Mercury in Capricorn:
With Mercury in Capricorn, natives have a strong need to compartmentalize the impressions they get from the world. These people often speak and write in a slow and methodical manner. They appreciate structure and order. Although they are resourceful people, Mercury in Capricorn natives easily get frustrated when too much information is thrown at them at once. They break down ideas into manageable parts before arriving at a decision.
Usually quite mentally active, these natives are generally productive people who prefer taking on projects that will yield tangible and practical results.
These natives can have an authoritative air about them. They don’t appreciate being taken off guard, and they quickly become impatient with people who don’t honour their word. There’s a bit of a law-maker in this position of Mercury. These people can seem a little strict and judgmental with others, and they are easily bothered when others don’t seem to “get” things that seem extraordinarily obvious to them.
Mercury in Capricorn people can have seemingly crystal-clear judgment, although their thought processes can be skewed to the practical world. They too easily reject what they deem to be frivolous ways of thinking. Not given too much fancy, these natives’ minds are oriented toward practical tasks.
There’s a bit of a doubting Thomas in Mercury in Capricorn people. Many squint often–not due to poor eyesight, but because their first reaction to information is scepticism! It’s more of a furrowing of the eyebrows, and it’s often seen in people with a strong Saturn (Capricorn’s ruling planet) in their charts as well.
With the Sun in Sagittarius or Aquarius, these people may come across as more hard-nosed, conservative, and practical than they actually are! The reason for this is that they process, and relay, information (Mercury) in a Capricorn way.
The style of communication with this position of Mercury is generally of the no-frills variety. Capricorn is a sign that hates waste and values time. They will generally be rather concise in their presentation, although they do take their time when they make plans–they consider it an investment in the future.
Many natives of Mercury in Capricorn choose their words carefully. They fear coming across as inept, so they may be a little stiff in presentation. Even those who seem quite smooth (and many of them do achieve this) will likely have fretted over their ability to communicate and their intellectual abilities.
Communication difficulties are generally most obvious when Mercury in Capricorn people are relating to those with Mercury in Aries (who find them too deliberate and methodical), Mercury in Cancer (who think they’re too cold and practical), and Mercury in Libra (who find them too judgmental). Mercury in Capricorn people have their gripes with them too–Aries seem too hasty, Cancer too emotional, and Libra too argumentative. The key to overcoming these challenges lies in understanding that they are the result of differences in styles of communication. (Source).