@downsyde
You have to imagine fantastically high tempertures that turn deeply buried coal (carbon) to become carbon gas deep down at some time.
The gas pressure would have been phenominal as gas leaked out through hard rock. As the pressure in the cracks reached higher (imagine one of those hot-dog ballons as you blow it up - the little tail slowly grows and expands.
Solid carbon subliming into gas would be a fantastic expansion difference between the solid and gas volumes especially at those temperature too.
At lower weight/pressure of rock at shallower depths the carbon gas would lift entire areas of rock above and open up cracks. The rock being colder condenses carbon, replaced by hotter gas and expands the crack etc .
By and large you will tend to get all the sheets /ribbons/veins running the same direction in a given area. The reason is tectonic stress will be stong in say x-axis and weaker in y-axis and cracks opening is favoured in one axis, the one perpendicular to the stronger tectonic strees i.e. easier to open up against the lower force.
In the end you need to think about graphene veins as sheets, much thicker in one axis than in another, which will generally be tiddlers. However, not like mica sheets. Like water running through Grand Canyon some areas of rock will allow slightly higher gas flow, and once that happens even more gas flow opens it up further through erosion of surrounding rock. This becomes the preferential flow route. The cross section I guess will be some oblate shape, but we mine in square tunnels. The last bit will be when a man can barely/safely squeeze into a narrow axxes and how long his arm is, but realistically very squarish tunnels.
Finding side stringershat never came to much, found during drives will be indicative of maybe a pipe type structure nearby-ish or maybe not near but in which direction and how close? This is why spid and I mentioned drilling from underground to find the main source rather than drill from surface - something SL miners never had the money or technology to do.
There also likely be erosion as carbon gas melts and gouges out or erodes molten rock and pushes towards the surface, and probably explains why the mined veins get wider as they go deeper, more erosion at depth by hotter gas for longer.
Carbon sublimes and is a gas above 4,600 C and was likely much higher.
Silicates melt at about 1200c. (means granite also).
So over tiime the available carbon is depleted and condensed higher up, pressure drops, carbon deposits cool very slowly because of the fantastic heating held in the surrounding rock. The pressure and heat allows carbon to form crystal structures, unique - only occurs extensively in SL.
At least, thats what I think is the story.pieced together from memory from various articles and some interpretaion on my part of mechanisms in play.
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