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Identity HelpHydroxylherderite? Adularia? with tourmaline from Skardu, Pakistan

9th Jan 2020 18:35 UTCNick Gilly

Hi all.

Another new specimen arrived today, which was sold as herderite+tourmaline+mica, from Skardu, Pakistan. Now the last two are definitely correct (some really nice terminated green elbaite pencils in there), but I'm not so sure about the 'herderite'. This is in the form of many colourless transparent wedge-shaped crystals that cover many areas of the specimen. I tried a hardness test, and they do scratch glass with difficulty, which is similar to the behaviour of my known herderite specimen from Brazil. However, I'm wondering if they could be adularia crystals. The shape looks like it could be either. Hopefully someone familiar with the minerals from the pegmatites in Skardu/Pakistan might recognise this.

Looking at pics of hydroxylherderite from Pakistan, many of the specimens on Mindat show green crystals, although there are one or two colourless examples that look similar to the crystals on my specimen. None of the hydroxylherderite or adularia specimens on Mindat show an association with tourmaline though. The approximate size is 40 mm by 30 mm by 25 mm, and the weight is about 26 grams.

Whatever it is, I can't complain as this really is a beautiful specimen, which only cost $9.99 before shipping.

Pics now follow (taken from the auction). Thanks for any help with this.

9th Jan 2020 18:37 UTCNick Gilly

Photo 2.

9th Jan 2020 18:37 UTCNick Gilly

Photo 3.

9th Jan 2020 18:38 UTCNick Gilly

Photo 4.

9th Jan 2020 18:39 UTCNick Gilly

Photo 5.

9th Jan 2020 18:39 UTCNick Gilly

Photo 6.

9th Jan 2020 18:40 UTCNick Gilly

Photo 7 (last one).

9th Jan 2020 20:01 UTCNicholas J. Rose

The larger crystal in the upper left corner of your first photo definitely looks like Hydroxyl-Herderite. The small faces on the left and right corners of the crystal is commonly seen in herderites from California, Brazil, and Pakistan. I assume that most of the other crystals are also Hydroxyl-Herderite. Neat specimen - especially for the price.

9th Jan 2020 19:52 UTCFrank K. Mazdab 🌟 Manager

Adularia is just a morphological variant of K-feldspar that's pretty typical of the formation conditions present in some vein environments; in a pegmatite, I'd probably expect more typical perthitic microcline/orthoclase as the more likely "type" of K-feldspar present.

If you buy a lot of unknown or suspect minerals online and don't have easy access to a SEM/microprobe to ID them for you, I'd perhaps also suggest you may want to invest in a copy of Frederick Pough's "A Field Guide to Rocks and Minerals", which offers a variety of relatively easy-to-do "old-school" blowpipe and chemical tests on common and semi-common minerals. Small quantities of the more useful chemicals and the fairly basic equipment are also likely available on eBay, and with a bit of practice on known materials are not especially difficult to do.

Under tests for herderite, for example, Pough notes:

"Thermoluminescent, glowing briefly with a blue-white phosphorescence on the charcoal just before become incandescent. After light heating (enough to only crack and slightly whiten the specimen) it is usually fluorescent in longwave UV light. Fuses with difficulty, becoming white and opaque. Dissolves slowly in acid, giving the phosphorus test [a yellow precipitate forms when a few drops of a nitric acid solution of a phosphate are added to a solution of ammonium molybdate]".

If positive, however, whether you have herderite or hydroxylherderite is unfortunately beyond the scope of simple at-home tests, most especially if you're near the composition boundary.

9th Jan 2020 20:09 UTCNick Gilly

Herderite-Hydroxylherderite Series

Yuno high mines, Shigar Valley, Shigar District, Gilgit-Baltistan, Pakistan

Thanks guys. That is even better if it is hydroxylherderite.

In fact the wedge shape of the larger crystal on the first pic is indeed a dead ringer for a hydroxylherderite posted by Mark Wrigley (see above). It's not mentioned but it looks like there are a couple of small green tourmaline crystals attached too, so perhaps it's from the same deposit.

Frank that sounds like an interesting book to get hold of, but that test is a bit destructive for a lovely specimen like mine, lol.

9th Jan 2020 20:16 UTCFrank K. Mazdab 🌟 Manager

Hi Nick,

Certainly in some cases agreed, but I based my suggestion of chemical testing on chipping off a small discreet crystal from the hidden back of your specimen that you described as:

many colourless transparent wedge-shaped crystals that cover many areas of the specimen

Presumably you must have found a similar discreet hidden back corner to do your scratch (hardness) test... hopefully you didn't try to scratch the big central showy crystal, right? right? lol

9th Jan 2020 21:54 UTCJosé Zendrera 🌟 Manager

Another vote for hydroxylherderite.
Could be from many localities: Shengus and Stak Nala in the Indus valley or anywhere in the upper Braldu valley, the only localities near Skardu where green tourmaline is found, aside of hydroxylherderite.
All of them belong to the old Skardu district but today divided in four new districts still not updated in Mindat.

9th Jan 2020 22:47 UTCAndrew Debnam 🌟

Agree Jose, it certainly looks some specimens I have seen from Stak Nala. For ten dollars why not take a chance if you do not have a piece with Tourmaline and

hydroxylherderite as long as shipping is only a few dollars

9th Jan 2020 23:03 UTCNick Gilly

Thanks again guys. Stak Nala definitely looks like the most likely of the sites mentioned as there are plenty of pics of the bicolour green elbaite from there. I've not found any pics online associated with hydroxylherderite yet, but it appears that they are out there.

Shipping to the UK was only $5 so a very nice find. I don't know if the shipping cost to the US would be the same or not.

10th Jan 2020 02:40 UTCAndrew Debnam 🌟

a piece that size regular Pakistan post is about 8-10 dollars to Canada depending on the province. So cheaper than our postal system.

4th Mar 2020 15:51 UTCNick Gilly

Attached are a couple of photos (from the seller) of a new, larger specimen that arrived here today. The colourless hydroxylherderite covers the top area of the specimen with groups of parallel, fibrous-looking (etched?) tourmaline prisms on the side. Dimensions are approx. 65mm by 40 mm by 35 mm, and the weight is about 147 grams.

Some very interesting specimens coming out of this location.

4th Mar 2020 15:52 UTCNick Gilly

2nd photo showing the main tourmaline group.

4th Mar 2020 22:24 UTCNick Gilly

Would my idea about etching be correct? I've attached a photo (not very good in this light) of the remnants of a tourmaline prism that is now hollow. It's the roundish shape dead centre. Did it grow like that or has the middle been dissolved away? I'll have to see if I can take some better pics at the weekend if we get some sunny spells.

There is a group of tourmalines nearby with partial prisms, with the interiors coated in a buff/greyish powder. Maybe something similar was going on here too.

5th Mar 2020 13:31 UTCHarold Moritz 🌟 Expert

I'd vote for hydroxylherderite. Herderite is super-rare (early finds have been shown to be the hydroxl kind - much like nearly all "amblygonite" has been found to be montebrasite), so Pough's test should be valid. Adularia does occur in pegmatites, but as a late secondary mineral associated with Be secondaries like bertrandite (along with secondary albite and muscovite), all fine-grained and easily overlooked, so I doubt that's what you have here. Hydroxlherderite is a secondary phosphate, so you need a primary phosphate source in the peg, like triphyllite or lithiophilite, and some beryl, and then both need to be altered by late pegmatite solutions so the required elements can react and precipitate hydroxylherderite. It can be any color, that comes from impurities. But here's a tidbit from its mindat.org page:

May be blue-green or blue in daylight, lavender or light violet in incandescent light

Interesting that at Stak Nala there is another secondary phosphate, eosphorite, but no primary. Bertrandite and beryl are present.

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