Oxygen is the eight element on the periodic table. When you encounter oxygen in it's standard state, it is a diatomic element - two oxygen atoms are combined to form one molecule of O2. Oxygen gas (as opposed to oxygen the element), is a colorless and odorless gas. It comprises about 20% of our atmosphere and is pretty important to life as we know it. Breathe in, breathe out - you've just taken in about 0.15g of oxygen from the air around you.
Conversely, oxygen is the most abundant element in the crust of the earth - it is about 46% by mass oxygen.
Why the difference? Almost all of the minerals that make up the earth are oxides - other elements combined with oxygen. Sand is primarily silicon dioxide, SiO2 (one atom of silicon bonded to two atoms of oxygen). Hematite, the main ore that iron is mined from, has the chemical composition of Fe2O3 (two atoms of iron bonded to three atoms of oxygen).
Figure 1 - common oxide minerals (Image credit: https://openpress.usask.ca/physicalgeology/chapter/5-3-mineral-groups-2/)
Where did all of this oxygen come from? NASA funded studies have suggested that oxygen has been present on Earth for at least 2.5 billion years. Around this time, there was a dramatic increase in oxygen levels in the atmosphere that researchers are calling the Great Oxidation Event. Studies suggest that this oxygen was in the diatomic form and was probably due to the rapid evolution of biological lifeforms.
Cache information: The cache is a magnetic nano so bring your own pen. Should be wheelchair accessible.
Sources:
https://en.wikipedia.org/wiki/Great_Oxidation_Event
https://www.nasa.gov/home/hqnews/2007/sep/HQ_07215_Timeline_of_Oxygen_on_Earth.html
https://www.quora.com/What-is-the-mass-of-oxygen-inhaled-during-one-normal-and-full-breath
http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/elabund.html
https://openpress.usask.ca/physicalgeology/chapter/5-3-mineral-groups-2/