A precise crystallisation age for the scourie dykes, and a new chronology for crustal development in north-west Scotland
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Pigeonite
Pyroxene
Anorthosite
Summary A preliminary account is given of an optical and electron-microprobe investigation into the pyroxene relations of Hawaiian hypersthene-bearing lavas. It is shown that the clinopyroxene jackets to the phenocryst hypersthenes consist of strongly zoned augite, not pigeonite as formerly believed; the compositional range of this pyroxene is approximately the same as that of the groundmass augite phase. Groundmass pyroxene relations are complex; in one case two distinct pyroxene phases, augite and pigeonite occur, in the other all compositions lying between augite and pigeonite are encountered.
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Pigeonite
Pyroxene
Diopside
Norite
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Pyroxenes in the rapidly-cooled upper portions of thich layered komatiite lava flows have significantly different compositions and structures from pyroxencs in the cumuliate lower portions of the same flows. Rocks in the flow tops contain composite pyroxene mcgacrysts made up of needles with cores of pigeonite and margins of subcalcic, aluminous augite. The pigeonite is more magnesian [Mgl(Mg + Fe) = 0.841 than any previously reported in terrestrial rocks. In the cumulate zon€s, the pyroxenes are bronzite and augite. The pigeonite probably formed under conditions of strong undercooling developed during the solidifcation of the upper parts of the flows, whereas the cumulus pyrox€nes formed under conditions approaching equilibrium in the more slowly-cooled ccntral parts of the flows.
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Abstract— Ibitira is a strongly recrystallized and unbrecciated noncumulate eucrite. We measured Ca compositional profiles of Ibitira pyroxene by electron microprobe and computed the cooling rate and burial depth from pyroxene exsolution profiles to gain information on early thermal history of Ibitira. Pyroxene begins to exsolve at 1082 °C and cools down to 550 °C at a rate of 0.02 °C/year, forming an augite lamella about 7.0 μm in width. A notable characteristic of the Ca profile of augite lamellae in Ibitira pyroxene is a gradient near the interface between augite and low‐Ca pyroxene (pigeonite). This profile suggests that after thermal metamorphism Ibitira pyroxene experienced a sudden temperature rise to above solidus temperature of pyroxene (∼1082 °C), and subsequent rapid cooling. The 39 Ar‐ 40 Ar age of 4.485 Ga for Ibitira, which is the oldest 39 Ar‐ 40 Ar age for noncumulate eucrites, may date this reheating event.
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Pigeonite
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Hess objects to Macdonald9s (1940) classification of a microphenocryst of pyroxene with an optic angle of 45 degrees as pigeonite. He prefers the term augite, restricting pigeonite to clinopyroxene having an optic angle of 0-30 degrees . He states that the most common pyroxene in basalt has an optic angle of about 45 degrees .
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Achondrite
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Abstract— Experiments on a Martian basalt composition show that D v augite/melt is greater than D v pigeonite/melt in samples equilibrated under the same f O 2 conditions. This increase is due to the increased availability of elements for coupled substitution with the V 3+ or V 4+ ions, namely A1 and Na. For this bulk composition, both A1 and Na are higher in concentration in augite compared with pigeonite; therefore more V can enter augite than pigeonite. Direct valence state determination by XANES shows that the V 3+ and V 4+ are the main V species in the melt at f O 2 conditions of IW‐1 to IW+3.5, whereas pyroxene grains at IW‐1, IW, and IW+1 contain mostly V 3+ . This confirms the idea that V 3+ is more compatible in pyroxene than V 4+ . The XANES data also indicates that a small percentage of V 2+ may exist in melt and pyroxene at IW‐1. The similar valence of V in glass and pyroxene at IW‐1 suggests that V 2+ and V 3+ may have similar compatibilities in pyroxene.
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