Is spadonium listed as a scanned resource?

Spadonium is in fact listed when you scan a planet. This is how I found mine.

When you're on a planet, Spadonium will be indicated by a purple blip with a white rectangular block in it after performing a scan.

Here is a screenshot:

Keep on looking!

Based on the top comment on this post, it looks like if a planet has Spadonium, it will show up in the scan:

You can scan the planets from space. There is always one violet resource so if there is no Spadonium scan another planet.

Posts in this thread also say that it will show up in scans:

I got Spadonium - had to jump 6 systems to find a planet where it was listed as a resource on the scanner.

However, it's worth noting that Spadonium is found in cacti in desert biomes. From the Spadonium wikia page:

this element can be recovered from specific cacti on desert biomes.