Credit: IMOTHY A. CLARY/AFP/Getty Images

Mariah Carey, like millions of fans worldwide, is in mourning.

The R&B superstar took to Twitter Saturday immediately after news broke that her friend and peer Whitney Houston died at age 48.

"Heartbroken and in tears over the shocking death of my friend, the incomparable Ms. Whitney Houston," Carey wrote. Often framed as rival divas, Carey and Houston joined forces, belting it out together in the 1998 song "When You Believe."

"My heartfelt condolences to Whitney's family and to all her millions of fans throughout the world," Carey's tribute continued. "She will never be forgotten as one of the greatest voices to ever grace the earth."

The cause of Houston's death is not yet known. The Grammy-winning "I Will Always Love You" singer sold over 170 million albums worldwide, and is survived by daughter Bobbi Kristina, 18.