so about this whole planet thing

Feldspar wrapped in feldspar This is a texture found in a few granites around the world that is cool enough I bet some people out there have it in their countertops (if anyone stops at the McDonalds in Mojave, California, take a hand lens with you into the restroom, the countertops are great!) These are Rapakivi Granites. The distinguishing feature of this type of granite is the big, several-cmsized spherical grains surrounded by thinner sheaths of a lighter-colored mineral. The main mineral in the cores of those spherical grains is a K-feldspar, usually orthoclase, and the lighter, often colorless mineral surrounding it is usually plagioclase.  Both of those minerals are common in granites but the pattern is very unusual. According to Bowen’s reaction series, K-feldspar minerals form after plagioclase, but in this texture, the K-feldspar forms the core of the grains and the plagioclase forms rims around it, suggesting that the plagioclase grew after the K-Feldspar formed. One hint at how these form is that the edges of the big K-feldspar grains are rounded into spheres; usually minerals grow in the shapes of crystals but they become rounded if they are re-melted or eroded. This texture is created by a specific sequence of events. A granitic magma forms big crystals of K-Feldspar as it cools, but then something happens to the magma. Either it mixes with another batch of hotter magma, which causes some of the K-Feldspar to remeltand plagioclase to begin crystallizing, or a the magma interacts with a fluid which moves some elements around and changes which minerals form. Rapakivi granites are really cool when they’re found but they require these rare conditions to form. The most well-known outcrops of them are probably in Finland, where they are quarried for things like countertops. JBB Image credits: Joy de Lanoy http://www.flickr.com/photos/32412693@N00/3183933030/ http://commons.wikimedia.org/wiki/File:Rapakivi_granite.jpg Readings; http://petrology.oxfordjournals.org/content/35/4/963.abstract http://www.sandatlas.org/2011/11/rapakivi-rotten-stone/

May Lake, Yosemite National Park by AGrinberg on Flickr.

Looking at this picture isn't illegal, right?

basaltic dyke in a granitic rocks.