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Anorthite and Native Brass

Last Updated: 21st Oct 2010

By Alfredo Petrov

ANORTHITE and NATIVE BRASS from MIYAKE-JIMA, TOKYO
by Alfredo Petrov, July 2010

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Public transportation to Miyake arrives in the rain.
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Accomodations on board.


Miyake-jima, an 8 km (5 miles) diameter active volcanic island located in the Pacific Ocean about 200 km south of Tokyo, is considered to be the classic locality for anorthite feldspar crystals of almost end-member composition (generally 96 to 97% Ca-plagioclase and only 2 to 4% Na-plagioclase). Millions of these "floater" crystals, usually 1 to 2cm in size and rarely exceeding 2.5cm, all coated with a thin skin of black, brown or brick red lava, were blasted up into the skies over Miyake as mini volcanic bombs (Don't call them that when going through airport security!) during the July 1874 eruption of Oyama volcano. The rest of the world did not hear about these little marvels until 15 years after the eruption, when Japanese mineralogist Y. Kikuchi reported them in the literature in 1889. Within a few years, in the late 19th and early 20th centuries, many hundreds of them were distributed to museums and systematic collectors around the world.

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Typical Miyake landscape: old lava flow, partially forested.


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Forest killed by acid gas cloud in 2000. Compare this new shrine gate with the old one in the next photo...


Later decades saw the mineral collecting hobby increasingly competitively driven toward the colorful and spectacular, and these little aesthetically challenged feldspars fell into obscurity until, in the 1980s, dedicated Japanese field collector Shinsuke Goto took his tent over to the island and spent two weeks hunting for anorthite crystals. In addition to floater crystals as specimens, Goto-san was also interested in the possibility of using them for lapidary work. Over the course of the two weeks, he managed to collect about 10 Kg of crystals, some of them perhaps cabochon quality, but he noticed that a few (less than 1%) had gemmy interiors and would be good for faceting. And a small proportion of those, only about a half dozen crystals in all, were pink to deep red inside, sometimes with a metallic schiller! Goto-san provided some of the red material to researchers in Japan, and had a few facetted stones cut from the rest, which of course were highly prized as great rarities by the rare gem collector community in Japan. (The only foreigner who ever managed to wangle one such red gem away from the Japanese was Dr. Fred Pough (poughite), already retired curator of the American Museum of Natural History.) A fabulous uncut 2cm deep red crystal is on display at the National Science Museum in Tokyo (specimen #165 in the Japan mineral room).

I met Goto-san at the Osaka show in 1994 and he showed me the one facetted red anorthite that he'd kept for his personal collection. That same year, coincidentally, a research paper was published on the metallic inclusions causing the rare red color in anorthites from Miyake and the nearby island of Hachijojima (Nishida et al (1994) Native zinc, copper, and brass in the red-clouded anorthite megacrysts as probes of the arc-magmatic process. Naturwissenschaften, 81, 498-502). Naturally I felt inspired to get on a ship right away and hunt for this material myself, but didn't get around to planning it until the year 2000, at which point (Murphy's Law!) the trip had to be indefinitely postponed because of a catastrophic disaster on Miyake.

In July 2000, Oyama volcano on Miyake started erupting again. Normally no big deal on an island where the inhabitants are accustomed to periodic ash falls and lava flows, but this time there was a difference: an enormous sustained belching of massive quantities of sulphur dioxide gas, with lesser amounts of H2S. All 3,000 inhabitants were evacuated, miraculously without the loss of a single human life, but sadly all the wild fauna (except birds), and all the farm animals and pets, and even many of the trees, died in the gas. It is touching to see the little shrines, still tended with flowers and offerings, to the souls of annihilated cows and pets. The Miyakeans were initially housed in schools and other public shelters in the city of Tokyo. Then, when it became clear that the gas eruption was not going to end anytime soon, they found apartments in Tokyo and had to learn a new way of life. The only long-term American resident of Miyake, Jack Moyer, an ecologist who had devoted his life to protecting the rare wildlife there, suicided in his Tokyo apartment, suffering from ill health and perhaps also depressed that Nature herself can be more brutal than human rapaciousness.

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Shinto shrine gate buried by the 2000 ash fall.
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Sign explaining the fate of this shrine gate.


In 2005, the gas eruptions diminished enough that the population started to trickle back home, ferry service and flights recommenced on a sporadic basis, gas and weather allowing. Electronic SO2 monitors were set up all around the island, with sirens warning the public when to put on their gas masks. Gas masks are occasionally still necessary, depending on wind direction and the volcano's mood. Real time gas updates can be checked on http://www.miyake-so2.jp

Currently, over 80% of the previous population have moved back to Miyake, although many homes on the east side of the island - which is most frequently the downwind side - are still abandoned. The economy of the island now depends mainly on weekend invasions by sports fishermen from Tokyo, weather and gas permitting, and (like most of rural Japan) on heavily subsidized make-work construction projects. Agriculture is still moribund, perhaps because no one wants to invest money with the insecurity of potential future gas eruptions and no one makes gas masks for chickens and cattle.

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Shredding my shoes on a 1983 scoria flow.
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Remains of automobile fossilized in 1983 lava flow.


In 2009 Dr. Carl Francis of Harvard University reminded me that Miyake anorthites were classic research material of which Harvard had had plenty, a long time ago, but the supply was now greatly shrunken by a century of research requests and in need of replenishment. And I had not forgotten those elusive native Cu-Zn alloys. At the same time, Japanese rare gemstone collector Yuko Tanaka, also well known as an intrepid cutter of the most difficult-to-facet gems, was desiring the extremely rare red anorthite gem rough. Since it was now possible to visit Miyake island again, we decided to join forces and hunt at the source. Together with a couple of her assistants, Tanaka-san and I set off for Miyake on a cold rainy and windy week in December 2009. The first part of the voyage goes through Tokyo Bay, a sheltered body of water that was greatly missed after the ship got out into the more than 3 meter waves of the so-called "Pacific" Ocean. (Don't believe the english mineral references that erroneously place Miyake-jima "in Tokyo Bay". It's far outside the bay, as one fully realizes after staggering to the bathroom a couple times to throw up.)

After arriving on Miyake and recovering from seasickness, we set off to study the local geology. In my dreams I had imagined thousands of sharp floaters sprinkled over the landscape, remaining just as they were after exploding out of the ground in 1874, waiting for us to merely bend down and pick them up. The reality of course was quite different and much more difficult. Lava flows and ash beds of many different ages stream down from the summit caldera, which is currently 1.6 km (1 mile) in diameter and 600 meters deep, and from parasitic cones and fissures on the flanks, in a chaotic patchwork. The first lava beds we checked out had only millimetric feldspar phenocrysts. Obviously there was much variability in the size of feldspar crystals in different eruptions. We decided it would be best to concentrate on the 1874 eruption, that being the one that produced the well-known classic specimens. The 1874 lava flow itself was easy to locate, being clearly marked on the geological map, but as we trampled over it we realized that the sparse larger phenocrysts in it were frozen in the hard solid basalt from which we couldn't remove them without shattering them into cleavage fragments. We would need to find the 1874 ash, not the lava, in order to find the famous floaters. And the ash wasn't on top of that lava. Think about it: Lava flows along the nearest downward sloping depression; ash goes wherever the wind pushes it. Picture shaking a bottle of coke and tipping it out on a windy day: the liquid from the bottom of the bottle falls at your feet, the foam blows away in the wind. That's what volcanic "ash" really is - nothing "burnt", just the foam from a pressurized magma chamber after the top of its "bottle" is opened. July 1874 weather reports, please, anyone? To complicate matters further, much of the 1874 ash beds are now buried under younger ash layers from more recent eruptions. And even where the 1874 ash is exposed, one has to dig below the topsoil to find decent specimens, because 135 years of weathering have not been kind to those floaters exposed to the humic acids of the soil horizon.

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Lone pine tree struggles to colonize 1962 ash.


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1874 lava exposed from under its overburden of younger ash by a stream channel.


The first day passed just finding out where the 1874 ash went. Another day was spent sampling different parts of it with test holes to find one horizon that we settled on as looking the most promising. Then we had a day and a half left to do some serious "mining" which, for ecological reasons, we limited to a couple of construction sites where the ground surface was being destroyed anyway. Final production came to a couple hundred ordinary crystals, and a few tens of nice floaters with various twinning habits, a half dozen or so gemmy colorless pieces for facetting, and... not even one single red one.

Not wanting to give up, we went back to the same area in early May 2010 and poked around some more, spending 3 days sifting through tons of ash, with results pretty much identical to those of our first expedition as far as feldspars were concerned, although we were also happy to find a couple of olivine phenocrysts far larger than any previously reported from Miyake. We regretfully came to the conclusion that none of the red anorthites with copper-zinc alloy inclusions had been spat out of the volcano during the mineralogically famous 1874 eruption and they must have formed at a different period of the volcano's history.

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Tanaka-san ascends an almost vertical mud slope to check out old diggings in 1874 ash.


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Winnie-the-Pooh herself digs a sampling hole in promising ash layer.



We were satisfied now with our study of the 1874 ash, and the fresh supply of anorthite floaters and colorless cutting rough acquired, but still not wanting to give up on finding the red ones. So back to questioning Mr. Goto on how he'd discovered his tiny quantity of red ones. He explained that the red ones were not found as loose floaters in ash but rather inside the centers of larger volcanic bombs - blobs of hot plastic lava that take on a variety of elliptical, aerodynamic, coprolitic or ribbon-like shapes as they cool and harden while flying through the air. He had had to smash these bombs with a hammer to find the red ones occasionally found in the centers, like peach seeds.

So, armed with more geological clues, we undertook our third trip to Miyake in mid-June 2010, with calmer seas but hot muggy weather on the island. Thanks to Goto-san's help, we found his original locality for the anorthite-bearing volcanic bombs. I won't bore my readers with the details of our geological speculations, but Tanaka-san proved to have a "sixth sense" for geological interpretation, and an inexplicable ability to divine where sharp crystals were hiding. We concluded that these were xenocrysts that had originated in some prehistoric lava flow, dragged up from the depths into younger volcanic bombs. The inhabitants of Miyake have detailed records of eruptions going back almost 1,000 years, to the eruption of the year 1085 in our western calendar. But almost half the island is composed of lava flows more ancient than that, of unknown age, and it is in one of these that we finally found the red anorthites in situ. As with the 1874 floaters, here too one sees that anorthite is easily wetted by lava, so it is perhaps impossible to completely separate a crystal from its enclosing opaque lava shell. Consequently, crystals need to be broken so that the cleavage can exhibit the interior transparency, red color and any metallic inclusions.

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"Native carbon" (charcoal) found in 1874 ash.


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Single anorthite crystal from 1874 ash, unusual because most are twins or groups.
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Anorthite from 1874 ash, cut by Colorstone Mithrill, Tokyo.


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Anorthite crystal floater from 1874 ash, with thin skin of lava.
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Dangerous high crumbling cliffs of 1962 scoria on a hot humid summer day.


Looking back on our three expeditions, what have we accomplished? A pessimist would say that we suffered the physical discomforts of seasickness, cold wind, cold rain and, later, hot humid sweaty days, and that we ruined our shoes and spent more than $10,000 on hotels, rental cars, ship tickets, food and drink - yes, Japan is not exactly the world's cheapest country to travel around in - which we will never be able to recover from our paltry supply of specimens and gems (not that we really want to part with them anyway). But we are optimists, not pessimists, so we'll put it differently: We had a great time, got a few fascinating specimens and gem rough for our collections, plus a few extras for our friends and fellow researchers (NASA already got some for an experiment with analogies to Moon rock), and we learned a lot about the geology of a historically interesting volcanic island, including some new data on the source of the alloy-bearing anorthites. The local population was helpful and friendly, their local seafood is delicious, and we did our part to support their economy in the difficult environment in which they find themselves. Not to be regretted for a moment.

Those who are not interested in technical discussions about the validity of microscopic rare mineral species can stop reading now and just scroll down and look at the pictures. For the "systematik" species die-hards, I'll discuss here a bit about the varieties of "native brass":

The mineralogical classification of so-called "native brasses" is complex and poorly described. Industrial brass is an artificial alloy of copper and zinc. In Nature there can be no simple solid-solution series between copper and zinc because native copper has an isometric structure whereas zinc is hexagonal. Industrial brass comes with many different Cu-Zn ratios with different physical properties, different names, and suited for varying applications, and there is an enormous literature on their structures and characteristics. Natural Cu-Zn alloys have also been reported with different Cu:Zn ratios, at least 10 so far, but how many of these constitute true mineral species is an open question. In order to be other than just a "zinc-bearing variety of native copper" or a "copper-bearing variety of native zinc" one would have to show some ordering of the Zn-Cu atoms or a structural difference from plain copper or zinc. Unfortunately, for many of these phases only the composition is known, not the crystallography. So far the IMA has approved two brass species, Danbaite (IMA 1981-041) and Zhanghengite (IMA 1985-049), and three others are considered "unnamed-valid", meaning that they probably would qualify for species status but there isn't enough data available to properly characterize them: UM 1980-01-E:CuZn, UM 1980-02-E:CuZn, and UM 2003-03-E:AgAuCuZn. Three others, Tongxinite, Native Brass sensu stricto, and "Zinccopperite" have been widely used in the literature, but their official status is "published without approval". To further complicate the matter, different writers have used the names "Native Brass" and "Zinccopperite" inconsistently, for more than one composition. (Avoiding such confusion is of course precisely the reason authors should collaborate with the IMA and not invent new names for incompletely characterized material.)

Moving on to the Cu-Zn alloys found specifically on Miyake-jima and Hachijo-jima, Nishida et al (1994) reported 5 different compositions for 8 analyses, in descending order by proportion of zinc roughly: one native zinc, four Cu60Zn40 (corresponding to the industrial alloy "Muntz metal"), one Cu75Zn25 (which corresponds to the industrial "Prince Rupert's metal" and a lot of common brass), one Cu86Zn14 (the industrial "Tombac" or "Rich Low Brass"), and one native copper.

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Anorthites cut by Colorstone Mithrill. Note native Cu-Zn alloy flakes included in the red one.


The strong preponderance of compositions grouped around Cu60Zn40 might perhaps indicate that this tends to form a distinct phase, although it does not correspond to any of the previously reported terrestrial brasses. However, Pavel Kartashov has reported a lunar brass of this composition from the Luna-24 landing site (American Mineralogist, 85, 1845 (2000)).

Nishida et al attribute the cloudy red color to innumerable micron-size particles of native brass. We can assume that the red (or pink when only lightly colored) is due to optical scattering effects as it is obviously not the color of "brass" itself. Not all red specimens show eye-visible metallic inclusions but, when they do, there are typically 1 or 2 such layers per crystal, parallel to a cleavage. Occasionally, a faintly pink or almost colorless crystal without the "red cloud" can show a discontinuous metallic pink copper layer viewed perpendicular to the feldspar cleavage, and be transparent colorless or a peculiar light blue in transmitted light parallel to the metal. The feldspar crystals very commonly have inclusions of transparent brown forsterite which, together with the incipient feldspar cleavages, greatly reduce the amount of material that can be considered cuttable. Nishida et al also found hydrocarbon inclusions in the feldspars, so they obviously crystallized in a reducing environment. As for the possible source of the hydrocarbons, they might have been distilled out of organic-rich seafloor mud on top of the Pacific plate as it was subducted under the Philippine plate. The entire Izu island chain is of course the product of this subduction.

Looking at the other islands in the Izu chain, very similar anorthites come from Hachijou-jima, (An 96, Ab 04) in composition - similar to those from
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1962 scoria on a misty humid summer day - For scale, note Tanaka-san digging up on the upper left side of the hill.

Miyake-jima. The Hachijo anorthites also can include native brass, zinc and copper, but are more opaque than the Miyake specimens, so less interesting from a gemmological perspective. White opaque anorthite phenocrysts up to 3cm size are found on Oushima island, but so far without reports of metallic inclusions, although some copper minerals like tenorite and atacamite have been deposited by fumarole gases there. At the far south end of the chain, Iwo-jima (the island that became infamous in World War II) has yielded a large number of little twinned feldspar floaters as volcanic bomblets, widely distributed in mineral collections, but these are of andesine composition, not anorthite, and native metal inclusions have not been reported. At the far north end of the Izu island arc volcanism, on Hakone volcano on the mainland, copper or brass inclusions have been found in anorthite phenocrysts, but much tinier than the Miyake specimens. We intend to undertake a systematic search for metallic microinclusions in feldspars from other islets in the Izu chain. Mineralogists should be on the lookout for native brass inclusions in feldspar phenocrysts from other island arc volcanos too.
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Windy day! "That way looks good." "No, the other way looks better!"
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30 meter high basalt cliffs from prehistoric eruption.
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Steam explosion crater, now largely vegetated, in southern Miyake-jima.
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Approx 3cm twin, highly unusual because very little adhering lava skin.
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Anorthite crystal embedded in scoria.
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...Goodbye!




Article has been viewed at least 20595 times.

Discuss this Article

29th Jul 2010 19:14 BSTRob Woodside Manager

What an interesting article. Thanks so much. It is hard to do single xl work on micron sized particles. So unless larger bits are found or our technology changes this is probably where the mineralogy remains. Do you think the Cu/Zn was exsolved from the anorthite as it cooled? Sadly, I'm ignorant of the phases in this system.

29th Jul 2010 19:30 BSTAlfredo Petrov Manager

Thanks for your kind comment, Rob. Not all the metal is only micron sized - Some is visible even to the naked eye, as you can see in the cut red stone, but I suspect the layers are indeed extremely thin. Not sure what the necessary minimum size is for structural work.

29th Jul 2010 21:38 BSTHarjo Neutkens Manager

Great article Alfredo, thanks!!

31st Jul 2010 07:08 BSTKeith Compton Manager

Loved the article Alfredo.
Just feel sorry you didn't find more.
But as we all now its all about the journey !!
Thanks for sharing
Cheers

1st Aug 2010 10:19 BSTRicardo Pimentel Expert

Great reading! Thank you!

1st Aug 2010 18:23 BSTRobert Meyer Manager

Alfredo,
A very enjoyable article, thanks! The red anorthite is quite beautiful. You article did not indicate how many of the red crystals were found in the volcanic bombs.

2nd Aug 2010 00:26 BSTLogan Babcock (2)

the red anorthite caught my eye with those flaws!!! gotta love that bright red with beautiful shiny specks!!!!! GREAT ONES, ALFREDO!!!!!

2nd Aug 2010 01:51 BSTBrandy Naugle Expert

I can hear your voice in my head as I read it. :P

2nd Aug 2010 09:03 BSTNeil A. Richards Expert

One of the best papers ever written. A need to read by young people who are starting their mineralogical journey. Love it.

2nd Aug 2010 15:35 BSTLinda Smith

Excellent article. I enjoyed reading the descriptions of the geology and the photos are great.

25th Mar 2013 19:55 GMTDave Crosby

No more comments from 2010 'till now? Informative, provocative, FUN read.
Thank you!

11th Apr 2014 04:30 BSTAlfredo Petrov Manager

An update to this story: In April 2014, I went to another island in the Izu chain, Aogashima, located 70 km south of Hachijojima and best reached by helicopter from Hachijojima, together with fellow Mindat manager Dr. Amir Akhavan and inveterate strahler Michel Meder, and we found another as yet unreported locality for these red-clouded anorthites with copper (or Cu-Zn alloy?) inclusions. Photos of the first couple of samples here:
http://www.mindat.org/gallery.php?loc=256825&min=246
One must wonder how many more intra-oceanic subduction zone volcanos in this chain would yield such feldspars... perhaps all the way south into the U.S. territory of the Northern Mariana islands?
 
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