Hollandite-strontiomelane solid solutions coexisting with kanonaite and braunite in late quartz veins of the Stavelot Massif, Ardennes, Belgium

In a newly found type of quartz vein crosscutting the famous "viridine"-bea ring phyllites at Le Coreux, hollandite, ideally BaMn8O16, was discovered f or the first time at this locality and in Belgium. Because the crystals con tain up to 60 mol% of the Sr end member, this is also the second occurrence of strontiomelane. The coexisting "viridine" (= kanonaite) contains the hi ghest amount (88 mol%) of the ideal end member MnAlSiO5 ever found worldwid e. The hollandite-type minerals are intimately intergrown with braunite con taining appreciable Ca and Mg. Ba-bearing muscovite, Fe-poor excess-Al clin ochlore (not quite trioctahedral), and albite are the remaining accessory m inerals in the dominant quartz matrix. Microprobe analyses of all phases sh ow rather extreme element fractionations: nearly all K is located in muscov ite and none in the hollandite phase despite the existence of the end membe r KMn8O16 (cryptomelane). Similarly, nearly all Na is in albite and not in hollandite (no NaMn8O16=manjiroite component). Nearly all Mn resides in the two oxide phases and in kanonaite. Mg is strongly fractionated into chlori te. The small amounts of Fe and Ti present are predominantly partitioned in to the hollandite phase, which also accommodates most of the Ba and Sr. Ind eed, the hollandite phase is stabilized by the latter two elements relative to other Mn oxides. Kanonaite is stabilized by Al. Although no requisite s ites are available in its crystal structure, braunite always contains small amounts of Ba and Sr. However, the Sr/ (Sr + Ba) ratios in braunite are sp urious and unrelated to those of the directly adjoining hollandite phases. The conditions of formation of these veins may be well below 300 degreesC a t low pressures (1-2 kbar), in agreement with the experimental results that the maximum Mn contents in kanonaite increase with falling temperatures.