How about cork? A glue down cork tile (even our thickest cork tile of 8mm = 1 lb per square foot) is going to be some of the lightest material available for use on a counter top. I've had several people use cork on their floors, walls and counter tops in their tiny homes. I've also seen this done in RVs, boats, buses and vans. When weight is a concern, cork glue down tiles can be used in lieu of heavier counters.

Here is what I'm talking about: 1 box of 4mm cork (1/8") = 85sf = 37 lbs. That is more than enough cork to lay all of our counter tops in your space.

As for the "stone" products, they should all be withing a few grams per square foot of material in weight. The biggest difference will be between the laminate and stone...and the butcher block and the stone.

Laminate is mounted on fibre board (which is wood mache = almost the same as solid wood). The wood counter tops and the laminate will be close to each other in weight. Of course the thinner the material, the lighter the product. In theory you can look at acrylic surfaces (very thin and very light).

Wood butcher block is the least heavy and soapstone is the heaviest.

Laminate should actually be the lightest.