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[marco campos-venuti]

Hi all,
I would like to show you an alteration trend I noted in feldspar crystal from a granitic rock.
There is a subvolcanic porphyritic granodiorite rock in Elba Island (Italy) of the same composition of the famous pluton that is rich in pegmatite veins that have given the name to elbaite and many other minerals.
This rock is disseminated by big feldspar porphyritic crystals up to 15-20 cm long.
NEAR SEASHORE
It is interesting to note that near the sea shore, the alteration of the granodiorite crumbles the granodiorite leaving the feldspars in relief. It was very easy for collectors to remove the feldspar crystals from the host rock, and now they are quite rare to find in good relief.
IN THE HILL
Far from the sea no crystals are found isolated from the matrix. In this case on the contrary, is not rare the case that the feldspar crystal is completely altered and leave its void cast.
I suggest that salt alteration has a physical impact to crumble the granular fraction of the granodiorite, but acid alteration in soil has a chemical impact on the feldspar alteration.
What do you think?

[CirrusClouds]

Very interesting photos! Feldspars are an interesting mineral. Would the large crystal growths be considered Phenocrystals?

[marco campos-venuti]

Of course they are Phenocrystals. It is a kind of subvolcanic rock related to the main pluton and it had time to crystallize.