If a gram of antimatter iron were placed on a sidewalk in the absence of ordinary iron or any other matter, it would not immediately annihilate. Annihilation occurs when antimatter encounters its corresponding matter counterpart, leading to the conversion of mass into energy.
In this case, antimatter iron would not undergo annihilation because there is no ordinary matter iron to interact with. Instead, the antimatter iron would simply exist as antimatter, with its own distinct properties.
However, it's important to note that storing or handling antimatter is extremely challenging due to the potential for annihilation upon contact with ordinary matter. When antimatter comes into contact with matter, mutual annihilation releases an enormous amount of energy in the form of gamma rays, which can be highly destructive.
In practical terms, producing and containing significant quantities of antimatter is currently beyond our technological capabilities. It requires complex and specialized facilities such as particle accelerators, where antimatter can be produced in controlled environments and stored using magnetic fields to prevent contact with matter.