonite.

As a result we have concentrated on synthesizing suitable materials in the laboratory, with compositions near Wo10 (Wo = 100Ca/(Ca+Mg+Fe)) and with X (X=100Fe/(Fe+Mg)) ranging from 20 to 60. Final synthesis for each X must be carried out at temperatures above the minimum stability of pigeonite, yet below the solidus. The optimum process involves: 1. Weighing and homogenizing mixtures of dried CaSiO3, MgO, SiO2, Feo, and Fe2O3; 2. wrapping the mixtures in Ag foil, sealing in evacuated silica-glass tubes, and “pre-reacting” at ~900oC for 10-30 days*; 3. grinding the “pre-reacted” product for 2-3 hours; 4. loading the ground material in iron capsules which are then sealed in evacuated silica-glass tubes; 5. Heating the samples for 7-15 days at temperatures ~50oC below the solidus; 6. repeating steps 3 to 5 at least once. The final product consists of 97-99% pigeonite crystals close to the target composition and 5 to 50 microns across. [*900oC is below the minimum stability temperature of all but the most Fe-rich pigeonites, so “pre-reaction” (step 2) typically produces a mixture of augite, olivine, and silica but no pigeonite. However, it is necessary to minimize the amount of starting materials isolated as inclusions within growing pigeonite - which happens if the oxide mixes are directly reacted within the pigeonite stability field.]

For some purposes (e.g. mid-IR spectroscopy) larger crystals are necessary. We have attempted to grow these by heating samples synthesized as above to just above the published solidus for each X (Huebner and Turnock, 1980, Am. Min. 65, 225-271), then cooling to 10-20oC below the solidus and crystallizing for several days. We have succeeded in growing pigeonites close to 100 microns, but to date, all these pigeonites are depleted in Ca and Fe (relative to the target composition) and are accompanied by ~10-30% Fe- and Ca-rich melt. We are investigating the possibility that the published solidus temperatures may be in error by 20-30 deg; we are confident that, once this issue is resolved, we can grow 100 micron pigeonite crystals on the target compositions.

Co-Authors: 
(1) University of Latvia, FOTONIKA-LV, Riga, Latvia (vid.beldavs@fotonika-lv.eu), (2) National Space Society, Washington,
SSERVI Identifier: 
NESF2016-150

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