Granite Laying on a Concrete Floor

You need to provide more information before we can even contemplate answering your questions.

1st Q: First off, where are you located, as in what country, or state if in the USA? Each state in the USA has a State Building code that will govern the live (movable, as in the weight of people, furniture, appliances, etc) and dead (immovable objects, like the weight of the floor construction and utilities) that a suspended floor must support...you're bet off talking to your local Building Inspector/Code Enforcement Officer before proceeding any further...they can give you guidance, but not determine what reserve carrying capacity your existing slab is capable of carrying (that may be an unlawful act in your locale).

2nd Q: Is the slab on the ground, or is it suspended meaning that it has a space underneath it.

3rd Q: Do you know if this concrete slab was designed by a Licensed/Registered Structural Engineer? If yes, then that Engineer should have the design calculations for that slab and can tell you what the reserve load of the slab is. If no, you're going to have to have a certified testing agency come to your house and perform what is referred to as a "Windsor Probe Test". It is a non-destructive test that a gun shoots a hardened steel penetrator (dart) into the top of the concrete floor slab. This is required to determine the concrete compressive strength, so as to perform analysis calculations to see what the safe carrying capacity of a suspended slab is. No less than 3 Windsor Probe shots must be performed per 500 square feet of floor space. You may need more depending on the size of the area you want to install the granite floor tiles. You also need to have the testing agency perform a sonographic analysis to determine: the minimum slab thickness; the size of the reinforcing steel bars, their depth in the slab, and their center-to-center spacing. All of this information is need to perform the structural analysis. Without that information no engineer will give you an answer to your problem, unless they want to make a wild arse guestimate...no good engineering practice and should the suspended slab fail or crack that engineer will most likely be fined, lose his/her engineering license, and possibly jailed. They'll also be liable in a court of law for any and all damages caused by a slab failure.

4th Q: Is there any floor material covering the slab now?

5th Q: What is the approximate age of the slab?

6th Q: Who built the slab?

Aside note: If the existing concrete floor slab is on grade (meaning it is resting on soil), I highly doubt that the additional weight of new granite floor tiles will adversely affect the soil's carrying capacity, unless there is a void under the slab or the slab is resting on organic soil, silty soil or clay soil. 6 or 7 Kg per square foot is not a heavy load to bear (on good soils) as it amounts to only 15.5 Pounds/SF of force.

Why did you mix-up the the weight of the tiles (Kg, as in Metric system) with the area (Square Feet, as in English measurement system)? Mixing up the two units is not standard practice....instead you should have used compatible, or like, system units such as Pounds-force per Square foot (Lb.-f/SF).

Have a great day!