i'm just replying based on my current research，dont take it to be the absolute truth as in research information always continue to update
Questions:
What is Zhuni clay?
zhuni clay is not simply fine ground clay. if you have seen the example of the "slipcast" yixing pots in '58 to '60, that is ultra ultra ultra fine hong ni, but does not have the zhuni texture, density, and surface characteristic.
in essence, proper real zhuni might actually occur as certain underground patches when abundant, in anoxic conditions, but nowadays, with its scarcity (with occasional rediscoveries), it appears that it is found as egg shape globs. the raw ore is yellow color, and the spoken difference between zhuni and hong ni from the exact same area is the fineness of the material, and the color, hongni takes on an ochre hue. it is said that raw zhuni if left to erode will turn into raw hong ni color.
i speculate the formation of zhuni to be a "precipitation", i.e. dissolved iron or iron compounds coming out of water, and that it is in a unique hydration state that gives it a certain color. this is why it is found as a eggie glob. there are also mention that raw zhuni can dissolve in water, whilst hong ni doesnt.
Where was it mined specifically in Yixing,
the earliest zhuni item if i'm not wrong is from end ming dynasty, and based on zhou gao qi's writings etc, zhuni is from zhaozhuang in yixing.
What is the clay characteristic, in raw form, and how did the clay processed for making tea pot.
the raw clay characteristic would probably have a very high content of a particular type of iron oxide in a certain hydration state that gives it that "yellow" color. the processing is complicated, some would wash the clay and allow it to become a suspension, allow the suspension to settle or the dissolved components to "dry out", to give an ultra fine paste for work. this is the "Historical" zhuni.
What is the limitation of this clay in making teapot.
zhuni is absolutely non-plastic, so a 100% zhuni pot is probably unlikely, it may be blended with a lower percentage of other good clays to give it plasticity. shrinkage is also a problem.
Is there any differences in this clay in different eras and what it their different characteristics in different eras?
yes there is! any zhuni collectors lament that modern zhuni pots are never at the level or quality of the type of zhuni seen in mid qing, late qing, ROC. that sort of texture, color, density, feel, has yet to be replicated in modern days.
How do we identify this clay when looking at teapot. I am sure it is more than high pitch ringing sound. 
actually it depends on how you want to define zhuni and the individual boundary for acceptance. it all boils down the percentage of zhuni content since zhuni 100% does not allow it to be fired into a stable object.
to me, zhuni is refinement, the feel is like good creamy nephrite jade in texture and density! if you drink from beakers often you will know how glass feels, and how pots that have been overfired to achieve sillica glassification would feel like, dead, dense, metallic.
How do we tell if the clay/teapot is fired in dragon kiln or coal kiln.
the only tell tale sign is if you chip or bust the pot
since the dragonkiln is wood fired for a very long duration, like 2 weeks or more, the pots inside undergo cyclic oxidation and reduction. by looking at the broken edge of a pot one can deduce a little. but its still better to go by the "era" of the pot based on clay material, shape, seal and other factors. like Dr Lu has said, after the 60s it is no longer dragon kiln-ing.
and since pots are fired in a protective ceramic housing that shields the pots from flying stuffs, their surface is usually pretty clean. if you see black spots, these are "molten" iron spots, which is also pretty cool to note that certain rare iron oxides in the clay are more easily decomposed. these spots are more common in zini and qingshuini of the 70s.
Is this clay particularly good for any tea and what are those tea?
AT333 wrote:Now that we have the honor of Dr Lu joining us, I like to post is few question on this clay, Zhuni. I am sure expert like kyarazen, chrl, tingjinkie and many others have much to share too.
So, let's start the ball rolling Thank you!
i should be the last to join the list of experts, since i dont think i have been collecting for a super long time
