This situation is actually really messy
From this accepted answer from a question at Physics.SE we read:
If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works.
and the answer goes on to explain that issue.
Therefore, a big part of your problem is from whose perspective is the atmosphere purple? Keep in mind that from space you don't see a blue atmosphere. You see blue oceans due to light scattering and reflecting in the water.
It's your story, so ultimately the perspective is that of your readers. Your readers are human. So the atmosphere is purple from the perspective of humans. Except that we really stink at seeing purple in an atmosphere that we can breathe, which means you're only asking this from the perspective of humans in space suits where the faceplate doesn't get in the way of seeing purple light. And whatever is causing the purple must be whomping powerful because we're not designed to see it in the first place (rainbows notwithstanding, go read the entirety of that Physics.SE answer).
The next problem is that atmospheric color has almost nothing to do with star color. We have a really good question on this site about exoplanet sky colors with breathable atmospheres (Terrestrial Exoplanet Skies – I've Built a Visual Sky Chart. Is it Correct?) and what that question demonstrates is that with the exception of clouds and twilight, you'd still see a blue sky even with a red star. This makes sense when you remember that sky color is due to Rayleigh Scattering of the predominant chemicals (in our case, oxygen and nitrogen). This means that you'd need an atmosphere that naturally scattered the purple spectrum and not anything else because we really stink at seeing the purple spectrum.
User Rafael is correct that Iodine would be a good candidate. Remember, it must be the predominant chemical in the atmosphere. If significant amounts of either oxygen or nitrogen exist, we'll see blue because that's how we're designed. And if there's enough iodine in the atmosphere to do that, it would probably eat through the seals of your space suit.
As for what volcanoes and oceans would look like? Well, lava would look red and oceans (assuming they're full of water) would look blue. Atmosphere has nothing to do with what either looks like. Rayleigh Scattering is not a lens that we peer through.
Rafael's comment (which should have been an answer) is correct that Iodine will solve your problem: kinda. No human will ever breathe the atmosphere, and it would only look purple to humans and might not to any other creature — but there you are.