Ammonite Fossil
Image: <a href="//commons.wikimedia.org/wiki/User:Tomomarusan" title="User:Tomomarusan">Tomomarusan</a> (CC BY 2.5)
Mineral NameAmmonite Fossil
Chemical FormulaCaCO3 (original shell, often recrystallized); SiO2 (common infill)
Mohs HardnessVariable, 3-7 (depending on infilling minerals)
Streak ColorWhite to light brown
CleavageVariable, perfect rhombohedral (if calcite is present)
FractureConchoidal to uneven
Specific GravityVariable, 2.6-2.8
Colorsbrown, white, grey, translucent
TransparencyOpaque to translucent
Type LocalityWorldwide in marine sedimentary deposits
Mineral GroupFossil (composed primarily of carbonates and silicates)
Usescollector's item, scientific study, decorative item
Associated Mineralscalcite, quartz, chalcedony, aragonite (original shell material), pyrite
Description
A polished cross-section of a spiral ammonite fossil, revealing internal chambers filled with translucent white to grey mineral growth, likely quartz or chalcedony, surrounded by a matrix of brown and white fossilized shell material.
Geological Context
Ammonites were marine cephalopods that lived during the Mesozoic Era. Their shells were preserved as fossils in sedimentary rocks, often undergoing diagenesis where original shell material is replaced or internal chambers are filled by minerals like calcite or silica.
