Abstract

Two Al-rich amphiboles were found in a corundum- and spinel-bearing calcite marble together with minor phlogopite, margarite, anorthite, and sphenb or rutile. Microprobe analyses give half their unit cell contents as: Amphibole AA-5,Na0.54K0.16ca1.99Mg3.35Fe0.01Ti0.16AIVI1.47AIIV2.26si5.74OHl.4F0.40CI0.07; Amphibole HA-l/2,Na0.67Ko.17Ca1.99Mg3.62Feo.07Ti0.27AIVI1.12AIIV2.25Si5.75OH1.05F0.58CI0.02 Amphibole AA-5 is the most aluminous yet found in nature (22.6 wtpercent AI2O3) and contains essentially no Fe (total iron as FeO = 0.05 wt percent).

Both amphiboles are too rich in AlIV (viz, high Al2O3, combined with low SiO3) to consider them as mixed crystals strictly intermediate between pure pargasite and pure tschermakite. AI-rich pargasite appears to be a suitable name for amphiboles of this composition, owing to the factthat more than 50 percent of their structural formulae can be recalculated in terms of a hypothetical end-member molecule AI-pargasite, (Na,K)Ca2Mg3AIVI2AIIV3Si5O22(OH,F)2, which conforms to the limiting AIIV/AlVI ratio of Ca-amphiboles statistically evaluated by Leake (1965, 1971).

The water determination and X-ray powder data are given for one of the amphiboles but not for the other owing to insufficient material. Conditions for the crystallization of the Al-rich amphiboles are briefly discussed.

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