STILPNOMELANE

Stilpnoimelane is a rare mica mineral. It falls in the brittle mica group and is semi-related to another mica mineral called margarite. The chemical composition of Stilpnomelane is K(Fe,Al)10(Si12O30)(OH)12. It is quite rare to find and it always has been found in sphereoidal sprays like this specimen. The process of formation was light to medium metamorphism of fine grained ocean sediments. When the Pacific Plate was subducted underneath the North American plate the fine grained, silica rich ocean floor sediments were compressed and heated to form the chert. During the heating and compression, the newly forming rock was also inundated with quartz veins that carried in a couple of other elements. Through further metamorphism (heating and pressure) the quartz veins were also converted into quartzite/chert. Finally, the extra potassium, Aluminum and Iron Ions recombined with some of the silica and water molecules to form the stilpnomelane. In fact, that piece was collected from the type locality for stilpnomelane. In other words, it came from the place in which that mineral was first discovered and identified. So, it is a rather excellent specimen, and it is quite unlikely that you will find another one anywhere else.

(Joseph George)

Hardness: 3

Other characteristics.