Final answer:
The reaction to dilute acid is indicative of carbonate minerals like calcite or aragonite in fossils. The fizzing reaction occurs as these carbonates react with the acid. Apatite, which is found in altered fossils, does not fizz in acid as it is harder and not a carbonate.
Step-by-step explanation:
The student's question pertains to the characteristics and identification of fossils and the minerals present within them. The fizzing reaction in the presence of dilute acid indicates the presence of carbonate minerals such as calcite or aragonite, which are common in unaltered fossil shells. When these carbonate minerals come into contact with acids, they undergo a chemical reaction producing carbon dioxide, which leads to fizzing. This reaction is exploited by geologists and paleontologists to identify carbonate minerals in rocks and fossils.
Fossils, including those from organisms like bivalves and corals, can become altered, where the original biologically produced minerals are replaced with different ones. The mineral apatite, with a hardness of 5, does not fizz in acid because it is not a carbonate and is generally harder than glass. Apatite can sometimes be just a coating, with the entire fossil being replaced by this mineral in some instances. Unaltered fossils containing calcite, which has a hardness of 3 and does fizz in acid, are typically softer and can be indicative of recent creatures or not fully fossilized remains.