Mercury in Aquarius, the independent thinker
Mercury in Aquarius means an original, independent mind that thinks in systems and isn't moved by what everyone else believes. You see the pattern behind a problem, you argue from logic, and you'll take the unpopular side if the reasoning holds.
Mercury is how you think, talk and learn. Aquarius is fixed air, traditionally ruled by Saturn, with Uranus as its modern ruler. The mind here is cool and structured, and unconventional with it, and when it settles on an idea it stays settled.
Devil's advocate by default
When everyone in a conversation agrees, you feel an itch. You ask "but why does it have to be that way?" and mean it as a real question. You're fascinated by how things work underneath: the rules of a game, the logic of a city's bus network, the reason a tradition started. Explaining one of these to someone who didn't ask is one of your favorite things.
You learn on your own terms. You taught yourself more from the internet at two in the morning than from any classroom, and you resisted teachers who said "because that's how it's done". Your messages are thoughtful, sometimes long, and include a link to something nobody else in the chat has heard of.
What you notice first is the flaw in the system: the rule that makes no sense, the process everyone works around instead of fixing. You notice it in your first week at a new job and say so in your second.
Right, and alone with it
The honest tension is detachment. Someone comes to you upset, and you offer a framework. You're trying to help, and the logic is sound, but they wanted company in the feeling first. People call you hard to read, then find you're one of the most loyal friends they have.
The other tension is that Aquarius is fixed. You're open to any idea in the world except changing your own, and an argument with you can go on for a week by message. Being early with an idea also costs you: you said it two years before it caught on, and nobody remembers that you did.
With your Sun and the rest of your chart
Mercury stays close to the Sun, so your Sun is in Capricorn, Aquarius or Pisces. With a Capricorn Sun, a conventional-looking person holds startling opinions and voices them calmly. With an Aquarius Sun, independence runs through everything you think and are. With a Pisces Sun, a soft, dreamy person turns out to have a sharp, clear logic when it counts.
The opposite placement, Mercury in Leo, argues from the heart and takes every point personally, which is exactly what you don't do. The Mercury sign guide shows how to find yours.
Questions
How does Mercury in Aquarius communicate?
Logically, frankly and with a lot of questions about why things are the way they are. You prefer ideas to small talk and argue for the fun of it.
Is Mercury in Aquarius smart?
It's an original, systems-level intelligence: brilliant at seeing patterns and questioning assumptions. It's weaker at reading the emotional side of a conversation in real time.
Is Mercury in Aquarius stubborn?
About its own conclusions, yes. You'll consider any idea, but once you've reasoned your way to a view, you need a better argument, not more pressure, to move.