How Good Is A Marble Slab at Deadening Vibration

Sunnyjim,
from my experience with marble I am with Geoffkait on this one - a thick slab of marble (which is 2" thick in your case) should be a good material to damp the vibrations. On its own marble (& granite) ring like crazy when struck (as Geoffkait indicated) so the key is not to suspend marble or granite by itself (for ex. use marble or granite as a shelf in an audio rack). you are doing the right thing by placing marble on top of a wooden surface (your plant table). this will prevent the marble from ringing. I suggest, further, that you place a 3mm cork sheet (you can buy a roll of cork from Staples & cut it to size with a scissors) in between your plant table wooden surface & the under-side of the marble slab. This will for sure deaden the marble (if there was even an outside chance that it would ring when placed on a wooden surface) & the cork is another fantastic material to deaden vibrations. I've used a marble tile on top of my CD player w/ a cork sheet in between - works great!

To Bombaywalla, Interesting suggestion about the cork, though 3mm does not seem very thick, still sounds like a good idea.

I've used the 3mm cork mat for damping vibrations on top of a CD player. Also several TTs use this same thickness mat on their platters.
I am also using this 3mm cork mat on my amplifier platform & the amp is 110 lbs.
If you put a heavy marble slab on top of the cork mat, if it makes you feel better, get the next size up in thickness. Staples has that too but maybe in 12"X12" tiles & not a roll. Since cork is pretty rigid & the surface area of the slab is pretty large the cork should not compress under the slab or components. So, I'm thinking that a 3mm thick cork mat will be fine.
The idea is to damp the marble. I believe that thickness is secondary as even a 3mm mat has sufficient damping capacity from my experience.
The good thing about cork is that it stays the same for a long, long time & does not stick to the surface. Dynamat is also a great mat but ensure that heat & humidity cycles don't make is stick to the surface(s) or disintegrate over time.
For me it really works well.
In fact, when I had a Music Hall MMF5 TT, I threw out the felt mat & replaced it with a cork mat. Much better performance from the get go & no static on the LPs.....

BW, where did you purchase the cork platter matt.

I made it myself. Bought the cork roll from Staples, put the MMF5 felt mat on it & traced out a circle, cut it out with an Xacto knife, made a spindle hole also with the Xacto knife & I had a cork mat ready to go....

The downside of placing the cork board under the marble is I would have to very carefully lift the marble slab which weighs alot to position the cork material.

maybe I am misunderstanding this - you place the cork mat first & then put the slab of marble over it (2-man lift on the marble slab). You don't lift the slab with one had & shove the cork mat under it. Yeah, that's asking for trouble!

06-28-13: Sunnyjim
TO BW: The cork mat sounds simple enough to make..... Unfortunately, I decided to go ahead and try the marble slab before I read any of the posts, so therefore my concern about lifting it again. Surely, a two man job. I will let you know if I decide to install the cork.

fair enough. The only thing I can say here is that do NOT make a judgement until you have tried the marble slab with the cork sheet under it. Thanks.