NEW YORK (AP) â¿¿ Shares of Stillwater Mining Co. tumbled Thursday, after the palladium and platinum producer announced plans to sell $300 million in notes. THE SPARK: The Billings, Mont.-based company wants to sell $300 million of convertible senior notes due 2032. The company also plans to grant the offering's underwriters a 30-day option to buy up to an additional $45 million in notes to cover possible overallotments. THE BIG PICTURE: The notes will pay interest semi-annually and will be convertible, under certain circumstances and during certain times, into cash, stock or a combination, at the company's choosing. Stillwater plans to use net proceeds from the offering to repay some debt that may come due in March. Any additional proceeds may be used for general corporate purposes. THE SHARES: Down 88 cents, or 7.2 percent, to $11.25 in midday trading, after dipping as low as $11.10 earlier in the session. Over the past 52 weeks, the company's shares have traded between $7.47 and $15.24.