Tungusite in the Santa Monica Mountains, Los Angeles and Ventura Counties CA
Robert M. Housley
rhousley@its.caltech.edu
Background I first confirmed the presence of
tungusite in the central Santa Monica Mountains in early October of 1996, using semi-quantitative SEM-EDS analysis and a form of powder XRD, on a sample given to me by Fred DeVito. Based on his visual comparison with tungusite from the type locality and the similarity of formation environments and mineral associations Fred had been convinced for seven years that his sample was tungusite, but he needed the proof. I had also for 4 or 5 years thought that similar material I had collected might be tungusite, but did not follow up because the EDS compositions showed Mn and Al, not present in the idealized tungusite formula.
Almost immediately after the confirmation Fred and I drafted a short note announcing the discovery and describing the locations where we knew that this tungusite occurred. We quickly submitted the note to Don Howard, editor of the Microprobe, a biannual publication of the Northwest Micro-mineral Study Group. Don in turn published the note in the Fall 1996 issue of the Microprobe, but with considerable editing and no chance for further review. (Microprobe, Volume VIII, number 4, pages 2-5).
In December of 1996 shortly after confirming the identity of tungusite from the central Santa Monica Mountains I discovered another small, but very prolific, locality for it at the far western end of the mountains in Newbury Park.
During the Spring of 1997 Don Howard visited Southern California and Fred DeVito and I showed him the various tungusite localities that were most accessible. At the Newbury Park site Don collected a great deal of material. In late June of 1997 Don put most of that Newbury Park tungusite out on the give away table at the Northern California Mineralogical Association meeting. I do not know what happened to Fred’s tungusite after he died.
In any case as pictures posted on Mindat demonstrate tungusite, particularly from the Newbury Park site, which has never yet been described, has made its way into collections around the world. It therefore seems timely to write a fuller account of the original identification and of the major Southern California occurrences of this mineral.
Identification The first good English language description of tungusite from the type locality appeared in 1995 (Ferraris, Pavese et al. 1995). This paper argues that tungusite is bimodal in composition with an Fe-poor pale green form and an Fe-rich dark green form. Our semi-quantitative analyses correspond within the uncertainty to the Fe-rich form.
This paper also give and XRD powder pattern for tungusite, but for tungusite of unknown, or at least unspecified, composition. They used magnetic separation to get tungusite largely free of gyrolite and reyerite, which they show that it readily inter-grows with. This cleaning procedure would suggest that their XRD sample is of the Fe-rich tungusite and would correspond to our material. To avoid preferred orientation effects, which they say are pronounced for a flat sample, they then put their powder sample in a glass capillary.
I decided to adopt a different strategy, taking advantage of the strong basal cleavage rather than fighting it. I first tested the idea by mounting a single flat sheet of gyrolite on the diffractometer. It indeed gave strong reflections for the 001, 002, 005, 006, and 007 gyrolite lines with nothing in between, and with an inferred c-axis lattice constant of 22.3 Angstroms.
I then mounted a few clean looking flakes from Fred’s sample similarly flat on the diffractometer. From them I obtained a pattern showing the 003, 004, 005, 006, and 009 lines of tungusite in almost identical positions to those observed by Farraris et al. It also showed a week 002 line of reyerite, which they too saw. None of the tungusite lines could be confused with gyrolite lines. Thus tungusite in the Santa Monica Mountains was confirmed on 10/7/96.
After finding the tungusite in Newbury Park I again had to verify its identity. On 1/9/97 I used the same technique, as with Fred’s sample, but took more time to make sure I had a good coverage of clean uniformly colored mineral plates on the difractomer. This time I saw 001, 002, 003, 004, 005, 006, 009, 0010, 0011, 0012, 0014, and 0015 reflections of tungusite and no other lines. These crystals are pure tungusite with neither reyerite nor gyrolite inter-grown.
Locality 1a The central Santa Monica Mountain location where I believe Fred found his tungusite and where I found most of my early samples is near the westernmost crossing of Cold Canyon Road and Mulholland Highway in Los Angeles County. In a region about half a mile in diameter centered a little west of this intersection nodules weathering out of exposed rock in cliffs and canyon walls are apt to contain tungusite. In particular exposed rock along a small branch of Cold Creek that crossed Mulholland Highway about 0.3 mile west of this intersection are very apt to contain tungusite.
In this area the most common minerals are gyrolite, apophyllite-(KOH), analcime, pectolite, and calcite. When it occurs tungusite is always the outermost mineral next to cavity walls and never projects into an open cavity. When it occurs next to gyrolite, as is the most common case, the contact appears to be gradational, perhaps due to inter fingering as seen by Farraris et al.
In this region the above minerals always occur in hyaloclastic breccia or pillow basalt, which are basal units of the middle Miocene Conejo Volcanics. In slightly higher flow units natrolite and thomsonite also occur in vesicles.
Locality 1b This locality is a mile or so south of number 1a. It is just north of Monte Nido along Cold Canyon Road. It is in the banks of a small tributary to Cold Creek just east of where it crosses the road. Here the rock is a basalt breccia and the tungusite occurs with pectolite, subordinate gyrolite and apophyllite-(KOH) in near vertical veins. The tungusite formed along the walls, and the veins then filled primarily with pectolite. As can be seen in the pictures below the tungusite contacts with the pectolite appear sharp.
Tungusite, locality 1b FOV 5 mm
Tungusite, locality 1b FOV 6 mm This is the Newbury Park locality about 10 miles northwest of localities 1. It is in a small road cut near the end of an unused street that was part of an aborted development. It is most easily reached by walking about 100 feet north from the end of Marion Street, or can also be reached by parking behind industrial buildings at the north end of Tourmaline Street. Although this locality was prolific when freshly opened, the host rock is a very hard flow breccia and all the easily broken material has been removed. It is probably that no one has collected here since 1997. The most abundant minerals here are quartz, apophyllite-(KOH), and mordenite, along with tungusite. Pyrite is less common and analcime is rare. The tungusite again formed first in the veins and vugs. Only quartz and apophyllite-(KOH) grew into open cavities.
Even though this site is a very good source of pure clean tungusite it is very important to caution that not all the nice looking green blades at this locality are in fact tungusite. A number of blades that are too low in Ca and too high in Mg to be tungusite or tungusite/gyrolite solid solution also occur and they have very similar appearances. I have not found any way to visually distinguish them and myself use SEM-EDS to tell which is which. Out of 10 good looking balls I have tested 6 proved to be good tungusite. I have not done XRD on the other compositions and have not done thin section work. It is not clear whether these low Ca composition result from alteration, intergrowths, or perhaps are another original mineral phase. However the fact that the low Ca blades are generally inhomogeneous in composition favors alteration.
Ferraris, G., A. Pavese, et al. (1995). "TUNGUSITE - NEW DATA, RELATIONSHIP WITH GYROLITE AND STRUCTURAL MODEL." Mineralogical Magazine 59(396): 535-543.
