Again Along the Spanish Trail

Saturday, September 14, 2019

Picked up an odd rock today along the Old Spanish Trail, not far from the Junction of Highway 160. in Clark County, Nevada. What Macrostrat.org calls young fan alluvium. Lots of limestone and sandstone bits and pieces. Didn’t see anything like this rock. It has notable pockets around the rock and then lines of heavily marked material evenly radiating out from them.

There is chunk of what may be limestone at the bottom of each pocket. It reacts little to acid but I think I see some activity. The white material, less that 2 on the Mohs scale doesn’t react because it is so porous the acid soaks it up before doing anything! The white material fluoresces a strong orange/red while the pockmarked lines, full of circle and bubble shapes, does not do anything under UV.

It strikes me somewhat as being like the calcite material which I saw recently at the Kokoweef Cavern Mine complex. There are a few dead springs in the surrounding area, I am wondering if this has something to do with being formed or acted on by a spring. But I don’t know why these lines would radiate from those pockets. Or why there would be pockets in the first place.

September 16, 2019

I’ve been thinking this over by myself. No one on Mindat.org of FB has made any suggestions as to what formed the rock, other than it might be gypsum. Which it is not, much too soft for gypsum. I have a reference specimen of rock gypsum, unweathered, and a self-collected piece, weathered, from Shark Tooth Hill in Kern County, California. Both those rocks are chalky and too soft to compare with the mystery rock.

Here is my working theory. The white material is calcite, a mass of which formed around several limestone rocks. At some point the mass became tumbled and rounded in the small desert wash or channel that I collected it from. Being very absorbent, the round mass picked up rainwater which would contain carbon dioxide. A weak carbonic acid would develop when the limestone reacted to that. The limestone is now eating away at the calcite, dissolving the material over the centuries.

The pockets you see are all centered on where a piece of limestone is located. The residue seen in the lines is the acidic precipitate left by this slow destructive process. Thus, that residue or the limestone does not respond to UV, being limestone and limestone related, while the calcite (the white mass or host) responds to UV as most calcite does.


Under SW UV. Forgive the “blue bleed”, a common sight when photographing glowing rocks.



R.C. has just checked in. “If it’s calcite, no surprise. Rainwater dissolves limestone, which is calcite. Limestone typically has fractures which gradually get enlarged as rain water makes its way through. This is how cave systems form. The calcium carbonate (calcite) dissolved in the rainwater is what forms stalactites and stalagmites in caves, and it commonly fills in fractures between blocks of limestone.”

So, do we have a rock that was once a fractured piece of limestone, now with calcite accreting around the remaining pieces? In other words, this may not have been a mass of calcite to begin with, rather it was a block of limestone that started this all off. And then it transformed into what is seen today. Hmm. Still pondering.

 



Kokoweef material self-collected on mine dumps by permission. Limestone rock on top and “popcorn calcite” underneath, the calcite carbonate deposited in the form you see here. It is unknown whether this rock came from the cavern roof, its floor, or a wall.

This popcorn calcite responds weakly under SW, producing an uninteresting cream color. It does have a short but notable phosphorescence or afterglow.


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Moving On in a Desert Wash

While my book lies dead or dying, I continue to collect and learn. The world is too fascinating to continue mourning.

On Friday I collected from two desert washes near the colorful art installation called Seven Magic Mountains outside of Las Vegas.

This first photo shows mud cracks in stone. Slate or some such. Does not fizz readily under acid like the next rock.

This second photo is a poor example of limestone with a surface solution, collected that Friday just for illustration. Notice how much finer the lines are than the mud cracks, a closer knit group of spider webbing. Limestone with a surface solution is common throughout the Southwest. Keeps scrolling to see a reference specimen.

This third photo is a reference specimen of limestone with a surface solution. I bought it from professional geologist and former instructor R.C. de Mordaigle. He runs Geological Specimen Supply in Olancha, California. It’s the best place to start your reference collection of common and uncommon rocks. R.C. was a great supporter of my book.

R.C. explains these cracks in limestone this way, “In arid areas both limestone and dolomite develop a rough dissolved surface from the action of weak carbonic acid as atmospheric water picks up carbon dioxide . . . .”

What the collector really wants to find in wash are ripple marks, distinct marks in stone caused by wave action millions of years ago. This specimen is mudstone from somewhere outside Barstow, California. Look closely at the second photo. This isn’t just a piece of somewhat striped sandstone. Notice how it has real relief. Ripple marks.

Notice the relief in this second view.

I was on a Barstow area field trip one summer and saw a woman collect a nice ripple mark specimen from a desert wash. That arroyo had just been hit by a summer rain. Alas, I myself did not find a ripple mark but I did collect a nice piece of sandstone with dendrite markings. Another story for another day. Keep looking!

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Keep That Spray Bottle Handy!

Found this crazy looking piece on or near the east border of Gold Butte National Monument in Nevada, where limited, non-commercial collecting is allowed. In response to the photos I posted to Facebook, Joel Coombs on The Rockhound Connection responded, “I believe that rock is slightly metamorphosed limestone. I have found smaller pieces with the same colors. Put a drop of pool acid on it. If it effervesces it is limestone.” I did put some acid on the back of the rock and it did indeed fizzle. This all agrees with Jim Boone’s identification of plain looking limestone in the same location.

I found it on the side of the road, a dusty and dirty rock but one showing interesting markings. My spray bottle revealed what I thought might be great material for a cab or a slab. (If I wanted to do that since I rarely get around to lapidary.) A friend pointed out, though, that the red areas were rusty splotches, possibly iron or hematite related and that being soft would not easily cab up. Something like Superglue might be applied to those areas, then the whole piece sanded later on.

I decided to keep it as it was. A former rock shop owner suggested clear glossy spray lacquer which could always be sanded off later. The first picture is of the rock wet, the second is after three coats of spray, now dry. The third picture is of the untreated back. Given the ease of effort, I think the spray worked out well for this particular rock.

How did I find it? I was volunteering on Tuesday to help fill in potholes on Gold Butte Road, the somewhat paved road that provides the main access to the Monument. Right now, under the direction of The Friends of Gold Butte, many volunteers are helping out on this multi-day project. Clark County and BLM are also supplying workers and cold patch. I found the the rock on a break. But I had my spray bottle nearby. https://www.meetup.com/Friends-of-Gold-Butte/

Poor, Poor Porphyritic

Every trade or hobby has difficult to pronounce words of art. Like porphyritic, an adjective describing large crystals in an otherwise fine grained rock. Pronounce it poor-fer-itic. The crystals don’t have to be overlarge or dramatic, just bigger than the groundmass or matrix that surrounds it. In my book I will have two examples, one quite pronounced, Chinese writing stone, and the other, andesite.

Difficult words act as roadblocks to learning. We stumble over them, and in recovering we try to regain our balance by moving on. Without learning. I am hoping, therefore, to retain interest in difficult words and concepts by including photographs of them.

This rock is causing me problems. It isn’t polished, so it looks less attractive  than it otherwise would. And the shadows are bad, although my new cameral setup can overcome that.  Perhaps I will  polish it at my lapidary club. Or perhaps I can put some oil on it to make it look slicker.

“Chinese Writing Stone Slab for Lapidary or Display – Chinese writing stone, or Chinese letter stone, is so named for it’s resemblance to Chinese writing characters. It is also know as Porphyry. Background is a dark greenish-black charcoal limestone matrix with Andalusite crystals – This Chinese Writing Rock form is from Australia.”