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March-April 2012

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Mineral Mysteries: Quartz Concentrated Around Altered Pyrite

As I am running low on topics for Let's Get It Right at the moment, I have long considered starting another series of columns for this journal, one that deals with observations of peculiarities relating to minerals—phenomena that are baffling to me and others, and for which there appears to be no reasonable explanation. Perhaps by exposing these to a wide audience they will reach someone who can adequately explain them. I am also hopeful that readers will propose other examples that can be included in this series. This does not mean that there will be no more Let's Get It Right columns; when new topics occur to me, I will write about them.

The introductory “mystery” is, I believe, an excellent one with which to begin the series. If everything that we observe to occur around a complex pegmatite during its crystallization appears complicated, look at what can take place in the vicinity of a pyrite crystal in phyllite when it is attacked by groundwater that has penetrated the matrix in which the pyrite originally grew. This mystery relates to samples of pyrite that I recently collected in a fresh road cut just south of York, Pennsylvania, less than 20 miles from my home. The rock that has been exposed there is part of the Harpers Formation, a very early Cambrian, in part very late Precambrian, unit consisting locally of phyllite and schist. There are zones within the phyllite, which is what is exposed near York, that are relatively abundant in pyrite crystals, simple cubes with no modifying forms. Some of these are quite large and make excellent specimens, such as that seen in figure 1. This piece was collected in an earlier road cut in the same area in 1968 by a friend who subsequently gave it to me. I did not find any of this size when I collected there in 2008.

John S. White, a consulting editor of Rocks & Minerals, operates Kustos, a museum/collector consulting business.

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