Help needed with weird rock texture

[g-21-lto]

Hey, anybody who knows geology...I collected a sample of rock, and I'm trying to figure out why it looks the way it does.

The rock is a gneiss (from northern Georgia, for background) containing bands of quartz and feldspar (not sure if it's plagioclase or potash) alternating with bands of biotite (and some muscovite). One piece I took from a quartz/feldspar band is of a very strange texture.

It appears to be crystals of quartz, some equant and some elongate, in a background of the feldspar. In places the orientation of the quartz is random, but in some parts of the rock, the elongate quartz grains have a slight foliation.

Can anyone tell me what causes this texture and what it may indicate about the nature of the rock? Thanks!

-- g21lto, geo major out of her depth

[hagiograph]

I'm no metamorphic petrologist and its been a looooong time since I was in a mineralogy class, but it might help to see a pic of the rock. Is the difference between the foliated quartz and the non-foliated very striking? Is it a big difference across a small region? Could it reasonably related to differences in pressure across the rock? Is there significant differences in the foliation in the layers above and below the non-aligned quartz?

Is it also possible that your "equant" quartz is not quartz at all? If you have nice prisms of quartz (its usual crystal habit), is it possible the "equant" crystals are not quartz (since equant might make more sense for plagioclase since it is not likely to form prisms so much, different crystal system)? Sometimes it is not easy to differentiate milkier quartz from plag.

A bit more info and maybe a pic would help. But certainly a better petrographer than I should speak up if possible.

-h.

[g-21-lto]

I'm no metamorphic petrologist and its been a looooong time since I was in a mineralogy class, but it might help to see a pic of the rock.
No digital camera :( But thanks.
Is the difference between the foliated quartz and the non-foliated very striking? Is it a big difference across a small region? Could it reasonably related to differences in pressure across the rock? Is there significant differences in the foliation in the layers above and below the non-aligned quartz?
I don't have enough of the rock to see foliation differences in the other mineral bands, but the aligned quartz is in a small region, and there is a marked difference in mineral shape and alignment over a space of several centimeters.

Is it also possible that your "equant" quartz is not quartz at all? If you have nice prisms of quartz (its usual crystal habit), is it possible the "equant" crystals are not quartz (since equant might make more sense for plagioclase since it is not likely to form prisms so much, different crystal system)? Sometimes it is not easy to differentiate milkier quartz from plag.
Hmm, I may have misidentified the quartz, but it is surrounded by a "milky" opaque plagioclase and it is seems more translucent and lustrous -- what I used to identify it. The equant and the more elongate "quartz" appear to be the same mineral -- identical lusters and colors throughout the sample.

A bit more info and maybe a pic would help. But certainly a better petrographer than I should speak up if possible.

-h.
Thank you though! I'm really quite stuck.

[flintknapper]

Granitization?

[llanitedave]

Sounds like a small region of pegmatite, which is where the quartz and feldspar components of a granitic fluid are separated as they precipitate. Some of these can get pretty big.