Venus doesn't have liquid water, which is needed for the reaction of silicates with CO2 ("weathering"). Without that reaction, CO2 just accumulates in the atmosphere. Most of the carbon on Earth (and there's a lot) is locked up in rocks.
There's also a biological effect. Here on Earth, silica in the ocean is scrubbed out by microorganisms that create silica shells; these tiny shells fall out into sediments, where (in deep ocean) they eventually form a kind of biogenic rock called "chert". Elsewhere, typically in shallow water, carbonate rocks are formed from the remains of other kinds of animals. Without these effects, the dissolved silicon concentration in seawater would be orders of magnitude higher, and the silica would react to form clays. This reaction would acidify the ocean and prevent carbonate formation.
Just such "reverse weathering" has been hypothesized to occur after the Permian-Triassic boundary, where CO2 levels stayed elevated for 5 million years. The extinction event was so severe it disrupted chert formation (a "chert gap").
Venus doesn't have liquid water, which is needed for the reaction of silicates with CO2 ("weathering"). Without that reaction, CO2 just accumulates in the atmosphere. Most of the carbon on Earth (and there's a lot) is locked up in rocks.
There's also a biological effect. Here on Earth, silica in the ocean is scrubbed out by microorganisms that create silica shells; these tiny shells fall out into sediments, where (in deep ocean) they eventually form a kind of biogenic rock called "chert". Elsewhere, typically in shallow water, carbonate rocks are formed from the remains of other kinds of animals. Without these effects, the dissolved silicon concentration in seawater would be orders of magnitude higher, and the silica would react to form clays. This reaction would acidify the ocean and prevent carbonate formation.
Just such "reverse weathering" has been hypothesized to occur after the Permian-Triassic boundary, where CO2 levels stayed elevated for 5 million years. The extinction event was so severe it disrupted chert formation (a "chert gap").