harry potter - What spell almost killed Hermione in The Order of the Phoenix?

The spell was cast without words, simply a flick of the Death Eater's wand, and a 'streak of purple flame' that lanced across her chest. It was later mentioned that the spell was weakened because of the lack of a verbal component, but still 'did enough damage as it was'. She was in the infirmary healing from the damage for some time. What spell was this?

Answer

The spell that Dolohov hit Hermione with was actually purple:

But the Death Eater Hermione had just struck dumb made a sudden slashing movement with his wand; a streak of what looked like purple flame passed right across Hermione’s chest. She gave a tiny ‘Oh!’ as though of surprise and crumpled on to the floor, where she lay motionless.

_{Order of the Phoenix - page 698 - Bloomsbury - chapter 35, Beyond the Veil}

I looked through the spells at the Harry Potter Lexicon and the one spell that seemed possible is a revulsion jinx. The revulsion jinx repels and emits purple light when cast. This is the spell Hermione used on Yaxley in Deathly Hallows to shake him off the trio, when he had grabbed hold of Hermione as the trio attempted to Apparate from the Ministry of Magic to No twelve Grimmauld Place after they infiltrated the Ministry and retrieved the Slytherin locket from Umbridge (Deathly Hallows, chapter 13, The Muggle-born Registration Commission). A revulsion jinx could be considered an offensive spell, probably as well as a defensive one.

It's interesting that Order of the Phoenix indicates a non-verbal spell might be less effective than a verbal spell. Snape doesn't mention this when he teaches non-verbal spells in Half-Blood Prince.