T. Boone Pickens Jr., famous oil tycoon turned evangelist for alternative fuels and American energy independence, has died at the age of 91 of natural causes.

Born in Holdenville, Oklahoma, Pickens made his fortune in the oil industry founding Mesa Petroleum and in the 1980’s became known as a corporate raider, buying and attempting to take over several companies such as Gulf Oil and Phillips Petroleum.

Later in his life, Pickens was known as an advocate for American energy independence through increased investment in alternative fuels and energy, especially for natural gas, which he felt was ideal for the commercial vehicle industry. He also supported renewable forms of energy such as solar and wind power.

He was also the co-founder of Clean Energy Fuels, a natural gas fuel provider who’s board he served on for 20 years.

“Words cannot express the sadness the Clean Energy family and myself feel at the loss of our co-founder, leader and friend,” said Andrew Littlefair, president and CEO, Clean Energy. “I worked closely with Boone for 32 years and he became a father-like figure and true mentor to me.”

Married and divorced five times, Pickens is survived by his two sons and three daughters.