Mercury in the 12th House is in house of friend Saturn. 12th House is the house of Monestaries, Losses, Imagination and Physical Intimacy. Mercury is not the most ideal place in 12th House especially when it is with Sun at a very close angle. Having said that it does wonders if Mercury is placed along with Lord of 3rd House or if Mercury also happens to be the Lord of 3rd House.

   Traits of Mercury like talkativeness, being over-smart, analytical and calculative approach as well as smart work is not welcome in the House of Confinement aka the 12th House. Generally speaking no planet is welcome in the 12th House apart from Ketu and Jupiter but planets like Mercury and Venus are all the more not welcome. Mercury maybe smart but knowledge is shallow. To understand the depth of 12th House is beyond its comprehension. Mercury with Sun or Venus in 12th House is all the more disastrous. Many divorce cases have Mercury along with Sun or Venus in the 12th House. 3rd House deals with Creative Writing and 12th House rules over Imagination. A person can be a great fictional writer if there is Mercury in 12th House and shares some association with 3rd House or is the Lord of 3rd House. Many famous authors like J. K. Rowling have placement of Mercury in 12th House and association with 3rd House. 

     According to B. V. Raman: "Native with Mercury in 12th House maybe poor, susceptible to mood disorders and wayword in approach. Native may have difficulties in getting progeny as well as be susceptible to extra-marital affairs". Mercury in 12th House may creates difficulties with communication. Mercury's 12th House placement along with Sun or Jupiter maybe great for Spirituality but not so great for marital life. Aspect on 6th House is not bad unless enemy planets are involved. 

   Mercury in 12th House may have some positives but mostly this is an undesired location for Mercury. Native can become a wonderful writer especially of fictional novels. If there are difficulties in marital life then it is best to pray to Lord Vishnu or Goddess Laxmi. 

 References:
Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.