Hello.
My mercury is in scorpio too along with pluto... But they are not conjuncting.
-Mercury in 9º of scorpio - 4th house
-Pluto in 21º of scorpio - 5th house
The combination you are talking (mercury pluto conj. in the same degree) seems very intense. That sounds a very paranoic mercury ready to go deeper and deeper in researchs and studies. It also seems a person who 'see' the reality. In contrast with mercury-neptune or mercury-moon (here i got the opposition), this 2 last aspects twist mind... make mind a bit infantil and dreamer... Mercury-pluto goes to the hard reality. Sometimes they can be very good watcher and very secretive, not trusting in people.
That's why it's associated with the mind of a detective, cause it can easily see what others don't want.
I share many of this traits but that's because my mercury is in scorpio.
If the conjunction happens in 8th then mercury would research about things associated with this house: tabus, sex, death, dark things...
Fragoso