Phlogging a dead horse?
Many of you are aware of my obsession with Blue Lace Agate, both from a lapidary point of view and with stones from the mine, as well as my curiosity as to how the mineral was possibly formed. Any new information that I come across, be it a new specimen or comment, has to be investigated. Recently it was Marco Campos-Venuti’s new book (Banded Agates: a genetic approach (2018) www.agatesandjaspers.com) which set me off on the trail again because it has a chapter on Lace Agates. Marco’s theory on their formation suggests two separate fluids were involved: firstly a saline brine solution, dependent on rainfall, and secondly a fluid from a greater depth associated with a low temperature hydrothermal system of probably less than 100 °C. Their interaction would be responsible for the formation of Lace Agates.
The words salt and brine made me think of the mineral
halite, and query whether that was perhaps the mineral that was
pseudomorphed by the chalcedony at Ysterputs, rather than fluorite. I bought a
new specimen of Blue Lace at the beginning of last year and comparing that to a
photograph of halite I found on the internet showed the appearance was so
similar.
(http://www.minersgallery.com/product/large-specimen-of-halite-crystals/)
Being of more a creative and imaginative nature than a true scientist, I sent these two views to Duncan, who agreed that the likeness was amazingly similar and suggested he post a query on the Mindat Mineralogy Message Board asking whether halite might be a possibility for replacement by chalcedony, seeing as it dissolved far more easily than fluorite (https://www.mindat.org/mesg-11-458346.html).This was duly done, and the single reply (from Alfredo Petrov) suggested it could possibly also be melanophlogite, and even if it wasn’t that, fluorite was the most likely mineral to have been replaced.
I had once heard of melanophlogite, having come across it when doing research for my website (www.namibianbluelace.co.za) but its looks and descriptions didn’t quite fit with examples in my collection. Likewise the profiles of the points of my specimens didn’t quite fit that of examples of fluorite I had seen either.
Melanophlogite was written up in an article by Marek Chorazewicz on where he had found it in the Santa Monica Mountains, Southern California (Bulletin of the Mineralogical Society of Southern California. Volume 87, Number 8 - August, 2014). In that article he also mentions melanophlogite could be the replaced mineral in the Blue Lace Agate specimens from the Triestia mine in Romania as well as Ysterputs. So perhaps my next step was to see what I could find out about Triestia, and whether its Blue Lace Agate was similar to that of Ysterputs? In so many ways it seems it is.
Ysterputs Blue lace Agate
This is what I found:
(https://www.mineralauctions.com/auctions/weekly-mixed-mineral-liquidation-auctions-ends-aug-26th-358/chalcedony-ps-after-fluorite-old-classic-17480.html)
The Hungarian Natural History Museum has
the following specimen and description:
Likewise from the Romanian Journal of Mineralogy of 1992:
Trestia (Gutii Mts). Occ. type: XII. Celebrated occurrence of the blue chalcedony, known since the 18th century. The sky-blue fragments (5-15 cm in size) are scattered in soil or alluvial material, being formed by solidification of some silica gel originating in hot springs. Sinter-like reniform masses, thin radial-fibrous or parallel banded structures, surfaces imprinted by irregular reliefs or plates and druses inlaid with cubic crystals. Though considered to represent quartz-chalcedony pseudomorphs after fluorite or galena, even a rare species of rhombohedral quartz or a new silica species named "cubosilicite", it is more likely that the blue cubes are quartz-chalcedony paramorphs after melanophlogite. (Rom. J. Mineralogy, 1992, 75, p. 1-51)
It appears that Trestia specimens do have similar-looking pseudomorphs of chalcedony after what is generally called fluorite, and that it has been mined and used as a gem since the late 1700s. SO, THE ORIGINAL MINERAL THAT MIGHT HAVE BEEN REPLACED BY THE CHALCEDONY HAS BEEN DEBATED FOR 250 YEARS! Oh dear, so our thoughts are nothing new, nor is the question I am researching.
What about melanophlogite – is this a newly discovered mineral, as Duncan had never heard of it before either? Nope, according to Wikipedia (https://en.wikipedia.org/wiki/Melanophlogite) melanophlogite was identified and named by Arnold von Lasaulx in 1876, even though a Mr Alessi had described a similar mineral in 1827. The first studies of melanophlogite were made in Sicily. The mineral was originally called Girghenti after an old Sicilian town, but was officially changed to melanophlogite in 1927, according to The American Mineralogist, Vol. 48, July/August, 1963 (Skinner B.J.; Appleman D.E. (1963). "Melanophlogite, a cubic polymorph of silica" (PDF). American Mineralogist. 48: 854–867.) I also see that the name melanophlogite describes the fact that the mineral turns black when heated.
And finally to bring me full circle, here are a couple of sentences from the Skinner and Appleman article mentioned immediately above: “Von Lasaulx described melanophlogite as a cubic form of silica with a hardness of about 6.5 to 7 and a specific gravity of 2.04 at 17.5 °C. Because of the perfection of the cube form of some of his specimens, he suggested that the mineral was pseudomorphous after fluorite or halite.”
This research has proven to be quite a wild gallop over several days, despite following internet links which are much easier to locate now than they were four years ago when I started on the Blue Lace Agate endurance trail. The ride turned into an interesting investigation but has resulted in no clear answers, which is often what happens in scientific enquiries, but it has opened new doors and I have learnt a lot. So, seemingly we are no further in identifying what really went on in the ancient dolerite crack at Ysterputs, some 54 million years ago.
Perhaps we
are just phlogging a dead horse?
In : Mineralogy
