Writing to mutable property for a struct record is not allowed in F#. Why?

When I have the following code:

[<Struct>]
type Person = { mutable FirstName:string ; LastName : string}

let john = { FirstName = "John"; LastName = "Connor"}

john.FirstName <- "Sarah";

The compiler complains that "A value must be mutable in order to mutate the contents". However when I remove the Struct attribute it works fine. Why is that so ?