Susan Buffett was president of the Buffett Foundation

Susan Buffett and her husband, believed to be the world's second richest man, were visiting friends in Wyoming at the time of her death.

Susan Buffett was a director of Berkshire Hathaway, the holding company controlled by her husband which invests in some of the world's largest firms.

Forbes magazine this year valued Susan Buffett's personal fortune at $3.1bn.

Rich ranking

The magazine ranked Mrs Buffett as the 153rd richest person in the world.

In a statement, the Buffett Foundation-of which Susan Buffett was president-praised her commitment to social justice and her work in the fields of education and reproductive health.

Warren Buffett is commonly known as the Sage of Omaha for his shrewd reading of market trends and his ability to invest in profitable and reliable companies.

Forbes magazine this year estimated his net worth to be $42.9bn(�23.5bn) dollars, second only to Microsoft co-founder Bill Gates.