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I need a planet that is a combination of chlorine world and vitriolic world. The planet is extremely hot and under high pressure, covered in oceans of pure sulfuric acid and a chlorine/fluorine atmosphere replenished by silicon-based autotrophs.
What are the optimal conditions for such a planet to form and remain stable? Surface temperature, atmospheric pressure, and so forth.
Honestly, if there is enough chlorine around to form a chlorine world at all, I would expect vitriolic worlds to also be chlorine worlds by default. Why? Because chloride salts are unstable in sulphuric acid. So once you have conditions that allow for the formation of a sulphuric acid sea, those are the same conditions that will result in the production and release of chlorine compounds into the atmosphere.
Even on Venus, hydrogen chloride and hydrogen fluoride are both present in the cloud layer in detectable amounts, in addition to the famous sulphuric acid.
So, what conditions are necessary for a vitriolic chlorine planet? The same conditions necessary for a vitriolic planet, plus a bunch of chlorine in the primordial cloud from which the planet condensed- which is what you need for a chlorine planet anyway.
Firstly, chlorine is not a very common element, so the chlorine world seems far-fetched. Second, there is a general lack of chlorine-sulfur compounds and chemical cycles involving chlorine and sulfur. Without processes that can 'buffer' the concentration of sulfur and chlorine in a geologically active crust/ocean/atmosphere, it is tough to imagine such a world.
Chemistry is not my forte, so I can't interpret this too well, but here is a report on a speculative pathways for a chlorine-sulfur dioxide cycle that could produce such a world.
I recommend you might try for a world with a better understood set of processes, such as the sulfur-iodine cycle which does take place at high temperature and pressure.
Since chlorine is quite a heavy element (heavier elements are produced in lesser quantities by stars), the star which produced enough chlorine to create a whole chlorine world/planet must have been a 9 solar masses+ blue supergiant star. This star exploded in a supernova and from its remains (the nebula) this planet could have formed.
Chlorine gas is not a very good greenhouse gas, meaning that your planet would not be a blistering world unless it was quite close to the parent star. Or if you prefer to have CFCs in the atmosphere instead of pure chlorine, then yes, you do get a highly greenhouse atmosphere with a large heat reserve.
A planet with an atmosphere primarily composed of chlorine is indeed possible, considering that there is nothing which makes it impossible to form. However, such a planet cannot be of a gas giant size. It would be a terrestrial planet (most probably Mars sized) with vast amounts of chlorine-containing chemicals in its crust. These chemicals either suffer decay or are processed (by organic processes) by microbes to release the chlorine in them.
Your planet couldn't exist, because it's too hot for sulfuric acid oceans.
If your planet is Venus-like, it can't have sulfuric acid oceans. Liquid sulfuric acid only exists in the upper portions of the Venusian atmosphere. Droplets do collect into rain, but this evaporates and then dissociates into other molecules as it enters the lower portions of the atmosphere. Your planet would need to be significantly cooler for oceans of sulfuric acid to form.
EDIT: The question changed since I answered it, so I'll come back and fix this answer at some point.
