Three new melanophlogite pseudomorph localities from Santa Monica Mountains, Southern California

By Marek Chorazewicz, Simi Valley, Southern California

    Originally published in the Bulletin of Mineral Society of Southern California, August 2014

INTRODUCTION

    The prolific little Southern California transverse range called the Santa Monica Mountains has provided many new mineral localities for researchers and rockhounds alike in the last 70 years. Fifteen zeolite species have been found in the range, including ferrierite-K first described from here. Numerous other minerals were found here in the last century’s '70's and '80's by Fred DeVito and Bill Rader on the mountain ridges and in the canyons, including quartz, agate, chalcedony, calcite, aragonite, dolomite, sphalerite, pyrite, gypsum, wulfenite, apatite, zircon, celestine and pseudobrookite. Some of the chalcedony found was showing cubic or rather pseudocubical shapes and crystals. These silica cubes were first reported from Encino by Murdoch in 1936, then later by Sharp, as pseudomorphs after fluorite, although, fluorite have never been found in the Santa Monica Mountains. Since 1936 science became more familiar with the mineral called melanophlogite, the tetragonal/cubical form of a silica clathrate with chemical formula of 46SiO₂ · 6(N₂,CO₂) · 2(CH₄,N₂).
Thanks to the recent research the cubes are now considered as chalcedony pseudomorphs after melanophlogite instead of fluorite. Similar research suggests that the Romanian blue chalcedony pseudomorphs from Trestia/Kötelesmező are also after melanophlogite, not fluorite. Possibly the same situation applies to Namibian blue lace chalcedony pseudomorphs from Blinkpan/Ysterputs Farm 254 as the crystal forms show very close resemblance to the Romanian and Santa Monica Mountains samples.
Last year Dr. Housley of California Institute of Technology published an article describing eleven melanophlogite pseudomorph localities in Southern California. Ten of those localities were found in Santa Monica Mountains. In this article I’d like to add three more occurrences to the list.
Dr. Housley’s article inspired me to look closer at the chalcedony I was finding already during my walks in the Agoura Hills and Calabasas area of the central Santa Monica Mountains. I was already on a lookout for zeolites before, now I decided to check out the chalcedony as well. I’ve visited a few of the described localities, found some pseudomorphs and got hooked. During 2013 and 2014 I’ve discovered some cubical forms at three previously unreported localities.

PROPERTIES

    So far no actual melanophlogite has been found in the Santa Monica Mountains. It is always found as chalcedony pseudomorph crystals. Based on Dr. Housley’s Raman spectroscopy analysis the chalcedony samples contain varying amounts of mogánite and crystalline quartz, and the ratio in the melanophlogite pseudomorphs is always similar to that in the adjacent chalcedony. That means the melanophlogite crystal structure has fully collapsed into the higher density quartz varieties.

LOCALITIES

    All the new localities are within one mile of each other around Lady Face Mountain in Agoura Hills, Los Angeles County, California.

Table 1. Melanophlogite pseudomorph location information

Name  Latitude  Longitude  Size  Host Rock  Kanan Quarry Ridge     34.1271926   -118.7717348   1.5 mm   Volcanic Breccia  Kanan Quarry  34.1290000   -118.7715200   1.8 mm   Andesitic Breccia     Cornell Rd  34.1382857   -118.7628337   6.0 mm   Andesite

Kanan Quarry Ridge

    I stumbled upon this location in November 2013 on one of my hikes through the Santa Monica Mountains when I picked up a rock with some sparkly quartz. One side of the rock had some druzy quartz points inside a small chalcedony vein, however the opposite site harbored some cubical crystals, which I recognized from the description in the Mindat article.
The host rock is a hydrothermally altered volcanic breccia, ranging in color from white, light grey, and yellow to dark orange. Milky to translucent chalcedony penetrated most of the spaces between rock constituents, in some specimens there is more chalcedony cementing the pieces together that the host rock itself. Complex intergrown mm-size pseudomorph crystals completely cover the chalcedony veins, however there are a few pieces showing the transition from botryoidal chalcedony to cubical crystals. Some veins have thin layers of crystalline calcite overlaying chalcedony.
The largest pseudomorph measured was 1.5 mm. I’ve found about ten pieces within a 3 meters radius of the original discovery, and I believe there is definitely more laying around.
The pseudomorphs from this location exhibit milky white fluorescence under both the short-wave and long-wave UV lamp.

Kanan Quarry

    Several weeks after visiting the ridge above I’ve focused on the quarry proper below. I thought I could find some similar pieces that weathered out of the ridge and tumbled down the slope. The abandoned Kanan Quarry is world famous as the co-locality for a very rare zeolite – ferrierite-K. Among other minerals the quarry has yielded clinoptilolite, sphalerite, quartz, calcite, dolomite, chalcedony, barite and agate to collectors over the years.


After some rummaging on the dumps I found some pseudomorph specimens, but their morphology is completely different compared to the ones from the ridge. The crystals are light milky gray in color and show complex spiral intergrowth with multiple faces twisted at small angles. This is very similar to Borges Quarry morphology observed by Dunning and Cooper, and similar crystals were observed in Encino samples by Murdoch, who described them as the “platy” type. The underlying chalcedony layer covers a thick vein of banded gray, honey and brown calcite. This calcite fluoresces nicely under the UV light. On top of the pseudomorphs sub-millimeter eroded crystals of tan dolomite can be found occasionally. In places where dolomite eroded completely the chalcedony has greatly increased luster from waxy to almost vitreous. A few cubical crystals are covered by small hemispheres of chalcedony.


Later on I’ve found here some simple cubical crystals on a gray botryoidal chalcedony. The crystals were scarce and isolated far apart from each other. This was found on a rock on the quarry floor, farther away from the main quarry dumps.
The largest pseudomorph found in the quarry measured 1.8 mm.

Cornell Road

    I’ve found this location in April 2014 while looking for some calcite geodes. In the area higher up the slope and to the East one can find calcite geodes up to 4-5 inches across with rhomboidal calcite crystals up to 1cm, occasionally covered with super sharp and completely clear, single and double-terminated quartz crystals up to 5mm. There is some blue chalcedony with tiny stalactites several mm long in addition to the agate as well.


The location is a red rock outcrop just above Cornell Rd, 250m SE from the Cornell Rd and Cornell Way intersection. I’ve found several specimens within a 30 meters radius of the outcrop. The area is also associated with ticks, poison oak, but also nice Dudleya pulverulenta plants and I’ve seen a nice night snake too (Hypsiglena torquata).
The host rock is a dark gray dense glass-rich slightly brecciated basaltic andesite belonging to Conejo Volcanics. The rock looks heavily eroded on the outside and resembles red scoria, but just a few millimeters under the weathered layer it is dark and hard. Botryoidal chalcedony veins cover the seams in the rock. Some of the chalcedony was covered with a thin layer of dried out tar, very hard to clean even with naphthalene. Some chalcedony is almost glassy clear; it superficially resembles hyalite opal, but the UV fluorescence is dark orange instead of green.
The largest pseudomorph crystal from here measured 6 mm. I’ve found only one crystal of this size, but also found several in the 4-5mm range. Those large crystals are mostly simple cubes, with one or two very thin extra parallel faces. About half of the larger crystals exhibit beveled edges, which I’ve identified as the tetrahexahedral “e” form {012} that was previously reported from the Mount Hamilton location by Dunning and Cooper. Some crystals are also covered by small isolated hemispheres of chalcedony.
The pseudomorphs fluoresce milky white, dark yellow and dark orange under SW and LW UV. Same response was shown by the botryoidal chalcedony as well.
One of the pieces contained globular accretions of reddish black goethite. Under magnification the globule showed as being fully composed out of square faces, so it is likely a goethite pseudomorph after pyrite. The crystals in the aforementioned sample show complex spiral intergrowths like Kanan Quarry crystals but are much larger, up to 2.5 mm.

ACKNOWLEDGEMENTS

    I’d like to thank Dr. Housley for his contagious enthusiasm about the Santa Monica Mountains and continuous inspiration to look for new locations and minerals in the area, and, in addition, for analyzing the samples, and for the effort of verifying locations in person. I’d like to thank the Southern California Chapter of the Friends of Mineralogy for hosting its Fall 2005 Mineralogy Symposium on Minerals of the Santa Monica Mountains. That symposium was an eye-opener about the little mountain range in my own backyard. I’d like to thank the editors current and past of the Bulletin of the Mineralogical Society of Southern California for great work in providing the members and non-members alike with great mineralogical content throughout the years and keeping track of the local mineral news. Last but not least the Santa Monica Mountains field collecting leaders from the past, Fred DeVito and Bill Rader, should get the honorable mention for their fieldwork in the 1970’s and 1980’s.

REFERENCES

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  Dunning, G. E. and Cooper, J. F., Jr. (2002), Pseudomorphic melanophlogites from California. The Mineralogical Record: 33: 237-242.
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  Housley, R. (2013), Ferrierite in the Santa Monica Mountains. Bull. Min. Soc. So. Calif.: 86(5) (May 2013): 5-7.
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  Skinner, B. J. and Appleman, D. E. (1963), Melanophlogite, a cubic polymorph of silica. American Mineralogist: 48: 854-867.