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July-August 2014

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Media Reviews

Exquisite Agates is yet another of the continuing offerings of beautifully done, coffee-table style, mineral picture books. As Dietrich Mayer proclaims in the preface, this is a personal agate book that features approximately nine hundred specimens in his private collection. The collection, assembled over about forty years, contains more than four thousand pieces, a number of which have been featured photographically in other publications. Localities known for aesthetically appealing agates are shown in deference to those of mere scientific curiosity. There is sparse text (both German and parallel English translations are given), although each agate is identified by location and size and described where appropriate. By design, no mention is made of controversial topics such as agate genesis.

The book begins with a short but interesting section on agate “characteristics.” These are actually common descriptive terms such as iris agate, plume agate, and the like. Each is defined or described, and illustrated with a color photograph of the feature. The bulk of the book, encompassing pages 14–423, contains the photo-essay. Agate photographs are arranged by country of origin with subsections for specific localities. Germany is treated separately followed by the remainder of Europe. Next, Africa is well represented with Morocco taking center stage, having been visited by Mayer some twenty times. Agate specimens representing nine separate Moroccan localities are featured. Asian localities include some that are not often seen such as Tajikistan and Uzbekistan. North America follows with twelve states treated separately. Central America is represented only by Mexico with nine localities or varieties illustrated. South American localities complete the tour and are primarily those traditional sites in Argentina and Brazil, although several interesting sites in Peru and Uruguay appear. The book closes with a list of references, acknowledgments, and a short biographical sketch of Mayer.

The book is of high technical quality in terms of printing, illustration color and crispness, paper, and binding, as one would expect of publisher Rainer Bode. The photographs, which are exceptional, were all taken by Andreas Wawra. Somewhat trivial points that I find interesting are the apparent optical illusion agate at the bottom of page 382, something quite alive peeking out of the agate at the bottom of page 103, and the acknowledgment of Mayer's wife who he claims is not interested in agates but is willing to tolerate them. (If we substitute the word minerals for agates, I fear many of us could say the same.) This is an attractive book that makes a nice companion to Johann Zenz's Agates I (2005), Agates II (2009), and Agates III (2011), also published by Bode Verlag, and is, consequently, highly recommended.

Genesis and Classification of Agates and Jaspers: A New Theory by Marco Campos-Venuti, Printed by Tophgrafia Luciani, Rome, Italy. Order from www.agatesandjaspers.com. 160 pages; 2012; $45 plus shipping (softbound).

Several decades ago most jasper and agate specimens were relegated to a back corner of the showcase—indeed their mineral pedigree was even doubted by many. Well, their microcrystalline makeup is quartz, a mineral, so they are certainly bona fide minerals. Today collectors have entire showcases devoted to these colorful and intricate manifestations of silica, and they have been commanding ever-escalating price tags, especially for the past fifteen years. A number of books, mostly catering to their great photogenic beauty, have been published that pay colorful homage to these types of quartz.

Since 2005 Marco Campos-Venuti, who earned a PhD in volcanology, has published a number of individual papers about the origins of various types of microcrystalline quartz, mostly agates. Based on these works and other information, he proposes various explanations for the genesis of jaspers, chalcedonies, and agates. He discusses textures such as moss and plumes in chalcedony, concentric and parallel banding of the more traditional agates, and the various orbs and breccia patterns found in the most spectacular jaspers. Throughout this volume, he has placed carefully chosen specimens to illustrate various textures being explained and described. Many of the specimens are from North American localities, although other examples from throughout the world are also shown.

According to Campos-Venuti, agates and jaspers are produced at near-surface temperatures and pressures, with no need for higher temperatures and pressures such as those associated with volcanic environments. From what I have read in other publications on agate genesis, the near-surface temperatures and pressures explain the geochemical setting for many agates, although some investigators still think that temperatures of formation can be quite elevated.

More problematic is Campos-Venuti's fundamental theory of agate and jasper formation's being highly dependent upon both desiccation and rehydration of the cavity. Although he claims this idea is novel, it has been around for more than one hundred years. Bauer (1904) thought the problem of agate genesis was simple and merely depended upon hot water dissolving silica from country rocks and depositing it in openings. When the water retreated and the void dried, a thin film of silica remained. Repeat the process enough times and agate fills the cavity. Linck and Heinz (1930) suggested that wet and dry seasons provided the principal mechanism for producing both concentric and horizontal banding. Silica was dissolved during the wet season and filled the cavity, and alkaline earth ions helped with the color and the concentric precipitation of bands; during drier times, concentrated silica (sol) was drawn up from below and resulted in horizontal banding.

To be perfectly honest, no universal agreement exists concerning the formation of agate bands, either concentric or horizontal, but it is apparently a fairly slow process that requires stable conditions. The alternate vacating and filling of a cavity with silica solution would seem to drastically interrupt delicate banding processes, either horizontal or vertical.

Another departure in this volume from a commonly held idea is that of the origin of so-called escape tubes or inflow tubes in agates. These are thought by most to represent passageways for the expulsion of fluids from the central agate cavity during times of over pressurization within the void. Conversely, they may represent infusions as fresh silica is injected back into a still not-so-hard silica precipitate. Campos-Venuti considers them as part of the original structure of an agate and a reflection of original incomplete banding in the cavity. Each band is hard before the next one is established; therefore, these tubes are held open by structural strength, not produced by soft-silica deformation. In other words, they remain intact throughout the formation of the agate. Well, maybe.

Campos-Venuti's discussion of moss versus plume agate formation is interesting and may be related to minerals such as celadonite producing the mossy or plumelike textures in a void. Subsequent silica sol infills the cavity and eventually preserves these delicate three-dimensional dendritic textures in agate or jasper.

All of the several hundred photographs are in color and have excellent color reproduction. The book is printed on high-quality stock, and the numerous diagrams and figures complement descriptions of the origins of agate and jasper textures. The text is easy to understand—even for relative beginners—with only a few problems in translation, such as those found in the last sentence above the photograph on page 132 and near the top of page 147.

I would encourage anyone seriously interested in agate formation, who reads this book, to also research the subject a bit more. The topic of how agates and jaspers are created is fascinating, and definitive statements about the most important physio-chemical processes at work are still equivocal. Certainly, numerous new books and professional papers are available on the subject. For example, a quite detailed treatise on agate formation by Jens Gotze was recently given as the first chapter in Agates III (Gotze 2011; see review in Rocks & Minerals, September/October 2012, pages 465–466). This “new theory” given in the present volume provides a set of interesting ideas that Campos-Venuti considers to be the best answer to the puzzling origin of agates and jaspers. Collectors of these colorful and desirable items should read the book, compare its ideas with those of other scientists, and decide for themselves.

REFERENCES

Bauer, M. H. 1904. Precious stones. London, UK: Charles Griffin and Company.

Gotze, J. 2011. Agate—Fascination between legend and science. In Agates III, J. Zenz, 19–133. Lauenstein, Germany: Bode Verlag.

Linck, G., and H. Heinz. 1930. Ergebnisse der Arbeit des Herrn H. Heinz über der Achate. Chemie der Erde 4:533–35.

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