質: when to pronounce in 2nd tone or 4th (Mandarin)? In Mandarin, sometimes 質 is pronounced with the 2nd tone (e.g., 質地, 特質) and other times it's pronounced with the 4th tone (e.g., 質疑, 質量, 品質).

Here are two words pronounced with the 2nd tone:


質地 http://www.cantonese.sheik.co.uk/dictionary/words/11242/
特質 http://www.cantonese.sheik.co.uk/dictionary/words/48232/


There doesn't appear to be a clear pattern.

Does anyone know any rules for when to pronounce 質 with 2nd tone vs. 4th tone?
You should always pronounce the 4th tone in the standard mandarin. 

Sometimes, I heard people wrongly pronounced the 2nd tone in the words of 质量，质地, which is probably acceptable to a native ear. But again that's wrong pronunciation.   

But if you pronounce the 2nd tone for 质疑，品质, it's really bad (at least to me). 

An interesting observation, in a online dictionary, I search for 质地， and it's marked the 4th tone for simplified and 2th tone for traditional one.  So, I doubt this could be regional thing as fefe suggested in his/her answer.