Friday, April 8, 2011

Recent Hike: A Feldspar Twin

On a not-so-recent hike, when snow had turned to mud (mud then dried and turned back to deep snow by yesterday), this small and rather unprepossessing rock caught my eye. Perhaps you're wondering why.
One side was glinting at me in the low afternoon light. I reached down and grabbed it, immediately recognizing it as a moderately sizable, though incomplete, orthoclase feldspar, one type of K-spar, with a Carlsbad twin. You can see this type of twinning in a sanidine crystal (sanidine is a related type of K-spar), and read more about K-spars at Life in Plane Light.
A view looking end on at the visible twin plane (above), and with light reflecting off the upper twin (below).
Two views with alternate twins highlighted by the sun, above and below.
I recognized this feldspar as being from nearby intrusive rocks that are often porphyrytic, and which usually contain large phenocrysts of dull pink K-feldspar like this one. Having never seen any exposures of porphyry dikes or masses on our hiking hill, I wonder if this feldspar has eroded from old alluvial fan material that formed from sources now eroded or sources farther up the main canyon than our side of the hill. Either way, it makes me want to look for unmapped porphyry.


Dana Hunter said...

See? This is why I love geology! It's not just a plain ol' rock with a little bit of shine, it's a rock with a story, and a bit o' mystery. Fantastic!

Thanks for this. Hope you get to explore out there!

David Orr said...

Seconded. Good luck in your earching!

Lockwood said...

That gif is very nicely done, then the following pics really help emphasize the idea. This idea about cleavage planes was one I had a hard time really getting; not sure if it was just me or if others did too.

Gaelyn said...

Certainly not a leverite. How did you do that cool pic in motion? Nothing dull about geology.

Silver Fox said...

@Dana I think I'll wait until the snow melts off - again!

@David Thanks! :)

Silver Fox said...

@Lockwood Those aren't really cleavage planes but twin planes of separate, side-by-side crystals. I think it's better explained by Elli at Life in Plane Light. Cleavage would be planes along which the crystal would break, usually those are closely spaced. I think you can see cleavage in the GIF on the larger twin: several planes running almost 90 degrees to the twin plane. It looks like a kind of blocky cleavage, because I can see some other lines of breakage at almost 90 degrees to that, subparallel to the twin line. I could be wrong about the cleavage.

Silver Fox said...

@Gaelyn If you click on the animated GIF, it will take you to one of several sites that you can upload photos to, then they make the animation, in this case from three photos. I didn't really plan on making a GIF when I took the photos - now I'll keep the possibility in mind!