by Adelheid Fischer

Laurence Garvie holds up a small jar that contains what looks like white sand. “This is every man’s worst nightmare,” he says with a smile. “It’s calcium oxalate, the stuff of which kidney stones are made.”

Garvie is referring to the medical condition in which crystals of calcium oxalate form blockages in the urinary tract. Those blockages leave the afflicted, mostly men, writhing in excruciating pain.

But he didn’t obtain his sample from the wards of area hospitals. Garvie is a mineralogist and self-described desert rat. He harvested the crystals from the decaying bodies of giant saguaro cacti found in the desert around the Phoenix and Tucson metropolitan areas.

For the past 10 years, Garvie has peered into the rotting innards of these newly fallen giants looking for signs of mineral formation. In the process, he’s discovered a whole suite of crystals that grow in the hot, putrefying pockets of downed saguaros.

Some of these crystals are rare or never before found in deserts. For example, one of Garvie’s is nesquehonite. It’s been found in mines and caves and is a common weathering product of meteorites found in Antarctica. Others do not match any known crystalline structures and are altogether new to science.

This scanning electron microscope (SEM) image shows fungi (blue) draped over nesquehonite crystals (purple). Color was added by the researcher to improve display.

 

Garvie has published his landmark discoveries in scientific journals. But his work—as well as his career path as a scientist—is anything but typical. An expert in electron microscopy, Garvie works a day job as a faculty research associate in the School of Earth and Space Exploration at Arizona State University. One of his more recent assignments has been to study minerals in meteorites for clues to the formation of the solar system.

In his spare time, however, he can be found roaming the Maricopa Mountain Wilderness south of Phoenix. On a remote rocky outcrop, he manages a weather station for a study on desert lichens. He’s quick to point out that the Sonoran Desert, which encompasses some of the fastest growing cities in North America, is every bit as intriguing and in need of study as rocks from outer space.

Garvie is living proof of that. The ASU researcher has found that as saguaros age, the structure of their spine clusters changes dramatically. He is the first to document that fact.

“It’s not some great scientific discovery,” he says. “For me, it was just a little puzzle that I’d seen for years.”

But saguaros aren’t his only research interest. In 1998, on a foray into the desert south of Phoenix, Garvie came across a small prehistoric potsherd covered by different kinds of lichens. ASU lichenologists Tom Nash and Matthias Schultz examined the plants under a microscope. They determined that one of the specimens was the first known occurrence of that particular species in the United States.

On another trip, Garvie returned with samples of a large, lumpy black lichen he discovered growing on a limestone outcrop. It turned out to be a whole new species.

“If I can find these species, and I’m not a lichenologist, then it shows what lichenologists would be able to do around here,” Garvie says. “It’s the same with people who study other desert microorganisms. Somebody once said that if you go walking just about anywhere in the desert, you’ll find several new species of insects.” And that’s not counting the multitude of bacteria and fungi waiting to be discovered.

“When I’m out there, I’m always aware of the fact that there are probably all kinds of weird and wonderful things that I’m not recognizing. When I walk back to my car after each visit, I ask myself, ‘What else can I see that’s interesting or that I don’t know about?’ The desert is bursting with research opportunities. Every time I go out there I find something new.”

Garvie is captivated by the sequence of events that is triggered when a giant saguaro topples over and crashes to the desert floor. He was a newcomer to the desert 13 years ago. But Garvie says he noticed chunks of bone-colored material, as light and porous as pumice, heaped around the bare wooden ribs of saguaro remains.

“I’m a mineralogist. So I asked myself, ‘What is this?’” He collected a piece that sat on his desk for a year.

“One day I had a few minutes to spare so I subjected the sample to powder X-ray diffraction, a method geologists use to identify minerals.”

The material turned out to be calcium oxalate.

The discovery was surprising, although not entirely unexpected. Groundwater, for example, contains calcium that settles out of atmospheric dust or leaches from dissolved rock. Too much calcium can be toxic to plants, including saguaros. Since plants can’t control the amount of calcium they absorb, they safely store it in their tissues in the form of calcium oxalate crystals.

Garvie’s curiosity was whetted. He was eager to learn more about the life cycle of minerals in saguaros. During his hikes, he started looking for saguaros in earlier stages of decomposition.

This SEM image shows the fruiting bodies of fungi. Garvie says they appear to be converting weddelite crystals into monohydrocalcites.

 

Gummy black liquid oozing from openings in a saguaro’s skin is a telltale sign that the cactus is near the end of its life, Garvie says. It signals that fungi and bacteria have begun to attack the plant’s moist inner tissues. In time, the weakened saguaro collapses or is blown over by the wind.

A saguaro’s internal flesh is composed of up to 90 percent water. Garvie calls it an “organic moist soup that’s teeming with life.”

Scientists at the University of Arizona dissected one cubic foot of a rotting saguaro. The interior temperature can reach a whopping 122 degrees F. Inside, they found 413 arthropods including beetles in both adult and larval stages, larval flies, pseudoscorpions, and mites.

“The whole thing is moving, heaving with bugs, worms, and all sorts of microorganisms. It’s quite exciting,” Garvie says.

But when the ASU scientist tore off a chunk of rotting saguaro, he found more than biological treasures.

“It was almost like a geode,” he explains. “The insides sparkle in the sun from all the new crystals that formed.”

Monohydrocalcite crystals spill from the ribs of a dead saguaro found in the South Maricopa Wilderness area.

 

Garvie did a series of laboratory tests to identify the crystals. The glassy jewels turned out to include more than just calcium oxalates.

Garvie found rare, magnesium-bearing crystals in the rotting saguaro flesh. Some had exotic sounding names. He found lansfordite, glushinskite, and nesquehonite. He says that at least a half dozen additional crystals were brand new to science. Most of these new minerals occurred only in decaying, not living, plant tissues.

Glushinskite (red) is a very rare mineral. These crystals formed during the decay of a saguaro's rotting flesh. Scientists last described glushinskite many years ago when it was found in a coal seam in Russia.

 

Garvie says that mining companies aren’t likely to bid for leases on decaying saguaros any time soon. Some of the minerals he found were very unstable. They broke down when exposed to ordinary light.

However, the work could have value in helping researchers to tackle the puzzling problems associated with global climate change. For example, scientists have studied how rainforests can sop up excess carbon dioxide in the atmosphere that leads to global warming. But little work has been done on what happens in desert environments.

Garvie says that calcium oxalates could provide valuable insight into how arid regions handle the cycling of carbon. Cacti concentrate large amounts of calcium oxalates. A year or so after the “gooey, squelchy, rotting material has decomposed,” Garvie says a specialized group of bacteria take over. These oxalotrophic bacteria break down the minerals into a stable compound called calcium carbonate.

Calcium oxalates are the most common biominerals in plants, Garvie explains. “But few plants [unlike saguaros and other cactuses] release or concentrate significant quantities of biominerals after their death.”

Planting cacti in place of grass and other vegetation along roadways and public spaces might have a measurable impact on transforming carbon into inert forms that can’t easily escape back into the atmosphere.

Garvie’s work began as scientific curiosity. His findings have raised interesting questions.

“We really don’t know where this research is going to go,” he says.
In the meantime, he plans to continue combing the desert with other, equally challenging goals.

“My real ambition is to find an unknown species of cactus."

For more information about biominerals in saguaro cactus, contact Laurence Garvie, Chemistry and Biochemistry Department, College of Liberal Arts and Sciences, (480)965-7250. Send e-mail to Laurence.Garvie@asu.edu