High Chromium and Vanadium in Andalusite, Phengite and Retrogressive Margarite in Contact Metamorphosed Ba-rich Black Slate from the Abercrombie Beds, New South Wales, Australia

Vincent J. Morand*
Department of Geology and Geophysics, University of Sydney, N.S.W. 2006, Australia
*Present address: Department of Geology, James Cook University, Townsville, Old 4811, Australia

Abstract: Graphitic, quartz-rich black slate within the Late Ordovician Abercrombie Beds, Lachlan Fold Belt, southeast Australia, has undergone contact metamorphism adjacent to the Siluro-Devonian Wyangala Batholith. This produced porphyroblasts of andalusite and cordierite, and smaller grains of pale green phengitic mica. Later regional metamorphism caused complete retrogression of cordierite and partial retrogression of andalusite, with margarite replacing some andalusite.

The aluminous minerals andalusite, margarite and phengite all contain V and Cr substituting for Al. Andalusite has up to 1.39% V2O3 and 1.09% Cr2O3, margarite has up to 1.07% V2O3 and 0.37% Cr2O3, and phengitic mica has up to 6.93% V2O3 and 1.52% Cr2O3. This mica also has BaO contents of up to 1.96%.

Chemical analyses reveal very high SiO2 contents for these rocks (about 89%), carbon contents of about 2%, and extremely low CaO, FeO, MgO and Na2O. Although V and Cr are prominent in aluminous minerals, their concentrations in the rock are only about average for black shales. However, Ba values range from 2000 to 6000 p.p.m., well above average for black shales. It is suggested that V and Cr probably precipitated from sea water, but Ba may have been concentrated by planktonic organisms such as radiolaria.

Keywords: black slate • andalusite • mica • barium • vanadium

Mineralogical Magazine; September 1990 v. 54; no. 376; p. 381-391; DOI: 10.1180/minmag.1990.054.376.03
© 1990, The Mineralogical Society
Mineralogical Society (www.minersoc.org)