Zeolites' isotopes defy nature
New finding could help inform how zeolites are used in carbon capture and storage

Date:
October 7, 2021
Source:
Northwestern University
Summary:
Researchers have analyzed ancient zeolite specimens collected
from the edges of East Iceland to discover that zeolites separate
calcium isotopes in a wholly unexpected way.



FULL STORY ========================================================================== Zeolites could be considered as nature's workhorse.


========================================================================== Filled with microscopic holes and channels, these ultraporous minerals
can soak up environmental contaminants, filter drinking water, manage
nuclear waste and even absorb carbon dioxide (CO2).

Now, in the first study of its kind, Northwestern University researchers
have analyzed ancient zeolite specimens collected from the edges of East Iceland to discover that zeolites separate calcium isotopes in a wholly unexpected way.

"Calcium occurs as multiple isotopes having different masses," said
Claire Nelson, the paper's first author. "Most minerals preferentially incorporate lighter calcium isotopes. What we found is that some zeolites prefer lighter isotopes to an extreme degree, while other zeolites prefer heavier isotopes, a rare and striking result." This finding could help quantify temperatures in both modern and ancient geologic systems,
as well as inform efforts to mitigate human-caused climate change by
carbon capture sequestration.

The study was published on Friday (Oct. 1) in the journal Communications
Earth and Environment, a new open access journal established by Nature.



==========================================================================
"We discovered something completely unexpected and new," said Andrew
Jacobson, senior author of the study. "It could have wide ranging
implications in the geosciences and across fields, especially
considering that zeolites have countless applications in industry,
medicine and environmental remediation." Jacobson is a professor of
Earth and planetary sciences at Northwestern's Weinberg College of Arts
and Sciences. Nelson recently earned her Ph.D. working in Jacobson's
laboratory and is currently a postdoctoral research scientist at Columbia University's Lamont-Doherty Earth Observatory. Zeolite expert Tobias Weisenberger, a geologist at the University of Iceland's Breid-dasvi'k
Research Center, was a key co-author of the study.

Rappelling for rocks Although they form in a wide variety of geologic environments, zeolites are particularly common in volcanic settings
that produce basalt. As lava erupted from volcanoes piles up over time,
the buried rocks compress and transform.

Groundwater interacts with these rocks to form zeolites, which
comprise aluminum, oxygen and silicon atoms linked together to make three-dimensional cage-like structures.

"The initial volcanic lava crystallized into primary minerals," Nelson
said.

"Then water rained down and infiltrated the rocks, dissolved them and
produced secondary minerals like zeolites and calcite." To collect
samples for the study, Nelson visited the Berufjo"rd-ur-Breid-dalur
region in eastern Iceland, where glacial erosion has carved deep valleys
and fjords into basalt rock to reveal buried zeolites. Nelson climbed
to the top of the fjord's mountains and rappelled into the river canyon
to collect samples from various altitudes, representing different depths
of burial and thus temperatures of metamorphism.



==========================================================================
A weighty surprise To analyze these samples, Nelson used a
state-of-the-art, highly precise method for measuring calcium isotopes developed in Jacobson's laboratory. Nelson and Jacobson were particularly interested in identifying mechanisms that fractionate (or separate)
calcium isotopes according to their masses.

"For decades, geoscientists have employed zeolites to understand the hydrothermal alteration of basalt, but until now, calcium isotope
researchers had neglected them," Jacobson said. "As it turns out, the
minerals show extremely large calcium isotope fractionations, much larger
than anyone predicted or even thought possible." The Northwestern team
found that the zeolites showed extreme calcium isotope variability, more
so than practically all other materials produced at the Earth's surface.

After further analysis, Nelson discovered that this behavior directly correlates with bond lengths between calcium and oxygen atoms within the zeolites. Zeolites supporting longer bonds accumulate lighter calcium
isotopes, whereas those with shorter bonds accumulate heavier calcium
isotopes.

"Basically, heavier isotopes prefer stronger (or shorter) bonds,"
Nelson said.

"It's more thermodynamically favorable for stronger bonds to
concentrate heavier isotopes. Longer bonds energetically prefer lighter isotopes. Such observations are rare and inform what we know about the
behavior of calcium isotopes in general." Hot potential The results
have wide-ranging implications, as zeolites have multiple industrial
and commercial applications. In addition, understanding the mechanisms
that fractionate calcium isotopes can help inform both existing and new
uses of the calcium isotope proxy. Because isotope fractionation can
be temperature-dependent, Jacobson and Nelson say that zeolites could
be developed into a completely new type of geothermometer, potentially
capable of reconstructing ancient temperatures in environments where
zeolites form.

"The bond length relationship indicates that the fractionations
are controlled by thermodynamics rather than kinetics," Nelson
said. "Thermodynamic, or equilibrium, controlled fractionation is temperature-dependent. So, with more research, the calcium isotope
ratios of zeolites could be used to quantify temperatures from the past."
The new understanding also has significance for using calcium isotopes
to trace basalt weathering, including its role in long-term climate
regulation and application in carbon capture and storage.

The study, "Large calcium isotope fractionations by zeolite minerals
from Iceland," was supported by the National Science Foundation (award
number EAR- 1613359).

========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Amanda Morris. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Claire J. Nelson, Andrew D. Jacobson, Gabriella D. Kitch, Tobias B.

Weisenberger. Large calcium isotope fractionations by zeolite
minerals from Iceland. Communications Earth & Environment, 2021;
2 (1) DOI: 10.1038/s43247-021-00274-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211007101022.htm

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