Clay Minerals (1976) 11, 147.

P A L Y G O R S K I T E - - B Y D I R E C T P R E C I P I T A T I O N F R O M A H Y D R O T H E R M A L S O L U T I O N

W. J. F U R B I S H AND T. W. S A N D O

Department of Geology, Duke University, Durham, North Carolina, U.S.A.

(Received 10 September 1975; revised 30 January 1976)

ABSTRACT: Palygorskite, associated with aragonite, occurs as a fracture filling in a dunite body that has been intruded by felsic pegmatites. The palygorskite formed by direct precipitation from solutions given off by the pegmatite. Aluminium, calcium and silicon were provided by the pegmatite solutions while magnesium was derived from the serpentinization of the dunite along the host fractures. No direct replacement took place.

I N T R O D U C T I O N

According to the literature palygorskite can form in one of three environments: marine or lacustrine sediments; during pedogenesis or as a result of pedogenic action; from hydrothermal solutions. The foremost question that remains, however, is whether it forms by alteration or replacement (transformation) of a pre-existing mineral or by direct precipitation (neo-formation) from solutions whose ionic content is derived either locally or elsewhere. To complicate matters, the distinction between the two modes of formation is in many cases a fine one. The required content and concentrations of the depositing solutions are also in question.

F I E L D R E L A T I O N S A N D M I N E R A L O G Y

The palygorskite of this study came from a quarry located in the west central portion of the Day Book dunite. The dunite body occurs about 5 km north of Burnsville, Yancey County, North Carolina and its outcrop, which is about 200 m wide by 650 m long, strikes in a northeast-southwest direction. Geology" of this body and others in the area has been discussed by Hunter (1941).

A number of zoned felsic pegmatitic bodies intrude the dunite in a direction parallel to its long dimension and are exposed on the floor and the working area of the quarry walls. They consist predominantly of feldspar and quartz with minor amphi- bole and mica.

The dunite in which the pegmatites are enclosed consists of fine to medium grained olivine with minor amounts of scattered chromite crystals and a few small veins of talc. A fracture system that is peripheral to the pegmatite bodies radiates outward

148 W. J. Furbish and T. W. Sando

FIG. 1. Scanning electron micrograph of palygorskite vein, containing unaltered aragonite crystals, in contact with altered olivine. Colloform banding parallels aragonite

crystal configuration.

FIG. 2. Scanning electron micrograph of palygorskite vein in direct contact with unaltered olivine. Colloform banding parallels fracture wall configuration.

Palygorskite from a hydrothermal sohttion 149

into the host rock for approximately 10 m from the pegmatite bodies. It is in the fractures of this system that the deposition of palygorskite took place.

Palygorskite has been deposited either alone or with aragonite. When aragonite occurs with palygorskite the aragonite crystals, or groups of crystals, either appear to be suspended in the palygorskite or are attached to the vein wall, indicating that they predate the deposition of palygorskite (Fig. 1). The aragonite crystal outlines are sharp and unaffected by solution. They are enclosed by palygorskite and each is outlined by an envelope of colloform banding (Fig. 1). When palygo~skite occurs without aragonite the colloform banding is parallel with the vein wall configuration (Fig. 2).

In some samples the contact of vein material with the host olivine is sharp and unaffected (Fig. 2). In others there is a zone of alteration in the olivine to clino- chrysotile (Fig. I). When this occurs, no replacement textures between the paly- gorskite and the altered material are discernible.

Identification of all material was by petrographic microscope, XRD, and differential thermal analysis.

D I S C U S S I O N A N D C O N C L U S I O N S

Recently Isphording (1973) reviewed ideas from the literature relevant to the deposi- tion of palygorskite in sedimentary environments. From his work in the Yucatan Peninsula area and the Georgia-Florida 'Fullers Earth' deposits and a review of the literature he decided that although some occurrences of paIygorskite may result from the alteration of clays and volcanic glass, many large deposits are more probably the result of neo-formation adjacent to an area undergoing tropical to sub-tropical weathering. His conclusion follows the lead of such authors as McLean, Allen & Craig (1972), on the palygorskite bearing sediment of the southern High Plains of Texas and New Mexico; Millot (1957, 1962), for West Africa and the French Tertiary basins; and Rogers, Martin & Norrish (1954), for the deposits of Queensland, Australia.

By contrast many authors have concluded that palygorskite originates by a trans- formation process in which the precursor is a smectite or volcanic ash. The idea dates back to Grim (1933) for the 'Fullers Earth' deposit of Georgia-Florida. It has been used since by such authors as Kerr (1937) for the Georgia-Florida deposits, Heystek & Schmit (1953) for the African Transvaal deposits, Loughnan (1960) for the Queensland, Australia deposits, and by Parry and Reeves (1968) for those of West Texas.

Pedogenesis, or the effect of weathering, may result in palygorskite formation and the process seems to account for the majority of palygorskite occurrences that have been described to date.

Several occurrences have been recorded where the mineral was thought to have formed by transformation or alteration of primary minerals in weathered igneous material. Longchambon (1935) found it as an alteration product of amphiboles and pyroxenes and Shaskina (1958) in the weathering of basalts. Similarly, Ovcharenko

150 W. J. Furbish and T. W. Sando

(1964) reported palygorskite from the Ukraine that showed relict amphibole cleavage. An occurrence of palygorskite in arenaceous, dolomitic limestone has been

described by Demangeon & Salvayre (1961). They believe magnesium was acquired from the dolomite whereas silicon and aluminium were supplied by the breakdown of an included quartz-clay fraction.

It is also clear that palygorskite can form directly from solution (neo-formation) in soils and weathered rock materials. Tien (1973) recorded the mineral formation in the joints of a dioritic rock and concluded that it crystallized from ground water involved in the weathering of both the host diorite and the overlying Triassic sedi- ments. Neo-formation by pedogenesis was also proposed by Millot, Poquet & Ruellan (1969) for palygorskite in the carapace level of the eastern Morocco soils. Other workers such as Michaud, Cerighelli & Drouinlau (1945), Muir (1951), Yaalon (1955), Elgabaly (1962) and Singer & Norrish (1974) also ascribe such an origin to paly- gorskite found in the soils of Australia, Egypt, Israel, Syria, and France, respectively.

Palygorskite is also recorded as being a direct precipitate from hydrothermal solutions or as a replacement product of hydrothermal solution action; this model is clearly most applicable to the Day Book occurrence. Probably the first clear evidence given to date for the deposition of palygorskite from hydrothermal solution was that from Van der Wel (1972). He reports its occurrence in association with silver minerals presumably by direct precipitation. More recently Andreas & Puffer (1975) reported palygorskite with calcite in a hydrothermal vein that had its origin in a nearby basic intrusive. Stephen (1954) argued that palygorskite had formed from a syenite body in the Shetland Islands by alteration involving high magnesium hydro- thermal solutions. Such a transformation origin was also proposed by Bonatti & Joensuu (1968) for palygorskite found in deep sea sediments with the precursor being montmorillonite altered by hydrothermal solutions. In many reports there is no clear cut evidence given as to whether the palygorskite was deposited as the direct replace- ment of a pre-existing mineral, by partial replacement of a pre-existing mineral or directly from a solution. This work shows that, given the correct set of geochemical prerequisites, palygorskite can crystallize directly from a solution.

In their work on the dunites of the Webster-Addic ultramafic body Condie & Madison (1969) proposed an equation for the process whereby MgO could be derived through the breakdown of olivine to serpentine. Such a reaction took place along the walls of some of the palygorskite bearing veins of this study, and, therefore, the reaction becomes a source for Mg in the formation of palygorskite. No replacement textures between palygorskite and serpentine were present, indicating that a high Mg content was in solution when palygorskite formed.

The Mg/A1 ratio would have been high enough to favour the formation of paly- gorskite over sepiolite (Singer & Norrish, 1974), and yet the A1/Si ratio would be too low for the formation of chlorite like minerals instead of palygorskite (Isphording, 1973). Further, aragonite instead of calcite was the stable phase in the palygorskite bearing vein. Bischoff (1968a), Lippmann (1960), and others have advanced the idea that Mg in solution tends to retard the formation of calcite and favour the formation of aragonite and the idea would appear valid in this case.

Palygorskite Jrom a hydrothermal solution 151

Proximal relationship of the palygorskite bearing veins, and their cont inued and direct connect ion with the in t ruding pegmatites Ieaves no doub t that Si, AI, Ca and HzO came from the pegmatite. Mg was added to that solut ion by the breakdown of olivine to serpentine, which also required H / O and Si f rom the pegmatite solutions for this reaction. The ratios and concent ra t ion of these ions in solut ion were such that palygorskite and aragonite were the stable forms able to precipitate directly from solution. This conclusion is further backed by results of petrographic thin section studies that show palygorskite crystallized in direct contact with unal tered olivine vein walls or that no replacement textures were present when it formed in contact with serpentine minerals.

R E F E R E N C E S

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SOMMAIRE: La palygorskite associte avec l'aragonite se prtsente comme une cassure remplie par de la dunnite qui a 6t6 introduite de force par des pegmatites felsiques. La palygorskite est formte par la prtcipitation directe de solutions produites par la pegmatite. Les aluminiurn, calcium et silicium ont 6t6 fournis par les solutions de pegrnatite tandis que le magntsium trait dtriv6 de la serpentinisation de la dunite le long des nombreuses cassures. Aucan remplacement direct n'a eu lieu.

152 W. J. Furbish and T. W. Sando K U R Z R E F E R A T : Palygorskit in Verbindung mit Aragonit kommt als Bruch- stellenftillung in einem Dunitk6rper vor, in den Felsit-Pegmatiten eingedrungen sind. Das Palygorskit beruht auf direckter F~illung aus yon dem Pegmatit abgegebenen L6sungen. Aluminium, Kalzium und Silizium wurden durch die Pegmatitl6sungen geliefert, w/ihrend Magnesium durch die Serpentinisation des Dunits entlang der Wirtbruchstellen gewonnen wurde. Es fand kein unmittelbarer Ersatz statt.

R E F E R A T A : La paligorskita, asociada con la aragonita, se presenta como un relleno de disyuncion en un macizo de dunita que ha sido objeto de intrusion por peg- matitas felsicas. La paligorskita se formo por precipitacion directa de soluciones desprendidas por la pegmatita. Aluminio, calcio y silicio fueron aportados por las soluciones de la pegmatita, mientras que se derivo magnesio de la serpentinizacion de la dunita a lo largo de las fisuras hospedantes. No tuvo lugar ningin reemplazo directo.