Secondary tungsten minerals in quartz veins in the Ishidera area, Wazuka, Kyoto Prefecture, Japan: anthoinite, mpororoite, and Fe-free hydrokenoelsmoreite

Abstract

Secondary minerals of tungsten that are the products of alteration of scheelite present in quartz veins in the Ishidera area, Wazuka, Kyoto Prefecture, have been examined using XRD, SEM -EDS, EPMA, and XRF. From the results, three tungsten minerals were identified: anthoinite, mpororoite, and hydrokenoelsmoreite. The two former minerals have not been reported to be found in Japan. This is, therefore, the first discovery of anthoinite and mpororoite in Japan. The two minerals form a white powdery mixture with pseudomorphing scheelite. Chemical analysis of the mixture shows that the Al/W ratio is approximate to 1 and that the Fe 2 O 3 content is very low, suggesting that the ideal formulae of anthoinite and mpororoite are WAlO 3 (OH) 3 and WAlO 3 (OH) 3 ·2H 2 O, respectively, even though the original mpororoite had a high content of Fe 2 O 3 substituting for Al 2 O 3 . In addition to these two minerals, another tungsten mineral was also found within the scheelite -pseudomorphs. It occurs as aggregates of regular octahedral crystals up to 50 μm in length. The XRD data are in good agreement with those for hydrokenoelsmoreite, but chemical analysis shows that the major components are WO 3 , Al 2 O 3 , and H 2 O with no Fe 2 O 3 . Up to this time, only Fe -containing hydrokenoelsmoreite, once termed ferritungstite according to the old nomenclature, has been widely reported to be found in Japan. This paper is the first to report the occurrence of such an Fe -free hydrokenoelsmoreite in Japan. It is likely that these three secondary minerals of tungsten at Wazuka were formed in an environment where the supply of H 2 O and Al 2 O 3 and the leaching of calcium ions from scheelite took place simultaneously. The source of Al is ascribed to the decomposition of muscovite in the quartz veins.

Conditions and Mechanism of Crystallization of Hydrous W-Fe Oxides with a Pyrochlore-Type of Structure (Elsmoreite Group) in the Oxidation Zone of Ore Deposits

“…Another possible candidate precursor for the formation of elsmoreite group minerals is anthoinite AlWO 3 (OH) 3 . In addition to the formation of pseudomorphs after scheelite, anthoinite often occurs together with hydrokenoelsmoreite (formal alumotungstite and ferritungstite) [2,6,14].…”

“…The use of electron probe microanalysis to study minerals of the elsmoreite group, particularly in articles published in the last 30 years, shows that the Fe/W or (Fe + Al)/W atomic ratios in these minerals are significantly less than unity. If we exclude the unique composition of hydrokenoelmorite □ 2 W 2 O 6 •H 2 O described in [4], then, according to most researchers [2,3,5,[10][11][12][13][14] the ratio (Fe + Al)/W is not strictly fixed and varies within the range of 0.27-0.51.…”

“…There is evidence that the minerals of the elsmoreite group are not formed directly during the destruction of the primary minerals but at a later stage at the expense of earlier alteration products. Among such earlier products of alteration of primary tungsten minerals and precursors of minerals of the elsmoreite group are Fe and Mn containing hydrotungstite WO 3 •2H 2 O replacing hübnerite [11]; anthoinite AlWO 3 (OH) 3 and mpororoite Al 2 O(WO 4 ) 2 •6H 2 O pseudomorphically replacing scheelite [14]; meymacite WO 3 •2H 2 O and Fe-containing meymacite WO 3 •xFe 2 O 3 •nH 2 O replacing scheelite [15,16]. Some data on the possible conditions of crystallization of minerals of the elsmoreite group can be derived from the conditions of synthesis of artificial tungsten pyrochlores.…”