Why is my particle transparent?

As you can see, the particle has an outer area, which is supposed to be transparent, and an inner area which is not supposed to be transparent.
I use a texture mask to try to create this effect. The white area should be 1 or above and thus completely opaque? Yet other particles can be seen behind it.
Why is that?

You’re adding 0.1 to it, so anything black is going to become 0.1 (a very dark gray) and therefore visible. Remove the add node if you want the fully masked effect.

I assume that in the end you plan on using a non-square shape, masking a square using a square is… wasteful to say the least.

No no, I want the black area of the mask, the outer part of the particle, to have a slight visibility, that’s why I add 0.1. Maybe there’s other, more efficient ways to… anyway, the white area of the mask, the center part of the particle, should not have transparency, yet it does. I don’t get it.