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Identity HelpCalcite on Prehnite?

1st Aug 2014 20:40 UTCMichael Otto

Are these Calcite crystals on the Prehnite? FOV @ 6cm. Collected at Sawteeth Acres Southbury, Ct Not far from O&G southbury Quarry.

1st Aug 2014 20:44 UTCPhil M. Belley Expert

Could also be apophyllite (tetragonal). If they are trigonal (hard to see in photo), calcite is likely.

1st Aug 2014 21:00 UTCWayne Corwin

Michael

Will they fizz if you put a drop of acid on one?

2nd Aug 2014 00:19 UTCsteven garza

Dear Mike;

You took an EXCELLENT shopt of that specimen, &, because you did, I can DEF say those are apophyllites! The angle was JUST right & one of the xls was just right, also. No need for the acid test, unless you really want to.

OK, enlarge the pic to 200% or so, then, look closely at the uppermost larger xl on the right; you'll notice some cloudy lines, straight as arrows, going perpendicular to likely "c". I'm guessing those inclusion lines are following the cleavage planes; since apophyllite has a basal cleavage plane, that's why I made that ID. I think most of the really tiny xls are apophyllite, also; get one of those (using a sewing needle to remove) to acid test, if you wish.

Your friend, Steve

2nd Aug 2014 12:19 UTCMichael Otto

steven garza Wrote:

-------------------------------------------------------

> Dear Mike;

>

> You took an EXCELLENT shopt of that specimen,

> &, because you did, I can DEF say those are

> apophyllites! The angle was JUST right & one of

> the xls was just right, also. No need for the acid

> test, unless you really want to.

>

> OK, enlarge the pic to 200% or so, then, look

> closely at the uppermost larger xl on the right;

> you'll notice some cloudy lines, straight as

> arrows, going perpendicular to likely "c". I'm

> guessing those inclusion lines are following the

> cleavage planes; since apophyllite has a basal

> cleavage plane, that's why I made that ID. I think

> most of the really tiny xls are apophyllite, also;

> get one of those (using a sewing needle to remove)

> to acid test, if you wish.

>

> Your friend, Steve

That was a very helpful and informative explanation, Steve. Thanks Wayne and Phil for your help. I have the other half of the piece that looks like the same Apophyllite crystals but in a mass. I'll post this other half for positive id from you guys. Had a tough time trying to get a good pic of this one. Thanks for the help.

2nd Aug 2014 12:55 UTCsteven garza

Dear Mike;

Although that last pic would help someone familiar with O & G/other Southbury apophyllite clusters (with some whitened terminations on some of the xls), it really isn't as good as your 1st pic, for ID purposes. The xls are SO close, it's impossible to see a single shape; you were right to show the individual xls, perched on the prehnite - you can see the single xl shapes, easily.

BTW, isn't that site now an upper-scale housing development, now?

Your friend, Steve

2nd Aug 2014 13:07 UTCMichael Otto

Thanks again, Steve. I knew that photo would be tough to id. There's still the acid test. What acid would you use to test? And yes the site is now and has been for a long time an upscale development.

2nd Aug 2014 13:25 UTCsteven garza

Dear Mike;

I was there, early on, when good amethyst was being found. The best acid would be vinegar (acetic), even at only 5%, but, if you want DEF results, get muriatic (hydrochloric) acid from a big hardware store (Home Depot/Lowes) &, using an eyedropper drip into a container with testing piece, watch after drip comes in contact with that bit. If calcite, it would fizz like crazy; if apophyllite, nothing visible will happen, but, if left there, over several hrs, the piece will dissolve away or gel. Make it a TINY xl; don't want to mess up that piece too much!

Your friend, Steve

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