The date 1891 would make it a Morgan dollar, so the mint mark is on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
By the mint mark on the reverse of the coin, but silver dollar coins with no mint mark are made in Philadelphia
The mint mark on any Morgan dollar is on the reverse above the letters DO in dollar.
No mint mark means the coin was minted in Philadelphia.
The mint mark (if it has one) on all Morgan silver dollars is on the reverse of the coin just above the letters "DO" in dollar.
above the DO in dollar on the back :]
By the mint mark on the reverse of the coin, but silver dollar coins with no mint mark are made in Philadelphia
No mint mark means it was minted in Philadelphia.
The mint mark on any Morgan dollar is on the reverse above the letters DO in dollar.
If there's no mint mark, then it was minted in Philadelphia.
No mint mark indicates that it was made in Philadelphia.
No mint mark means the coin was minted in Philadelphia.
The mint mark (if it has one) on all Morgan silver dollars is on the reverse of the coin just above the letters "DO" in dollar.
above the DO in dollar on the back :]
Not always, no. If it was minted in Philadelphia, then there is no mint mark.
For 1891, the value is really determined by the mint mark. One with no mint mark is worth $22 to $30, depending on condition. An 1891-O is worth $22 to $90. The '91-S is $22 to $50. Then the more elusive '91-CC is worth at least $50 and as much as $200.
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The US has never used a U mint mark. In 1891 Morgan dollars were minted in Philadelphia (no mint mark), New Orleans (O), San Francisco (S) and Carson City (CC). Please see the Related Question for more.