Installing a Graphics Card in a Mac Pro Tower (mid 2010)

I have a Mac Pro tower (Mid 2010) with ATI Radeon HD 5770 1024 MB card. I swapped out the card and installed a Radeon RX 580 4GB Metal GPU Video Graphics Card. My research said it was compatible with my tower. After installing and starting up the computer I see the boot screen and the Apple Progress Bar. For about 35 seconds the progress bar moves until it is halfway across and then the screen goes black. I waited 10 minutes and nothing happened. Tried several times. I reinstalled my original card and everything is fine. Any suggestions on how I can get the card to work. I watched a youtube video in which the installer used both the 8 pin and the 6 pin cable together. I tried this as well but got the same result. The Card was listed as being for Mac Pro. I checked the System requirements on the box and it says supported operating systems include Linux, WIndows 10, Windows 7. Would there be a completely separate card and packaging for a Mac compatible version or should this card work for PC and Mac?