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

I totally support Harald. Sanadinite facies is used for metamorphic assemblages developed at highT-lowP conditions. In other words, it is a space on P-T grid. I have never heard of sanidinite rock, although the possibility of clustering of sanidine into small pockets in trachyte or rhyolite magmas cannot be ruled out.  To my knowledge, no magma of sanidine composition has been reported.

Thank you! I am not sure what I have except sanidine is the is the principal mineral and only visible feldspar in this body of rock in a volcanic terrain. This "volcanic sanidinite" appears to be associated with the overlying basalt. Insofar as terminology I took "volcanic sanidinite" from Assbichler et al. "HFS Minerals in a Volcanic Sanidinite from a Gaseous Phase" Goldschmidt Abstracts 2015, 133. There is site about 50 km from my site of a basalt with very conspicuous sanidine inclusions to the extent that the ground is littered with transparent light yellow sanidine!

Dear colleagues,

I would like to draw your attention to a sanidine-bearing diamontiferous igneous rock in Mongolia. Sanidine is a common constituent in the Tertiary to Quaternary volcanic rocks and recovered as a by-product besides garnet and olivine: The economic figures for garnet are 1764 gr/m3, peridot 80 gr/m3 and for sanidine 4 gr/m3. The gemstones are accommodated by dark tuffaceous breccias representative of the diatreme pipe facies. Olivine, plagioclase, clinopyroxene and magnetite are rather fine-grained. Accessory minerals are ilmenite, apatite, zircon, chromium spinel, graphite. Inclusions may reach a size of as much as 3 cm and contain garnet, pyroxene, olivine, biotite and sanidine. The host rock was identified as a lamproite rather  than a kimberlite. The overall features of the basic source rocks are moderate trace element contents of Cr and Ni, presence of sanidine instead of leucite and a K2O/Na2O of far below 5.

You can dowload the paper from my list of publications from the RG server:

DILL H.G. , KHISHIGSUREN S., MAJIGSUREN Yo. BULGAMAA J., HONGOR O. and HOFMEISTER, W. (2004) The diamondiferous peridote (olivine)-garnet deposit Shavryn Tsaram, Central Mongolia, with special reference to its placer deposits.- Gemmologie, 53: 87-104.

I wish you much success

H.G.Dill

Thank you! If only my sanidine site here Mexico produced a few diamonds - they would greatly aid my meagre research budget!

 Dr.Wight Hoxie

You mentioned that sanidine is the principal mineral. What are the other (accessory) minerals?  Can you provide a photomicrograph of the rock under PPL and Xed nicols? The heat required for the formation of sanidine probably was provided by the overlying basalt.  What lies under the "sanidnite" bearing rock? Dr.K.N.Rao

Thank you for your interest and questions. I don't have access to a petrographic microscope but the principal secondary mineral appears to be magnetite with 10% or more by weight of the rock mass composed of magnetic material. Besides sanidine magnetite is the only other crystalline mineral that I have found in the samples that I've examined. The overlying basalt also is highly magnetic with plagioclase as the principal mineral.