Identity HelpSanidine? Or Oligoclase? Or Onorthite?
I've collected this material up near the top of Pacaya Volcano, near Guatemala City, Guatemala, for a while now.
Nice big crystals up to an inch or two in size.
I have always thought the crystals were Sanidine.
I just read that Sanidine typically forms in Felsic rocks and not Mafic, however. So now I'm not so sure. The only associated mineral I have found on the matrix tends to be Olivine-type minerals.
The internet says that feldspar is common in many volcanoes, ranging from Albite-Onorthite, to Oligoclase, and Sanidine.
Any opinion on what this could be? Would you consider it to be Sanidine? Or is another option more likely?
I'm not sure that you could tell which of the albite-anorthite series this might be without analysis. You can label it Albite-Anorthite Series of simply label it Plagioclase (the synonym of Albite-Anorthite Series). Without analysis that is about the best you are going to do.
These are found everywhere in the Columbia River Basalts where I often collect. I think the proper name for them is phenocryst, rather than crystal, unless they are growing within a vesicle (void) in the basalt. Someone will correct me if that isn't right. 
I have seen phenocrysts of both sanidine-anorthoclase and plagioclase in similar looking basalts elsewhere, hard to tell without an analysis or Petrography. Any geological reports available on the area? All I know from the local geology is from the Global Volcanism Program:
Major Rocks are: Basalt / Picro-Basalt, Dacite, Rhyolite, Andesite / Balsatic Andesite
Minor Rocks are: Trachyandesite and Balsatic Trachyandesite
