silicon based

Not sure but don't silicon races need a resource to grow.

My first guess is Durantium.

Not sure what exactly you are asking, but silicons work like this:

 - If they are below their planetary population cap, they grow at a steady rate
 - diffrently from aquatic and carbon life, they can't accelerate that rate buy planetary improvements (was that your question?)
 - if you hit the popcap, you need durantium, to build a city in order to raise the popcap

There are fun exceptions to this, like using the slaver ability, so you can also build work camps to increase your popcap.

If you want a higher growth rate, you can get the prolific ability an/or the growth trait.

Morale also helps a bit (up to 25%)

Yes, only with Durantium ou can build Cities. Without cities the standard planet has 4 pop max. Actual growth of pop happens without improvements with 0.1 pop per turn until pop max is reached. There are some, but not much improvements that help growth rate, i. e. Colonial Hospital.

playing onyx it says I can't buld a city without promethion.

Is that just a tool tip error or what?

That was correct last time I played a silicon race.

This bug requiring PROMETHION needs fixing, it wrecks races completely and the game itself , how could this go on for so long ??

The screen clearly says you need 10 Promethion to build a city

Promethium is not a bug silicon cant eat food they eat gas instead. It used to be duranthium. Now its promethium. Build a starbase to mine promethium.