quartz, inclusions, gems

This short communication describes an association of helvite, tourmaline and fine grunerite fibres enclosed in rock crystal from Governador Valadares, Brazil, and extends an earlier account of helvite inclusions in tourmalinated quartz. Prompted by P.J. Dunn’s 1975 description, the author re-examined cut quartz specimens that had remained unidentified in his collection for approximately twenty years. Their conspicuous greenish-yellow tetrahedral inclusions closely matched Dunn’s description of helvite. Microscopic examination also revealed dark tourmaline rods and fine fibrous or acicular inclusions, producing a distinctive association of three minerals within the transparent quartz host. A photomicrograph at 50× illustrates the contrasting forms of the yellow helvite tetrahedra, dark tourmaline crystals and slender fibres. Particular attention is directed to identifying the fine filaments, some of which were sufficiently large and exposed at polished surfaces to permit electron-microprobe examination without first extracting them from the quartz. Fibres in four specimens yielded iron, aluminium and silicon, together with minor magnesium and manganese; calcium was not detected. The author considers alternative silicate interpretations, noting that the elemental combination alone could be compatible with more than one mineral group. The low magnesium content and presence of manganese favour an iron-rich amphibole, leading to identification of the needles as grunerite. The accompanying manganese-bearing helvite provides additional context for the occurrence of manganese in the fibrous inclusions. Grunerite is characterized as an amphibole dominated by iron, with relatively little magnesium and aluminium and commonly some manganese. Its colour may range from colourless through green to brown, and it can form silky, fibrous and divergent aggregates resembling asbestos. The author notes that grunerite is not an uncommon inclusion in rock crystal. The paper thus combines renewed microscopic observation of older collection specimens with elemental analysis of accessible inclusions, resolving the nature of the fine fibres and adding detail to the previously reported helvite–tourmaline association. The identification rests on mineral morphology and qualitative chemical evidence rather than a reported crystallographic determination.