Every rock on earth is composed of minerals, with there being a very wide range of compounds
and native elements.
Since there are currently 6,031 known minerals, some are more common while others are less
common.
So, which mineral is the most common to be found by volume in Earth?
It turns out to be a mineral that despite being so common, the vast majority of the
world's population has never seen it.
I am not referring to quartz, the form of silicon dioxide which comprises 12% of Earth's
crust and I am not referring to the feldspar group of minerals which comprise 51% of Earth's
crust.
Instead, I am referring to the mineral bridgmanite, which by volume comprises 38% of our planet.
This means that 2.2699 sextillion metric tons of bridgmanite exists in Earth, so why did
it take until 2014 for this mineral to be discovered?
The answer is that bridgmanite is the primary constituent of Earth's lower mantle which
is so deep that its minerals essentially cannot be directly observed by scientists, specifically
at between 660 and 2900 kilometers or 410 to 1802 miles depth.
At these depths, bridgmanite comprises 75% of the mantle, with the other two primary
minerals present being ferropericlase and perovskite in decreasing abundances.
Before I describe how this absurdly common mineral formed, I want to elaborate on why
it took until 2014 to be discovered.
For context minerals in the very top portion of the upper mantle such as olivine are somewhat
commonly brought to the surface via the upward motion of magma which results in volcanic
eruptions, with an example of this occurring 580,000 years ago in Arizona's extinct San
Carlos volcanic field.
This emplaced what is today sourced as 90% of the world's peridot.
Olivine is fairly common and stable at the very top of the mantle which can be as little
as 5 kilometers depth all the way to 410 kilometers depth, with most of this peridot in the San
Carlos field originating in the upper mantle's uppermost section.
To get minerals which occur deeper in the mantle, you generally need kimberlite pipes
which not only transfer diamonds emplaced at 150 to 200 kilometers depth to the surface
from the thick lithosphere of cratons, but can also sometimes even transfer amounts of
olivine's deeper cousin wadsleyite at 410 to 520 kilometers depth or ringwoodite at
520 to 660 kilometers depth.
The reason the same chemical formula forms different minerals at increasing depth relates
to the extreme pressure and heat rearranging atoms into the most stable arrangement at
those conditions.
As for bridgmanite, it forms due to the decomposition of the mineral ringwoodite into the minerals
bridgmanite and ferropericlase at 660 to 2900 kilometers depth.
With a melting point of 2,780 to 5,330 degrees Celsius depending on pressure the mineral
experiences, it does not melt in even the lowermost mantle where temperatures reach
3700 degrees Celsius.
Bringing lower mantle minerals to the surface is near impossible except for one very infrequent
circumstance, meaning that to understand what our planet's lower mantle and core are composed
of we need to turn to a completely separate group of astronomical bodies; asteroids.
Asteroids represent material left over from the formation of our solar system, with some
fragments representing what was once the mantle or core of a long ago destroyed protoplanet.
While this is an extreme oversimplification, chondrites can represent the crust, pallasites
can represent the mantle, and iron nickel meteorites can represent the core of differentiated
asteroids where denser materials sunk into the core and lighter materials rose towards
the surface.
You might thus think that a specimen of bridgmanite was found in a pallasite or iron nickel meteorite,
but instead it was found in a chondrite meteorite which fell to earth in Australia in 1879.
While this class L6 chondrite would ordinarily not have contained the very high pressure
mineral bridgmanite, it did because long ago it experienced shock metamorphism from asteroid
impacts which created small amounts of this mineral.
As a final note, I would like to thank this channel's new patron TwCory for supporting
this channel!