Catechism of the Catholic Church §2383 — excerpt
The separation of spouses while maintaining the marriage bond can be legitimate in certain cases provided for by canon law. If civil divorce remains the only possible way of ensuring certain legal rights, the care of the children, or the protection of inheritance, it can be tolerated and does not constitute a moral offense.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: CIC, cann. 1151-1155