Many minerals commonly occurring in modern sediments and rocks are too unstable to survive in great abundance in older terrestrial rocks. For example, olivine was once abundant in many terrestrial Precambrian mafic rocks, but since the Precambrian era, most old olivine has been altered by oxygen, carbon dioxide, and water to make serpentine, iron oxides, and magnesite. Because olivine crystallizes in hot and dry magmas, it is thermodynamically unstable under much cooler and wetter surface and near-surface conditions. Because of olivine’s and pyroxene’s tendency to weather rapidly, detrital olivine and pyroxene are largely restricted to Cenozoic sediments and sedimentary rocks. However, Precambrian olivine and pyroxene occur in Moon rocks and meteorites that have been isolated from oxygen and water.
Other examples of minerals generally absent from older terrestrial rocks include tridymite, a hightemperature polymorph of quartz, and aragonite, a high-pressure polymorph of calcite. Tridymite is common in Cenozoic siliceous volcanics, including rhyolites, obsidian, and andesites. However, except in stony meteorites and lunar basalts, the mineral changes to quartz over time and is rarely found in rocks that are older than Tertiary age.
Many marine organisms excrete shells that consist of aragonite rather than calcite. Unless aragonite fossils are deeply buried, they will alter to calcite over time. The oldest known aragonitefossil is from an organic-rich shale of Mississippian age. Geologists have only found Paleozoic aragonite fossils in three localities. One of the rocks is tuffaceous, while the others are black shale and asphaltic limestone. The presence of abundant organic matter in three of the four known rocks with Paleozoic aragonite is probably responsible for the preservation of the aragonite. The organic matter coated the fossils and probably prevented water from reaching them and promoting their conversion to calcite.
Some non mineral materials are unstable and invert to minerals over time. Opal and volcanic glass are amorphous materials and not minerals. Over time, both weather or alter into more stable crystalline compounds, such as quartz. Obsidian is rarely found in rocks older than the Miocene. The oldest known volcanic glass is in a 70-million-year-old welded tuff. Opal is slightly more stable than obsidian. Reaction rate calculations indicate that opal will entirely convert to quartz in about 180 million years at 20°C (70°F) 50°C (120°F) , approximately 4.3 million years at , and in only about 47 years at diagenetic temperatures of 200°C (390°F) . Not surprisingly, the oldest known opal is of Lower Cretaceous age, about 125 million years old.
Source
A Text Book Of Geology, By- G.B Mohapatra