Dumb as a rock? A lumpy bit of earth reveals a geological timeline in Seymour

Editor’s note: Today’s blog comes to us from paleontologist and field volunteer Neal Immega.

You all know by now that the museum has a dig in Seymour where we are finding fabulous Permian fossils, including the toothy Dimetrodon and the weird boomerang-headed salamander Diplocaulus. We don’t dig in just one place; lots of people go prospecting for new sites (or maybe they are just looking for a private spot to do their business).

A previous blog topic was on a weird lump that turned out to be a caliche ball. Today we have another lumpy rock to look at. This specimen was collected by geologist Gretchen Sparks, who brought it in just to plague me.  Let’s see just how much information we can squeeze out of it:

CrossBeds

At first glance, it’s just a rock showing cross-bedded sandstone with low-angled bedding, doubtless caused by water deposition in the Permian creek that crossed our digging area. This is pretty normal stuff. We see cross-bedding everywhere at the dig site, because the sandstone is durable and stands in relief.

But why is it lumpy? The bulge in this picture is not exactly standard:

Profile

It gets better. The rock is too heavy to be only quartz. A heavy, light-colored sandstone is likely to be cemented by barite (barium sulfate). Let’s cut the rock in half and polish the face.

cutLumpLabeledThis is turning out better than I expected. You can see a seam of barite cutting the nodule vertically in half. The sandstone shows horizontal layers which correspond to the cross-bedding.

What about the red-colored area? The area we are working in North Texas is called the Permian Red Beds because everything got oxidized from prolonged exposure to the atmosphere. It was a really dry time, and the critters stayed close to the Permian creek which deposited these sediments. It is good for us because the fossil remains are concentrated in a small area (our dig site is just about the size of a tennis court).

Let’s consider this possible sequence of events.

1.    250 million years ago, sandstone was deposited in a creek. It is all cross-bedded.

2.    The sand grains were likely coated with iron oxide and thus turned red.

3.    The sandstone was buried by maybe 1000-plus feet of additional rock.

4.    Shales deeper in the geological section were heated by the normal geothermal gradient to hundreds of degrees and adsorbed water was squeezed out, taking with it the barium that was also adsorbed (from the ocean) on to the clay surfaces. The water moved vertically along cracks in the rock.

5.    When the barium reached the rocks we are digging in, the barium precipitated because the pore water is very “hard” with dissolved gypsum. The barium reacts with the local sulfate, producing barium sulfate which is essentially insoluble in water. It is the ultimate “hard water” scum.

6.    The barite precipitated as the vertical seam and filled the pore space in the red rock.

7.    A whole lot of rock was eroded in the next 250 million years to bring us to the present and the rocks back to the surface.

8.    10 to 100-thousand years ago or so, North Texas was in an Ice Age, and it was really wet with lots of vegetation. Decomposing vegetation created a reducing environment which dissolved iron right out of the rock. Barite is very chemically resistant, and this lump could have been at the surface through part of the ice age. The iron could have partially leached out of the lump at that time. You can see that the leaching went deeper into the lump where there are horizontal fractures in the rock.

All this from a lump of rock. Thanks, Gretchen!

Rock Steady!

In addition to the wonderful movies and exhibitions we put on at HMNS, we also offer unique learning experiences. Our paleo hall is a great place to pick up new skills, such as sifting through seashells millions of years old or learning how to cut gemstones.

Most days throughout the summer, a volunteer is stationed in our Paleo hall cutting facets in rock and explaining to the interested visitors how to carve the perfect gem.

Our volunteer starts with a mineral, usually quartz. They set the stone to the holder, and using a protractor measure out the angle at which they are going to grind their quartz. The spinning wheel, which is a diamond gritted lap (diamond is the hardest natural substance known and can cut through anything) is then used to grind the stone and create a facet.

This lap is used in the final stages,
to polish the almost finished stone.

After one side is cut, the stone is rotated so another side can be ground down. Our volunteers use different laps to make larger cuts, or to polish the stone as it nears completion.

Check out the beautifully cut quartz stones pictured below. Notice that the gems can be cut into different shapes with a different number of facets.

An experienced volunteer can craft one of these in a little over an hour.

Interested in becoming a volunteer and learning how to cut gems yourself, or how to lead tours or get more behind the scenes opportunities? Contact Sibyl Keller at 713-639-4656 or check online here.

Want to learn more about gems, diamonds and jewelry? Don’t miss Faberge: Imperial Jeweler to the Tsars, at HMNS until July 25.

100 Years – 100 Objects : Elbaite (on Quartz)

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from Joel, the Museum’s President and Curator of Gems and Minerals. He’s chosen spectacular objects from the Museum’s mineralogy collection, which includes some of the most rare and fascinating mineral specimens in the world, that we’ll be sharing here – and on hmns.org – throughout the year.

main-elbaite-on-quartzTourmaline Queen Mine, San Diego County, California.
North America has produced some extraordinarily beautiful specimens of elbaite, a member of the tourmaline group, but the most admired are the bright red-pink crystals with blue caps found in 1972 at the Tourmaline Queen mine. The 24-cm example pictured here is the finest of the 33 major specimens recovered and is therefore the finest North American tourmaline. The lustrous, lusciously colored, undamaged pair of crystals at the top grow from an undamaged quartz crystal and are accompanied by smaller tourmaline crystals. It has been nicknamed “The Rabbit Ears.”

Marvel at the world’s most spectacular collection of natural mineral crystals in the Cullen Hall of Gems and Minerals at the Houston Museum of Natural Science.

You can see larger and more detailed images of this rare specimen – as well as the others we’ve posted so far this year – in the photo gallery on hmns.org.



100 Years – 100 Objects: Rhodochrosite

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from Joel, the Museum’s President and curator of Gems and Minerals. He’s chosen spectacular objects from the Museum’s mineralogy collection, which includes some of the most rare and fascinating mineral specimens in the world, that we’ll be sharing here – and on hmns.org – throughout the year.

Sweet Home Mine near Alma, Colorado.

The superb 10-cm rhodochrosite crystal on matrix pictured here, known as “The Alma Queen,” is among the most famous specimens in all of mineralogy. It is the best specimen recovered from a pocket found in 1965 by John Soules and Warren Good. The exceptionally large, rich red-colored crystals perched on a crystallized matrix of quartz and tetrahedrite combine to create a level of sculptural composition and aesthetic beauty that make this one of the world’s truly great mineral specimens. Since its discovery, it has attained legendary status as a “Mona Lisa” of the mineral world – it is widely regarded as the finest example of rhodochrosite known – and among knowledgeable collectors and connoisseurs, it is considered to be the finest mineral specimen of any kind ever found.

Marvel at the world’s most spectacular collection of natural mineral crystals in the Cullen Hall of Gems and Minerals at the Houston Museum of Natural Science.

You can see larger and more detailed images of this rare specimen – as well as the others we’ve posted so far this year – in the photo gallery on hmns.org.