Hi mineralogical friends!
Let’s visit the less frequented areas near Acton for our next trip on Saturday, Oct 19, 2024. We will get a chance to see some classic rockhounding areas from a different perspective. We will look for titanite in the granite roadcut first, then we will visit the Free Cuba mine for the same, but also for some interesting copper minerals.
First, I have an update about our finds from the March Cady Mts trip. The purported pink thulite checked out at the Caltech lab as clinozoisite, so it actually makes it a “clinothulite” variety. Some prismatic gemmy crystals were found after etching the calcite with acid. They can reach a couple of mm in size, look a bit like tourmaline, and have an intense deep pink color, some had even a kind of hot pink tinge. I suspected they could be piedmontite, but the electron microscope EDS result at the NHMLAC showed they contained a very low percentage of manganese. Amazing what even a little bit of that underappreciated element can do to the color of a mineral…
Another piece of news is that the dark blue balls from the Old Dominion mine some of us collected that afternoon are not shattuckite as it was speculated before. Instead, the powder X-ray diffraction spectrum at the NHMLAC lab found a better match, plancheite, a quite rare copper silicate mineral with a slightly different structure. Thanks to George Rossman & Bob Housley at Caltech and Tony Kampf at NHMLAC for their excellent help with the mineral identification.
And now, back to the next trip — we will meet on Saturday, the 19th, at 9:00 AM south of Acton, in a dirt parking lot next to the roadcut, 2.4 miles south of the Crown Valley Rd Acton Exit off Fwy 14. It’s 55 miles from Pasadena. The driving time is 50 min with easy traffic in the early morning.
Here are the GPS coordinates of the meeting place: 34°27’25.5″N 118°11’54.2″W (34.4571, -118.1984). I’m not including the Google Maps link this time. The saved Maps link will try to lead you to a nearby ranch house, which has the same address you can see in the picture below. Search for the included GPS coordinates instead.
This fieldtrip was inspired by a very old write-up in the MSSC March 1955 Bulletin and also by the finds from Bob Housley’s visit to the mine about two years ago. See the invite on the webpage to read the 70-year-old description. There is an interesting local geology – an intrusion of a light-colored diorite. The titanite (sphene) is associated with the slightly darker hornblende and plagioclase rock. The platelets are up to a few mm, and the write-up mentions up to a 1/4 inch (good old days).
We will examine the rock at the roadcut first, then we walk down the slope to the nearby Free Cuba mine. The downhill is dusty but not too steep, slightly rocky in places. The copper veins were mined 160 years ago. The matrix is also the same diorite, so some titanite is present too. The Mindat location has a bit of useful info but its location is ways off: https://www.mindat.org/loc-81239.html
The Mindat page for the mine mentions cuprite and native copper and this is what Bob found when visiting. Excellent micros but need magnification to be seen well. There is also malachite and a nice blue chrysocolla. However, Bob also found an unknown dark bluish-green mineral, forming balls and layers. Bob identified it as pseudomalachite, a much rarer copper phosphate. Not too many places in California where you can find it.
For people wanting more there are quite a few other places in the Acton area for a post-fieldtrip side excursion. One such place would ideally fit into the backside exploration theme — there is a small trash dump at the back of the famous Acton Quarry. One can find quartz geodes and small heulandite & stilbite in the dark basalt.
See you soon on the trip!
MarekC