FMF - Friends of Minerals Forum, discussion and message board

[Josele]

Recently I find a book which explains extensively the mechanism of jasper and agate formation, from the simplest chert or flint until the most complex agates: GENESIS AND CLASSIFICATION OF AGATES AND JASPERS: A NEW THEORY, by Marco Campos-Venuti. The author is a PhD volcanologist and editor of the Rivista Gemmologica Italiana who is a passionate collector of agates and jaspers and has written innumerable scientific papers.

The book shows a new geological model that explains the formation of most of microcrystalline and amorphous varieties of silica. The use of a geological and stratigraphic innovative approach to the theme leads to the consequence that most of the concepts are original. The most important conclusion is that silica varieties take their origin in a meteoric environment with a strong influence due to the weather and the alternation of dry and wet seasons. Host rock, pH of water, latitude and many other parameters determine the numerous varieties of silica, which are explained with the help of many photos, drawings, diagrams and tables which maintain the interest of the reading.
A must for all silica lovers.

More information:
https://www.agatesandjaspers.com/

All images © Marco Campos-Venuti

[Mark Ost]

Really nice post. Look forward to following it as it progresses.

[Don Lum]

Great pictures and explanation. Thanks for posting.

[Rei]

Those are some real lovlies :) Not sure what they mean by some of those terms, though. For example, what do they mean by "onyx"? Onyx is agate that's been artificially colored black, typically by carbon.

[Josele]

[Rei]

Hmm, question, what does it say about the formation of the "white facies" in your picture above? Because that's something I've noticed with a number of recent specimens I've found, they form outward-in (crack-filling) in layers that match the contour of their vug, with opaque or milky, colorful chalcedony, but then they sometimes they abruptly terminate and switch to absolutely perfectly straight layers (like in your picture) of either bright white or transparent, no longer matching the exterior contours. I've been assuming that the transparent is quartz, but what is the white?

I found a really curious piece the other day which not only had one of those sort of transitions, but also had calcite in the middle. I'd wager that the calcite was from an entirely different formation event.

[Dale Hallmark]

[Josele]

[Rei]

Ah, fascinating! Thanks! :)

[Mike Wood]

Thanks Josele for bringing this book to our attention. Agates don't normally 'float my boat' but this book by Marco Campos-Venuti looks absolutely fascinating. Plus I'm a sucker for pretty pictures; those photo's of agate slices are mind-boggling! I'm definitely going to buy a copy as soon as possible.
Cheers,
Mike

[Ed Huskinson]

Yes. regardless of style of formation, many agates display horizontal banding. And they are just that - bands that indicate the orientation of the agate in it's matrix. The phenomenon is called a "geopetal" structure, usually enabling the observer to know which way was up for the agate or nodule.

We inherited the term from paleontology, believe it or not. Early paleontologists noticed that some hollow fossils had developed horizontal deposits of mud (or whatever) in the shells. Applying the law of uniformitarionism, they realized that these late layers were formed when silt or mud (or whatever) had dribbled into the hollow spaces of brachiopods or gastropods, and had settled to the bottom, so that one could deduce the "up" direction accordingly.

I think Dr Megaw may have covered this in an earlier post. Regardless, enjoy the Agates, regardless of the nomenclature applied to them.

Ed in Kingman

[Josele]

Just to comment that the current issue (July - August) of Rocks & Minerals magazine has some articles about Agates: https://www.rocksandminerals.org/index.html

I profit for posting an agate slice of my collection:

[Jordi Fabre]