Cloud Peak Energy Inc , a unit of Rio Tinto Plc, fell in its debut on the New York Stock Exchange on Friday after its shares priced below expectations in the U.S. coal miner's initial public offering.

Shares opened at $14.50, more than 3 percent below the $15 IPO price and fell as much as 6.7 percent. In early trade, Cloud Peak shares were being quoted for $14.56, down 2.9 percent.

Gillette, Wyoming-based Cloud Peak sold 30.6 million shares for $15 apiece and raising about $459 million in its IPO on Thursday. But Cloud Peak had expected them to price for between $16 and $18.

Almost all of the proceeds will go to Rio Tinto and the Anglo-Australian miner will retain a 48.3 percent stake in Cloud Peak.

Rio's shares closed the day down 1.9 percent in Australia, and were down 1.4 percent in trade on the London Stock Exchange.

Cloud Peak is the third-largest U.S. producer of coal and owns surface mines in Wyoming and Montana.

Cloud Peak reported sales of $1.06 billion for the nine months ended September 30, up 17.4 percent over the year earlier results, with a profit of $190.1 million.

The IPOs underwriters, led by Credit Suisse, Morgan Stanley, and RBC Capital Markets, have the option to buy another 4.59 million shares.

(Reporting by Phil Wahba and Clare Baldwin, editing by Dave Zimmerman)