U.S. stocks survived Greece's 'no' vote with only minor losses Monday, but oil prices weren't so lucky. All three major stock indexes closed lower, but the real turmoil was found in the commodities market. The drama in Greece fanned fears of lower global economic growth, which sent oil prices sharply lower by 7.4 percent to $52.74 a barrel. The looming Tuesday deadline for nuclear negotiations with Iran also put downward pressure on prices. This sent shares of energy stocks lower. Transocean (RIG) fell 4.2 percent. About 61 percent of voters in Greece rejected a bailout package demanded by Greece's European creditors and the International Monetary Fund. The 'no' vote on Sunday leaves the possibility of a 'Grexit' wide open, as experts said Greek banks have just days of liquidity remaining. A 'Grexit' would mean a return to the drachma for Greece or the creation of an alternative currency. Meanwhile, health insurance giant Aetna (AET) agreed to purchase Humana (HUM) in a deal announced Friday for roughly $230 a share, or $27 billion. Aetna was the worst performing stock in the S&P 500. Shares dipped almost 7 percent.