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Identity HelpCalcite ID in stones in Santa Rosa CA creek

26th Mar 2024 23:57 UTCPaul Hernday

07134050017114967078298.jpg
The 6 rocks arranged in a loop in this photo all have white deposits that fizz vigorously in 10% HCl. Also most of the white-spotted vessiculated lava that I tested also fizzed at the white spots. 
I'm finding that my hunch is pretty good at visually distinguishing calcite from quartzite prior to the acid test, but I'm not sure why. Maybe it's the context of the type or shape of rock, or the flatter white of the calcite or ??? Any signs others have noticed?  


Questions Answered
Can it scratch glass? : No
Can you scratch it with your fingernail? : No
Is it light/heavy for the size? : Normal

27th Mar 2024 01:20 UTCJohn Christian

Calcite veins are often below the level of the rock that it cuts because calcite is softer and it dissolves in rain. Quartz veins sometimes are higher than the surrounding rock because quartz is harder.

27th Mar 2024 03:12 UTCPaul Hernday

Thanks!

27th Mar 2024 05:21 UTCHenry Raab

not sure what it is but i find identical stuff up in san mateo county

27th Mar 2024 06:06 UTCJohn Christian

There are lots of calcite veins especially in the Franciscan Group rocks which were tortured and fractured as they were scraped off the descending plate and stuffed down the subduction trench. The fractures were filled with calcite and quartz. 

Newer rocks were fractured by the earthquakes caused by the plate motion and then filled with calcite.

See if the calcite veins are fluorescent.

27th Mar 2024 13:57 UTCEd Clopton 🌟 Expert

This is a technicality, but we love technicalities!  Quartz and quartzite are two different things:  Quartz is a mineral; quartzite is a metamorphic rock consisting mostly of the mineral quartz that started out as sandstone, which in turn started out as quartz sand.  Thin, intersecting veins in rock are far more likely to be quartz than quartzite; quartzite mostly occurs in thick, parallel layers or complete stratigraphic units.  And, in your case, the veins appear to be neither one but are calcite instead, as you figured out.

29th Mar 2024 14:38 UTCGregg Little 🌟

Ed Clopton 🌟 Expert  ✉️

mineral quartz that started out as sandstone, which in turn started out as quartz sand.
 Ed; I couldn't resist going further down your technical rabbit hole so do forgive me.  The word "sandstone" in this case should be replaced with "quartz arenite".  The term sandstone is actually a detrital rock grain-size term (1/16 to 2 mm) where the clasts could be any mineral or lithic fragment. Virtually everyone assumes it is mineralogically a quartz sand which is usually accurate, sort of by default as quartz being highly resistant usually is the last mineral standing in the erosional processes.

29th Mar 2024 15:52 UTCEd Clopton 🌟 Expert

Thanks for the clarification.  I'm aware that sandstones can consist of various minerals, which is why I specified quartz sandstone.  I've heard of arenite, but it's been a while and it didn't spring to mind.

Arenite must derive from the Latin arena for sand, so arenite = sand+stone.  Cool!  So "quartz sandstone" wasn't far off!

30th Mar 2024 03:47 UTCGregg Little 🌟

Right on Ed.  You did specify quartz at several points so you certainly clarified it sufficiently by emphasizing the parent material in the genesis.  A good teaching moment for differentiating quartz verses quartzite.

Cheers, Gregg 

27th Mar 2024 15:41 UTCKyle Beucke 🌟

Paul,

The rock on the lower-right may be "greenstone," i.e., seafloor basalt that was hydrothermally altered (while it was still on the seafloor?). 

Breaking some of these open would help in a rock ID as well as seeing the vein textures more clearly (e.g., avoiding effects of surface weathering).

Kyle 

27th Mar 2024 17:12 UTCPaul Hernday

Thanks all, this thread has been really helpful. -Paul
 
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