A mineral phase change in the Earth is a transformation where a mineral’s internal crystal structure rearranges itself into a different, more compact form. This phenomenon, known as polymorphism, occurs deep within the planet. The changes are not chemical reactions but represent a physical shift in how atoms are stacked together. These structural adjustments are responsible for major physical boundaries and processes that govern the deep Earth.
Defining Phase Changes and Their Triggers
A mineral phase change is a transition between two different solid forms, called polymorphs, that share the same chemical formula. For example, graphite and diamond are polymorphs of pure carbon with vastly different crystal structures. In the Earth’s interior, these structural shifts are driven by changes in external conditions.
The two primary forces that trigger these transformations are immense pressure and high temperature. As minerals are carried deeper into the mantle, pressure increases significantly, while geothermal heat causes temperature to rise. These conditions force the mineral to seek a new, more stable atomic arrangement.
The specific conditions under which a mineral is stable are known as its stability field. When pressure or temperature crosses a certain threshold, the mineral becomes unstable and quickly transforms. These thresholds define transition boundaries within the Earth, marking where one mineral phase gives way to another.
How Atomic Structures Rearrange
A phase change involves atoms restructuring into a more efficient, tightly packed arrangement. This rearrangement is a consequence of rising pressure, which forces the mineral to reduce its volume, making the resulting new phase always denser than the original.
To accommodate the extreme pressure, atoms shift positions to minimize empty space. This densification often involves a change in the mineral’s coordination number, which is the count of neighboring atoms surrounding a central atom. For instance, a silicon atom may switch from being surrounded by four oxygen atoms to six.
An increase in the coordination number allows ions to occupy smaller spaces, resulting in a more compact crystal structure overall. This microscopic adjustment is the physical basis for the significant density jumps observed in the Earth’s interior.
Major Transitions Inside the Earth
The most notable mineral phase changes occur within the mantle transition zone, located between 410 kilometers and 660 kilometers deep. These transformations are responsible for the seismic discontinuities used to map the planet’s internal layers. The most abundant mineral involved is olivine, which makes up a large portion of the upper mantle.
At approximately 410 kilometers, the alpha-olivine structure transforms into a denser polymorph called wadsleyite (a beta-spinel structure). This transition marks the upper boundary of the transition zone and results in a sharp increase in seismic wave velocity. The change is endothermic, meaning it absorbs heat, which influences local mantle flow.
Wadsleyite then transforms into an even denser polymorph called ringwoodite (a gamma-spinel structure) at around 520 kilometers deep. The major phase change occurs near 660 kilometers, where ringwoodite breaks down into two new minerals. This decomposition forms magnesium-silicate perovskite (now called bridgmanite) and ferropericlase, which make up the bulk of the lower mantle.
Impact on Planetary Dynamics
The density changes caused by mineral phase transitions affect the large-scale dynamics of the planet. Scientists detect these changes indirectly because denser material transmits seismic waves at a higher velocity. The sharp jumps in seismic wave speeds at 410 km and 660 km are direct evidence of these transformations.
The phase boundaries also control mantle convection, the slow churning flow of rock that drives plate tectonics. The 410 km transition is exothermic, releasing heat that encourages material flow across the boundary. Conversely, the 660 km transition is endothermic, meaning it impedes the vertical movement of material between the upper and lower mantle.
This effect on convection dictates how efficiently heat is transferred from the core to the surface. The mineral phase changes act as physical gateways, regulating the planet’s internal heat engine and influencing the cycling of material throughout the deep interior.