Natal house is the sign where a planet was originally placed during your time of birth.
If Mercury was placed in Gemini in your personal horoscope, then during transit, Mars forming conjunction (in Gemini), sextile (aspect of 60° which are Aries and Leo in this case), square (aspect of 90° which are Virgo and Pisces here), trine (aspect of 120° which are Libra and Aquarius here), opposition (aspect of 180° which is Sagittarius here) are discussed in this article.
Conjunction is transiting through same sign as in birth chart, sextile is transiting 3rd or 11th house, trine is transiting 5th or 9th house, opposition is transiting 7th house from natal sign.
Transit Mars conjunct Natal Mercury : In general this transit characterizes competition, debate and contest. The individual defends his ideas and view points in more aggressive and expressive fashion. The native is full of tremendous mental and intellectual energy which can be used beneficially by exercising some restraint. A good period to execute plans and increase the work efficiency. If the transit is under stress aspects then there is a tendency to nervous irritation, impulsive decisions and heated arguments which may cause occupational hazards and personal injuries.
Transit Mars Sextile Natal Mercury : The native asserts his personal ideas positively and dynamically and he is more inclined to work on the projects of his own benefit. This is a beneficial period for debates, intellectual contests, short trips and communications related to business, work and corporate affairs. A good period to enter into agreements related to insurances, taxes, joint finances, etc. Individuals in engineering, military studies, science and economics, display greater proficiency in such pursuits. The native is able to convey his ideas positively on sexual and emotional issues.
Transit Mars Square Natal Mercury : Irritable and touchy temperament, back-biting, sarcasm and heated arguments characterize this transit. The native defends his view points just for nothing and with heated arguments. Under this transit the native encounters difficulties in business communications and disagreements over joint finances, inheritances, taxes, insurances etc. Impulsiveness and irritation can cause automobile and fire arms accidents. Disagreements over work methodology may cause occupational hazards and accidents related to work and environmental noises and pollutions add to it. The native is in danger of infections and digestive problems.
Transit Mars Trine Natal Mercury : Correct decisions, planned actions and intelligently directed energy characterize this transit. A good transit for for initiative in study, writing, research, business communication, engineering and strategies for future goals. An excellent transit to enter into agreements. The craftsmen are ready to manufacture something worthwhile. A good period for police and military activities and strategies. The native can improve his health by proper diet, hygiene and exercise.
Transit Mars opposite Natal Mercury : Conflicts, aggressiveness and disagreements characterize this transit. Not a good transit for joint finances, corporate affairs, inheritances, and alimony. The native is irritated of his work environment and thus frustration mounts on. The native should exercise caution, in the use of sharp instruments and fire arms and while driving. The native is also in disagreement with his siblings and neighbours. The native is susceptible to infectious diseases and bilious disorders of digestive track.