^It's a sacrilege to tell people to heat ANY dope under ANY circumstances on BL! I do agree for the most part that in the vast majority of cases it's not necessary to heat dope, and I think in this case it's definitely not necessary.
OP, are you talking about like a rainbow colored "oil slick" floating on top like you'd see in a puddle with oil, if so I wouldn't personally worry about it too much, I've seen this occur with dope and with a lot of other things, certain beers for example. A micron filter will remove any particulate matter, but won't help you out with anything that completely dissolves into liquid. Street heroin is far from perfect...all you can do is be as safe and clean as possible. How safe and clean is up to you. The only people I've known who have ever had problems with bacterial infections or other related problems due to bacteria and other contaminants were either using extremely unclean IV practices or missing a lot of shots. To get a blood infection or endocarditis from using dirty equipment is very rare even if you're not an absolute fanatic about cleanliness....If you use a clean needle, clean filter and clean mixing receptacle every single time you shouldn't have any problems....
If the dope looks strange or you don't trust it, throw it away. The risk of IVing street drugs is that you have no idea what may be in them!
The whole idea behind "never heating heroin" is that heroin and most opiates(AFAIK) are extremely water soluble even at low temperatures. Anything that doesn't dissolve in water without heating ISN'T heroin, so anything left in the cooker after thoroughly stirring it around is just cut. Heating it until it all dissolves is just melting the solids into the cut, and they could re-solidify later on in your veins...which isn't good!
However, there are certain types of heroin and certain situations where I believe that adding enough heat to warm the water to a point below boiling that isn't enough to melt whatever cut is in the dope is helpful. For example: It's a cold day and you just copped a chunk of ECP that's as hard as a rock...you can't even break it with your fingers....Using hot water(not boiling) is the only way to get it to break down quickly enough to get it to dissolve. I've had ECP that did just sit there as a rock in water for a long period of time, the water will eventually penetrate, but it takes awhile...As far as tar goes, room temperature water will sometimes break it down instantly but a little heat makes it go much quicker! The thing is to use warm-hot water and not actually cook the shot!
I'm sorry if this offends anyone, because for some odd reason there's people on here who are so fanatical about this issue that they refuse to even discuss it or listen to anyone elses ideas on the matter, but the fact is that the vast majority of people who IV tar boil their shots, which in no way am I advocating, but to simply dismiss using any amount of heat when mixing heroin under any circumstances is ridiculous IMO....
Sorry for the rant....