Bill
Pictures are quite small but I think what you have is pieces of the Challenger Space Shuttle that fell in this area in the 1986 tragedy. I have some myself. Not to common but most my friends here have found some and samples were sent to NASA and verified. The metal is heaver then aluminum, but lighter than silver and very hard. I also have found silver in the form of ore here on the Treasure Coast Beaches.
I took a sample to Bill, the head of the Mel Fisher Conservation Lab here in Sebastian. He said that the Spanish usually brought refined silver back to Spain but they might have been carrying a small amount of ore. His only other explanation is that it is known that the survivors did burn some of the silver on the beach, that they could not take with them, to keep the Indians and Pirates from getting it. Their was at least one Dutch ship that raided the salvage operation and the Ayes Indians were always taking what they could.
Hope this helps
Bob