Clinoptilolite and Heulandite from Wheal Hazard, St Just, Cornwall.

Quantitative chemical analyses of a number of millimetre-sized crystals detached from specimens labelled 'heulandite' from Wheal Hazard, St Just, Cornwall show that most of their compositions lie within the closely related clinoptilolite series of minerals: specifically, potassium- and magnesium-rich clinoptilolite-Ca. An average of 28 analyses produces the chemical formula: (Ca 1.87 ,Ku 1.31 ,Mg 0.86 ,Na 0.12 ,Sr 0.07 ,Ba 0.04 ,)[AI 7.22 Si 28.81 O 72 ].nH 2 0. The crystals occur in fine grained metasedimentary rocks of the Mylor Slate Formation, in fractures that post-date the development of the metamorphic fabric. It seems likely that the crystals developed as a result of hydrothermal alteration of the rock during the final stages of the cooling of the Land's End Granite. On the basis of our analyses it is unsafe to label specimens from the well-known 'heulandite' locality at Wheal Hazard as heulandite.