My once and future Honors student Robin R. brought back some sweet wavellite [Al3(PO4)2(OH,F)3•5(H2O)] samples from her holiday travels*. Check out these beautiful radiating crystals! A penny will serve as your sense of scale.
The wavellite is a “crust” on top of a layered rock that appears to be a quartzite. (The layering in the quartzite is cross-cut by the wavellite layer.) It looks like the host rock fractured, then wavellite crystals grew in the fracture gap, radiating out from point nuclei, and meeting along fairly crisp edges. Ultimately, this team of wavellite crystals sealed the crack shut as a vein. When the rock was stressed again, it broke along the weak discontinuity between the wavellite vein and the host rock, baring their radial structure for all to see.
Seriously cool, right? Thanks to Robin for letting me photograph this pretty rock and post the images here. * She bought this from a rock shop in Amsterdam.
Small correction: it wasn’t a rock shop, but a sort of everything shop in the Albert Cuyp market. On the off chance that any readers will be in Amsterdam any time soon: look for a stall/shop that sells spices, bath salts, tourist tchotchke, kitchen implements, and other random things. I snagged the only wavellite, but there was a lot of nice zoisite, a pretty hunk of native copper, a few cases of small but quality minerals, and some other good stuff lurking near the back of the shop.