Lawsonite

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Lawsonite
CaAl₂(Si₂O₇)(OH)₂ •H₂O

Lawsonite 8,5cm specimen, Syros Island Greece	© Rob Lavinsky

Lawsonite is a mineral formed under water saturated high pressure/low temperature (HP/HT) metamorphic conditions (blueschist facies) typically formed in subduction zones. As lawsonite is not stable under "normal" pressure/temperature conditions at the depths it has formed, it is often metamorphosed to sericite, zoisite or other minerals before it is lifted back to the surface.

Lawsonite and pseudomorhps of Lawsonite has been found in most blueschist terrains around the world, including Ile de Groix, France, the Italian alps, Turkey and other places. It is nevertheless most of all a Californian mineral. It was here it was first described, and it is from here most lawsonite specimens has found their way into private and public collections. The recent finds from Syros challenges the Californian localities as the best in the world. Currently Lawsonite is listed at found at 101 localities, world wide (March 2013).

For those interested, I will recommend the articles, papers and books referenced in this article.

references:

W. T. Schaller and W. F. Hillebrand(1905): Notes on Lawsonite, Bulletin 262, Contributions to mineralogy by the USGC.

Werner H. Baur(1978): Crystal structure refinement of lawsonite, American Mineralogist, Volume 63, pages 311-315,

Eugen Libowitzky, Thomas Armbruster(1995): Low-temperature phase transitions and the role of hydrogen bonds in lawsonite, American Mineralogist, Volume 80, pages 1277-1285

Sean R. Mulcahy, Robert L. King and Jeffrey D. Vervoort (2009): Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes. Geology vol 37 pp987-990

Lawsonite
Greece
Aegean Islands (Aiyaíon) Department, Kykládes Prefecture, Cyclade Islands (Cyclades; Kikladhes; Nomos Kikladhon), Syros Island (Syra), Grammata Bay, Cape Marmari

Lawsonite 7,5cm specimen	© Rob Lavinsky

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Lawsonite 11,6 cm specimen	© Rob Lavinsky

Lawsonite 17,6 cm specimen	© Rob Lavinsky

The well formed green lawsonite crystals in the striking blue glaucophane matrix appeared on the mineral market in 2011/2012. The originate from an undisclosed locality on the small island Syros in the Aegean Sea between Greece and Turkey. Syros has been thoroughly studied by petrologists and geologists as it's suite of high pressure/low temperature rocks represents the crustal root of the Cycladic orogenic belt formed during a Mesozoic Eurasia-Africa subduction. Lawsonite and undeformed pseudomorphs of lawsonite can be found in a wide variety of rocks, including blueschiscts, marbles and graphite schists, predominantly in the northern part of the island. The occurrence of both unaltered and phengite/zoisite/chlorite pseudomorphs of lawsonite together with mineral assemblages of both blueschists and retrograde greenschist facies rocks has been used to investigate the pressure, temperature and fluid composition path during both the subduction and uplift of these rocks.

Lawsonite pseudomorphs stand in relief on the weathered surfaces of a number of blueschist outcrops on the island, in particular those belonging to the Kastri schists. They occur as white, seemingly euhedral, rectangular or diamond-shaped aggregates, 0.2 to 3.0 cm across. The pseudomorphs, some of which contain remnant lawsonite, consist mainly of a fine-grained mixture of zoisite and phengite with other minerals such as chlorite and/or albite. The blueschist matrix assemblage for the pseudomorphs is typically glaucophane + epidote + phengite ± garnet ± omphacite ± rutile ± titanite in rocks with the bulk composition of basalt.

Lawsonite occurs predominantly in the Kastri marble, which are intercalated with glaucophane schists. It is these glaucophane schist horizons whitin the Kastri marble that hosts the green lawsonite crystals, which are formed at methamorphic conditions at about 350-500 deg C and 8-17 kbar. Flickr photos of in-situ lawsonite crystals from Lawsonite Point (on the northern shores of Grammatta Bay) indicates that intact lawsonite crystals may reach sizes up to 3 cm.

references:

Lindsey Able(2001): Lawsonite pseudomorphs in the schists of Syros, Greece, 14th Keck Symposium Volume

John C. Schumacher, John B. Brady, John T. Cheney and Robert R. Tonnsen (2008): Glaucophane-bearing Marbles on Syros, Greece, Journal of Petrology Vol 49 No9 pp1667-1686

Arianne Sperry(2000): Pseudomorphs after lawsonite as an indication of pressure-temperature evolution in blueschists from Syros, Greece, 13th Keck Symposium Volume

Laura Frye-Levine(2004): Graphitic schists of Syros, Greece, 17th Keck Symposium Volume

Mélody Philippon, Jean‐Pierre Brun, and Frédéric Gueydan(2011): Tectonics of the Syros blueschists (Cyclades, Greece): From subduction to Aegean extension, TECTONICS, VOL. 30

David P. Miller, Horst R. Marschall, John C. Schumacher (2009): Metasomatic formation and petrology of blueschist-facies hybrid rocks from Syros (Greece): Implications for reactions at the slab–mantle interface, Lithos Vol 107, pp 53–67


USA
California, Marin Co, Tiburon Peninsula, Tiburon Uplands Nature Preserve, Reed Station



All Lawsonite localities on the Tiburon Peninsula lies in the Ring mountain area, not far from Reed Station, and this and the next locality has not been distinguished in this text.


USA
California, Marin Co, Tiburon Peninsula

Lawsonite 4,0cm crystals	© Rob Lavinsky

Lawsonite 6,9 cm specimen	© Rob Lavinsky

Lawsonite 5,7 cm specimen	© Weinrich Minerals, Inc.

Lawsonite was discovered and published by F.L Ransome in 1895 in material found “ as an important rock-making constituent of a rather massive outcrop of crystalline schist, which is exposed near the periphery of an extensive sheet of serpentine, on the Tiburon Peninsula,at a point about half a mile in an easterly direction from Reed Station, on the line of the San Francisco and North Pacific Railroad” ¹

According to Samuel Rice, the Lawsonite from the type locality occurred in a “ rock that is acolorful schist composed largely of blue glaucophane, green pyroxene, red garnets, and white mica.”

In places, white, tabular crystals of lawsonite have partially replaced the other minerals in the schist. However, it is from vein-like masses of green mica in the outcrop that Ransome collected the large pale blue lawsonite crystals that he described, and this is the environment of the best specimens. Individual lawsonite crystals observed at this locality range up to two inches in length.

Lawsonite occurs in a thin zone (10-15 meters) (the mélange zone) between the Fransiscan metasedimentary rocks at the rocks belonging to the Fransiscan complex and overlaying ultra-mafic rocks belonging to the Coast Range ophiolite, formed by local high pressure in the contact between the two formations.

Lawsonite crystals occurs in two distinct environments in this mélange zone, either in. high grade metamorphosed “exotic blocks” as for the type locality, or with albite in veins in Franiscan metasandstones. The latter type lawsinite crystals rarely exceeds 1 cm.

References:

¹Salem I. Rice (1964): The lawsonite type locality, Mineral information Services, Vol 17, No 6

Oliver E. Bowen (1972): Rocks & Minerals of the San Francisco Bay Region California Natural history guides 5 University of California Press, 3rd edition.

David A. Bero, Christen D. Rowe (?):Fault-related metamorphism along a remnant of the Coast Range Thrust, Ring Mountain, Marin County, California


USA
California, Mendocino Co.

Lawsonite 4 cm FOV	© Marin Mineral

Lawsonite can be found at several localities in the Mendocino County, and the fine euhedral tabular crystals of pale pink color, up to 2 inches in length described in“Minerals of California" from a creek near Covelo may be the best. The pictured specimens are probably all from a more recent find from a undisclosed locality.

Lawsonite is a rock forming mineral in several fine grained meta-sedimentary rocks in the Coast Range of California in rocks belonging to the Franciscan Complex. The Franciscan Complex represents an accretionary complex (meaning off-shore sediments trusted and subducted underneath a continental shelf) formed by long-term (40-160MA) subduction of an oceanic plate under the Western margin of the North American craton.

The Fransciscan complex consist of three distinguishable belts: the eastern belt the central belt, and the coastal belt. The occurance of lawsonite is largely connected with blueschist facies metamorphosis, and two principle occurances of blueschist are recognized, both of them lawsonite-bearing. The most widespread are regionally metamorphosed greywackes in the eastern belt with minor interbedded chert and igneous rocks. These are of little interest for mineral collectors.

The other are the “exotic blocks”; blueschists derived from basalts, eclogite and amphibolites in the melange zone in the Central Belt, most often near the contact to the Coast Range ophiolite. It is the more coarse grained of these "exotic block" that may contain larger lawsonite crystals. It is usually distinctly crystalline and is found in three different habits, either equidimensional, tabular, or prismatic. The crystals are found either embedded in muscovite or chlorite, or growing into open cavities in veins. The tabular crystals are found only as porphyroblasts in lawsonite-chlorite schist. Crystals of the other types are found in both modes of occurrence. The crystals are generally striated and rough. In size they vary from 1 millimeter or less to a maximum of 5 centimeters.

References:

Ernst, W. G. and R. J. McLaughlin (2011), Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino-Garberville-Covelo 30' x 60' quadrangles, NW California, Tectonics, (abstract)

Robert J. McLaughlin (1981): Tectonic setting of pre-tertiary rocks and its relation to geothermal resources in the Geysers-Clear Lake Area. Geological Survey Professional Paper 1141

Joseph Murdoch, Robert W.Webb (1956) Minerals of California , San Francisco Bulletin 173

R.J. McLaughlin1, S.D. Ellen1, M.C. Blake, Jr.2, A.S. Jayko1, W.P. Irwin1, K.R. Aalto3 (2000): Geology of the Cape Mendocino, Eureka, Garberville, and Southwestern part of the Hayfork 30 x 60 Minute Quadrangles and Adjacent Offshore Area, Northern California, Pamphlet to accompany Miscellaneous Field Studies MF-2336 Version 1.0

Maureen Berlin (2005): Tectonic Wedging in the California Coast Ranges: Useful Model or an Unreasonable Hypothesis? GEOL 5690, Tectonics of the Western U.S.,

Edgar H. Bailey, William P. Irvin, David L. Jones WILLIAM P. IRWIN, and DAVID L JONES (1964): Franciscan and related rocks and their significance in the geology of Western California. US Geological Services, Bulletin 183

Marc Cloos (2013): Field Workshop: Franciscan Subduction Complex, California USA- Accretionary Tectonism Near the Inlet of a Subduction Channel. First Notice letter.

George Switzer(1951): Mineralogy of the California glaucophane schists, San Francisco Bulletin 161.


USA
California, Sonoma Co , Healdsburg, Porter Creek



Switzer refers to the "Porter Creek Pumpellyite Locality", which was the first locality where pumpellyite was described in California. The following text is quoted from Switzer:

"The locality is part of a large area of schist, a localized zone in which mineralization was unusually varied. The outcrop from which the specimens described were collected is on a serpentine contact.
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There are several vein minerals, of which pumpellyite is clearly the first. It has formed in numerous veins, either concordant with, or transgressing
the planes of schistosity of the rock. Some of the veins have even, parallel walls and reach a maximum width of about 3 centimeters. Other pumpellyite veins pinch and swell and are not continuous for more than a few centimeters. They are frequently vuggy, but no good crystals were
found. The pumpellyite is fibrous and the fibers are roughly perpendicular to the vein walls.

The age of the lawsonite is clearly shown by veins of lawsonite cutting cleanly across pumpellyite veins. Composite veins of pumpellyite and lawsonite are also found, with lawsonite always in the interior and pumpellyite lining the vein walls. The lawsonite veins are in part parallel to, and in part cutting across the schistosity. The veins have sharp contacts and reach a maximum width of 1 centimeter. Good crystals were found in numerous cavities.

On one specimen a few small albite crystals were found encrusting lawsonite on the wall of a small cavity. Vugs in the pumpellyite and lawsonite veins are locally filled with calcite. "

George Switzer(1951): Mineralogy of the California glaucophane schists, San Francisco Bulletin 161.


USA
California, Sonoma Co, Valley Ford



Valley ford is a typical Franciscan lawsonite locality, where the mineral occurs both associated with glaucophane and micas in "exotic blocks" and in lawsonite bearing veins.

USA
California, Sonoma Co, Valley Ford, North of Valley Ford, Unnamed Glaucophane occurrence




Olav Revheim March 2013

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Edited 6 time(s). Last edit at 03/04/2013 10:35PM by Rock Currier.

Olav,
You are getting really good tweaking the images. The introductory image should have the locality in its caption as well as the species and the size because the header does not say where it is from. Something I have been fooling around with to make the pictures look a little better, especially when there are more than three images you want to use to illustrate the mineral from a particular locality is the spacing between the images.

Notice how the vertical spacing between images is a bit different than the horizontal spacing next to images. The spacing between the images is somewhat larger than the spacing separating the images below. Instead of using 400 pixel sizing on images, I am now using 405 pixels width per image. This pushes the images a bit closer to each other on a line and makes that spacing a bit closer to the spacing of the images below. Its not a big deal, but I think it makes the image presentation look a little better. I when there are an odd number of images, usually there is often an orphan image below the others that I have until now put on the left. I have recently put such orphans as centered images, and I think it makes the picture layout look nicer. However there is no requirement that you adopt this protocol for the images, but you can experiment around with it and if you think the end result is better you can use it.

I have always thought Harjo's image arrangement was slicker than mine and this may narrow the gap in the way they look. The revision of the Best Minerals, Fluorite, USA uses this tweak.

Rock Currier
Crystals not pistols.

Excellent work Olav,
I added a bit telling how many localities Mindat lists for the minerals.

Rock Currier
Crystals not pistols.

Thanks Rock