Hello Chuck
I can't give you an absolute answer Chuck because there are any number of variables and much about this subject is very theoretical.
Most coloured diamonds (fancies) are a specific colour due to the fact that they contain minute amounts of various elements:
Yellow diamonds result from the chemical substitution of nitrogen atoms for some carbon atoms, or by the clustering of nitrogen atoms.
Blue results from the presence of boron. The more boron there is, the deeper the blue. Green is a result of exposure to radiation during the formation of the diamond, perhaps due to proximity to uranium materials. Pink results from the presence of manganese.
High temperatures could then modify some of these primary colours.
One can artificially reproduce colour by bombarding (irradiating) a diamond with atomic particles and heat.
Whether the colour of brown diamonds has as much to do with their chemical composition as it does with the degree of their heat exposure is a matter that I would prefer to have an expert comment on.
Diatreme liberated diamonds come in quite a number of crystal shapes. I don't have the data in front of me, but I believe diamond naturally has three crystal forms (shapes). Rough is often found in a considerable number of shapes however. It is also found with coatings (chemical crusts), inclusions, and varying degress of clarity. How much any or all of this has to do with the purity of the original carbon source or with the chemical conditions within the kimberlite and the effects of heat and/or pressure, is also a matter that is best commented on by an expert.
It is my general understanding that black, rounded and/or complex crystals have undergone a considerable deal of heat/pressure attrition and possibly reabsorbtion.
Diamond size certainly could be the direct result of a degree of reabsorbtion, but it could also be the result of the size of the original carbon source, the physical conditions under which the diamonds were formed, and/or experienced in their transportation to the surface.
Each pipe appears to have its own unique set of conditions under which it was formed. It is not unusual for diamonds, even from erruptive phases within a common diatreme, to have different characteristics and certainly different grades.
One current theory has eclogitic diamonds sourced from subducted ocean floor limestone (calcium carbonate, CaCO3). Under heat and pressure, it is theorized that it undergoes chemical process that liberate CO2 (carbon dioxide) and C (carbon) that is formed into diamonds. I understand that this theory is based on the fact that the type of carbon found in eclogitic diamonds is also unique to such limestones.
Peridotitic diamonds are I believe, theorized to be sourced from graphite (a naturally occurring crystalline form of carbon).
Presumably the quantities of these two sources of carbon that are subjected to the correct pressures and temperatures, will determine, at least to some degree, the size of the diamond crystals at their time of emplacement.
So the causes of micro/macro sized diamonds I suspect are complex.
So far, we have ten pipes here in the NWT with uneconomic quantities and/or micro/macro diamond ratios, for every one that is economic and that is considered outstanding. The South African ratio is closer to 50 to 1.
Many uneconomic NWT pipes with low or high micro diamond ratios lie within hundreds of meters of economic pipes with economic micro/macro diamond ratios. Why these should differ is a mystery?
One other matter that may be of interest, and again, I do not have the data immediately in front of me, but as near as I can recall, most diamonds have ever been age dated younger than approximately 1.3Ba and the majority I believe have been dated as being formed somewhere around 2.3Ba to 2.7Ba ago.
Assumming this to be approximately correct, you can see why cratonic "root zones" have to have been stable to be prospective. If they had been tectonically active, their 1.3 to 2.7Ba old incubated diamonds most probably would have been destroyed long before kimberlites sampled them say 25 to 55 Ma ago in the case of Alberta.
This is a fascinating subject isn't it?
Regards |
