Dear Dr. Hoxie,

your volcanic rock showing a fine-grained grain size and volcanogenic in origin is denominated trachyte or quartz trachyte depending on the exact amount of the main constituents. Sanidinite facies is quite a different story. It is the metamorphic overprinting of a rock-type under the maximum temperature at very shallow depth or a minimum of pressure . It is characterized by minerals like monticellite, melilite, spurrite  or rankite, all of which have calcium as main component in their structure. These rocks are normally found near or included in basic dikes. They do not contain quartz but tridymite instead. Sillimanite is replaced by mullite. Grossularite is substituted for by wollastonite and anorthite due to the high temperature of approx. 800 to 900°C. Only a few minerals such as CPx  present under the highest grade of contact metamorphism may persist under these P-T conditions. Many metapelites tend to melt and partial or complete fusion with glass is common.

I hope it will be of assistance

H.G.Dill

(second entry)

Thank you very much for your reply. The outrcop caught my eye because its very light color stood out against an otherwise dark basaltic terrain. My first thought was granite intrusion? but no the rock is mostly sanidine, amorphous and crystalline, no quartz, a lot of magnetite, and some apparent gas-phase deposition. To me it is a curiosity. Grain size runs from 1 to 3 mm, a little coarse for a volcanic, perhaps. There are no phenocrysts of anything and I decided that it was other than what would be a "typical" trachyte but more of a "micro-syenite" except for its nominally volcanic association/origen. The "sanidinite" is overlain by a black vesicular plagioclase basalt with which it may be associated. No, it is not metamorphic; I took the designation "volcanic sanidinite" from a paper that used the term for a quartz-poor, sanidine-rich volcanic rock from Lacher See, Germany.

Dear Dwight

I propose you check the link below which deals with metamorphic sandinite facies

https://books.google.nl/books?id=IukdhqxEoq8C&pg=PR16&lpg=PR16&dq=metamorphic+sandinite&source=bl&ots=fcjchfCUP-&sig=UdJxqucBhRz8QJkZNDSYCIUZt3g&hl=en&sa=X&ved=0ahUKEwjW-prqs43NAhXTyRoKHY45D-kQ6AEIKDAC#v=onepage&q=metamorphic%20sandinite&f=false

Regards

Massih