18-rock-n-gem-magazine-big

Published Winter 2001

Contents: Stone circles, Journey to Tafilalt series, origins of agate, word search, sugilite, shows.

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R.E. Liesegong AND THE ORIGIN OF AGATE
The father and grandfather of Raphael Eduard Liesegang (1869-1947)
were pioneers in the photographic industry.
It is not too surprising that Raphael / followed in their footsteps to continue with the family tradition. However, Liesegang's achievements went much further than an interest in the science of photography and he made major contributions in the fields of physics, chemistry, and bacteriology. This combination of outstanding scientific talents and the family link with the photographic industry meant it was inevitable that he would be involved in the chemistry of gels. Discussions of natural rhythmic patterns had appeared in the scientific literature since 1850.
Liesegang's 30 papers on gel banding resulted in the phenomenon being named after him. By 1913, rhythmic banding had become known as the Liesegang rings. Liesegang rings can develop in rock systems within the molten magma as in the formation of orbicular rhyolite (Fig. 1) or by water percolation through an iron oxide rich sandstone (Fig. 2). Rhythmic banding on a steel plate demonstrates that Liesegang rings are not just limited to the mineral kingdom. The plate was originally a support backing for a quartz clock that has resulted in a beautiful, rhythmic brown pattern radiating from the spindle hole (Fig.3). Faint brown ghosts of etched finger prints can also be observed. A simple electrolytic cell caused the etching with the steel plate and brass spindle as the electrodes and perspiration residues acting as the electrolyte. There is the possibility that the clock battery could have provided an added driving force in the etching process. In some instances Liesegang rings can form as bands on previous deposits. Fig.4 shows the regular deposition and hardening of oil on the observation window of an ion beam thinner: the oil particles have aligned in different ways to produce an interesting rainbow pattern of Liesegang rings.
In 1915, Liesegang published Die Achate where he argued that the major agate patterns could be explained by a rhythmic diffusion mechanism. Gergens first observed spiral, twisting growths being produced in the Chemical Garden 1Il 1859. Liesegang took the experimenration further and added iron (II) sulphate to a sodium silicate solution and produced a moss agate-like growth. The simulated pattern allowed Liesegang to assume that moss agate was the result of iron compounds in solution seeping into a pre-existing silica gel. The thread-like forms are caused by osmosis and density differences between the sodium silicate and the iron silicate: the threads rise within the gel (Figs.5 and 6). Horizontally banded agate is found throughout the world and Liesegang was able to simulate these patterns by leaving concentrated hydrochloric acid on a layer of water glass in order to create rhythmic white disks. If the white-banded gel is then surrounded by ferric chloride, an alternating white and brownbanded gel is produced whose pattern is similar to horizontally banded Brazilian agate (Figs. 7 & 8).
Wall lining agate probably occurs in every country and is found on every continent. This is the most abundant agate type and as sections are similar to a plan view of a castle it is sometimes called fortification agate. Liesegang devised many simple gel experiments to demonstrate how this type of agate could form. He initially observed the fortification effect around tree roots in the Munzenberg sandstones where the rhythmic banding appeared long after the deposition of sand. Liesegang proposed that the banding in sandstone was due to an initial, even distribution of iron oxide that was followed by circulating solutions containing carbon dioxide. These field observations allowed Liesegang to organise experiments where he treated silica gel with iron compounds. He found that under suitable conditions rhythmic banding could be created (Figs.9 & 10). Liesegang accepted that his diffusion theory would not explain every agate but he believed that his theory offered a more likely explanation for agate genesis than the 200 year old Inflow theory. This hypothesis relied upon innumerable separate alternating iron free and iron containing depositions that would eventually fill the gas cavity. Although the patterns do bear a superficial resemblance to agate, interested research workers have never accepted Lisegangs work on agate genesis. The banding produced in gels are so striking that it has not stopped publications on colloid and geochemistry still mentioning Liesegang and the Origin of Agate. There are a number of objections to Liesegang's hypothesis such as the need for agate to form from a pre-existing gel. Furthermore, silica gels are over 90% water and the dehydrating gel results in the collapse of the banding producing a pile of stained and powdered silica gel. Unfortunately, the dehydrated gel does not bear any relationship to agate and Liesegang never did address this problem.

Terry Moxon has been investigating the problems of agate genesis all a part time basis at the Dept of Earth Sciences, Cambridge University for the past 3 years. He would be particularly pleased to hear of any readers who have collected agate from Derbyshire and Cumbria.

Fig.t The sample was purchased as orbicular rhyolite and shows that Liesegang rings can form within the magma. (Scale: the spherulites have a maximum diameter of ~ 4mm.)
Fig.2 Rhythmic banding in. the sandstone at Hunstanton.
Fig.3 A steel plate has been selectively etched. (Scale: the hole in the plate is ~ 1cm in diameter] Courtesy of Jolm Raehurn.
Fig.4 Rhythmic banding as the result of oil and other debris building up on a glass observation window of an ion beam thinner.
Fig.5 The thread-like growths are due to iron silicate growing ill a solution of sodium silicate.
Fig. 6 Purchased as a sample of Indian moss agate.
Fig. 7 Liesegang disc growths of copper chromate. The sample on the right has grown under the influence of all electric current and has produced more regular banding.
Fig. 8 Horizontally banded agate, Isle of Mull. (Scale: across the width 5 cm}. Photograph by Nick Crawford.
Fig.9 Ringed patterns as the resulting from the reaction between a gel containing potassium hexacyalloferrate (II) and added iron (III) chloride. (Scale: the drop is ~ 1cm diameter.)
Fig. 10 (see magazine cover) A fortification agate from the Cheviots. (Scale: the width is about 6 cm). Photograph by Nick Crawford.

 

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