Can the marriage contract of a zaaniyah be annulled?
Question

                If a married woman commits zina, is she still a wife, or is her marriage contract annulled and does she become divorced because of this action?.
Praise be to Allah.
    If a married woman commits zina, her marriage contract is not 
    annulled and she does not become divorced because of her committing this 
    sin, but if she does not repent and she persists in this evil action, her 
    husband is enjoined to divorce her, so as to protect his honour and his 
    children. 

    Ibn Qudaamah (may Allaah have mercy on him) said: 

    If a woman commits zina with a man, or her husband commits 
    zina, the marriage contract is not annulled, whether that occurs before or 
    after consummation, according to the majority of scholars. This was the view 
    of Mujaahid, ‘Ata’, al-Nakha’i, al-Thawri, al-Shaafa’i, Ishaaq and ashaab 
    al-ra’y. But Ahmad regarded it as mustahabb for the man to leave his wife if 
    she commits zina, and he said: I do not think that he should keep such a 
    woman, because there is no guarantee that she will not be unfaithful to him  
    and attribute to him a child who is not his. Ibn al-Mundhir said: Perhaps 
    those who said that keeping this woman is makrooh did not mean that doing so 
    is haraam, so it is similar to this view of Ahmad’s.  

    Ahmad said: And he should not have intercourse with her until 
    it has been established that she is not pregnant by waiting for three 
    menstrual cycles. 

    But it is more likely that this may be established by waiting 
    for one menstrual cycle. End quote. 

    Al-Mughni (9/565). 

    It says in Kashshaaf al-Qinaa’ (5/2): 

    If a woman commits zina before or after consummation, the 
    marriage contract is not invalidated, or if a man commits zina before or 
    after consummating the marriage with his wife, the marriage is not 
    invalidated by zina, because it is a sin that does not put one beyond the 
    pale of Islam, like stealing, but he should not have intercourse with her 
    until she has observed an ‘iddah, if she is the one who committed zina. End 
    quote. 

    Shaykh al-Shanqeeti (may Allaah have mercy on him) said: 

    It should be noted that if a man marries a woman thinking 
    that she is chaste, then she commits zina when she is married to him, the 
    more correct of the two (scholarly) opinions is that the marriage is not 
    annulled, and it is not haraam for him to continue with the marriage. This 
    was the view of some of those who did not allow marriage to a zaaniyah, as 
    they differentiated between continuing such a marriage and initiating it. 

    Those who held this view quoted as evidence the hadeeth of 
    ‘Amr ibn al-Ahwas al-Jashami (may Allaah be pleased with him) who was 
    present during the Farewell Pilgrimage with the Messenger of Allaah  
    (peace and blessings of Allaah be upon him). He praised and glorified 
    Allaah, and reminded and exhorted (the people), then he said: “I enjoin good 
    treatment of women, for they are prisoners with you, and you have no right 
    to treat them otherwise, unless they commit blatant sin. If they do that, 
    then forsake them in their beds and hit them, but without causing injury or 
    leaving a mark. If they obey you, then do not seek means of annoyance 
    against them….” 

    Al-Shawkaani said concerning this hadeeth of ‘Amr ibn 
    al-Ahwas: It was narrated by Ibn Majaah and al-Tirmidhi, who classed it as 
    saheeh. Ibn ‘Abd al-Barr said in al-Istee’aab in his biography of 
    ‘Amr ibn al-Ahwas: His hadeeth about the Farewell Sermon is saheeh. End 
    quote. 

    His hadeeth about the Farewell Sermon is this hadeeth, based 
    on the words, He praised and glorified Allaah, and reminded and exhorted 
    (the people). This reminding and exhortation refers to the sermon as is well 
    known. 

    Thus you may understand that the opinion of one who says that 
    if a man’s wife commits zina, the marriage is annulled and she becomes 
    haraam for him, is contrary to the correct view, and Allaah knows best. End 
    quote. 

    Adwa’ al-Bayaan (6/82, 83) 

    And Allaah knows best.