Brah-zeal

[arappaho]

So Steve, found this article surfing about agates.

It's long and technical, but about page 4 or 5 they are talking
about a "fibrous" growth in agates that might be "something" to
do with the pattern/s you're talking about.
I'm going to have to read it again myself, tomorrow.

Joe

[arappaho]

Oh yeah...........here's the link to the article;

www.originlab.jp/pdfs/moxon_terry_mineralogical_magazine.pdf

[earthnhands]

thanks for that link arappaho!
;D rebecca

[xtldggr]

The debate continues.

It should first be noted that I am fully aware that there are colloquial or provincial names for the materials I’m about to describe. When looking at this from the viewpoint of a scientist, however these terms are often confusing and in many cases erroneous and not in accordance with the original intent or etymology of the term used. That being said it has become more than obvious that within the gemological\lapidary community that use of these terms however inappropriate will continue. Not trying to offend or anyone but I belief facts are facts.

The scientific community through out the ages has wrestled with this problem and a careful study of mineralogy clearly illustrates this with the continued refinement of the nomenclatural system as applied to minerals.

The mineralogist is tasked with responsibility to apply a systematically methodical classification of minerals, old and new, as they strive for a better understanding of the materials they study. An example of this is the strict guidelines and criteria applied by the International Mineral Association (IMA) to classify such materials. The rules are not static and subject to change as our understanding becomes clearer. A good example of this process has been applied to Limonite, which has been and continues to be referred to as a mineral; although less today, than say twenty years ago. Mineralogical books published in the 50’s and 60’s will almost universally refer to limonite as a distinct mineral species a claim that would not be made in a contemporary manual of mineralogy. This same problem exists with the classification of the various quartz varieties.

A few terms to chew on:


Macrocrystalline - Texture of rock containing crystal distinctly visible by the naked eye.
Mesocrystalline - Texture of rock containing crystal between Macro and Microcrystalline
Microcrystalline- Texture of rock containing crystal visible only with use of a microscope
Cryptocrystalline- Texture of rock containing crystals too small to be visible with an
ordinary microscope but may be visible with a modern electron microscope.

Chalcedony is a cryptocrystalline variety of quartz, which is further distinguished from the other varieties by its fibrous nature and lower specific gravity (SG); non-fibrous quartz has a SG of around 2.65 and chalcedony, due to small voids between the fibers and has a SG of around 2.6. Furthermore, to truly be considered chalcedony the fibers must be oriented in such a way as to be normal (at a right angle to it deposition\formational plane). This is a scientific standard, which may or may not be applied within the lapidary\gemological community. In fact within the lapidary\gemological Chalcedony is a group name for all SiO2 formation as just described as well as the variety name for the commonly white or light colored massive or nodular forms of SiO2.

Agate is indeed a form of chalcedony, however not all chalcedonies are agate. As most of you know, agate refers to the banded form of chalcedony, with the exception of Moss Agate, which shows no banding, but rather dendritic inclusions within the chalcedony. Many in the scientific community have a problem with calling this an agate and would prefer to call it dendritic included chalcedony so as to limit confusion and maintain a strict standard of classification.

Although not universally accepted, the great majority of the scientific community believed the banding in agate to be a result of a phenomenon known as Liesegang banding a chemical process first described by the German chemist, R.E Liesegang in 1896. Liesegang banding is evident in the biological world as well, as commonly seen in tree rings, fungal rings and recently in human bone growth. The process, which occurs prior to lithification, is described as the differential deposition of precipitated solids from\within a colloid or gel due to electrochemical, density, temperature and pressure related variations within the gel or colloid, as it approaches equilibrium as governed by the laws of thermodynamics.


Let the debate continue!! As it enriches all of our understanding of this fascinating subject.


John-xtldggr-Makohon