49.

THE MINERALOGY OF A PHOSPHATIC HORIZON AMATUKU ISLET, ATOLL,

K.A. Rodgers Department of Geology, University of Auckland Private Mail Bag, Auckland New Zealand

ABSTRACT

A sample of phosphatic horizon on Amatuku islet, record ed by the Royal Soc iety expeditions of 1896-8, consists of a biocalcirudite cemented by a thin (0.02 - 0.05 mm) collophane crust. The only apatite mineral identified in the cement as a result of optical, electron-microscope and x - ray examination is the carbonate hydroxyapatite, dahllite. Although Tuvalu has little intensive agricultural production and the requirements for phosphorus fertilizer are relatively small, the cr ushed phosphate rock may be converted into a more efficient local fer tilizer by blend ing with sulphur.

INTRODUCTION

The prese nce of thin, localised, phosphate-cemented horizons within the o therwise dominant carbonate rocks and sediments of Funafuti atoll were noted b y several workers reporting on the findings of the Royal Society bo rin g expeditions of 1896, -97, - 98 (Cooksey, 1896; Sollas, 1904; David and Sweet, 1904; Judd, 1904; and cf . Cullis, 1904 p.392). While the chemistry of the mineral cement was ascertained, no identifications were made of the minerals present. S ubsequent studies of Tuvalu's phosphates ha ve not r e - exami ned the Funafuti deposits. Hutchinson ( 1950) overlooked any phosphate occurrences on the nine atolls, even those worked on Niu l akita in the late nineteenth and early twentieth ce ntur y. \vhite and Warin (1964), and Warin (1968), do c umented major deposits o n Nukef etau and as well as a smaller one on but

S. Pac. J . Nat. Sci., 1987, Vol. 9 49- 56 50. did not visit those known on Funafuti.

I recently received a sample of a localised horizon occurring on Amatuku islet from Mr S. Rawlins of Funafuti, with a request to determine the mineralogy as a means of checking any potential the rock might have as a local fertilizer. It is presumed that the specimen came from either the occurrence described by Sollas (1904) or a closely similar deposit. Its petrographic character conforms to his general description.

Petrography

The specimen (University of Auckland, Geology Department number 37469) is a somewhat leached, highly porous, fine biocalcirudite consisting of granule- and pebble-sized clasts in a grain supported fabric. The bioclasts are subangular to subrounded and well sorted, and consist of calcareous algae (Lithothamnium and ?Halimeda) and corals, with rare echinoid spines, gastropod and foraminiferan skeletal remains. The bonding cement is a thin (0.02-0.95 mm), reddish-brown collophane crust which throughout the bulk of the rock shows a brown, somewhat powdery surface. On less leached portions and freshly broken surfaces, the phosphate cement is light tan-coloured with a finely botryoidal surface and a vitreous lustre. It covers the surface of all clasts and partially infills pores and borings (Fig. 1).

Examination of moderately thick (0.04-0.05 mm) polished thin sections shows the cement to consist of from three to eight layers of extremely fine-grained, translucent phosphate which is tinted yellow to pale brown, and is weakly anisotropic with a r.i. greater than balsam. These properties are typical of collophane (e.g., McConnell, 1950). Individual layers are about 0.005 mm thick. All are of similar appearance, with a suggestion of laminate structure. There is no evidence of microfibrous texture, nor of pre-existing carbonate cement(s), nor of replacement of such cement by phosphate (cf. Braithwaite, 1968). The clasts themselves show no attack by the phosphate although most show some loss of definition of their internal 51.

Figure 1. Scanning electron micrograph of a fresh surface of collophane crust showing the fine, botryoidal habit of the cement and the puckered and rucked up nature of the surface which is also apparent in the transverse section of Fig. 2. White scale bar is O. 05mm long.

Figure 2. Photomicrograph of thin section of two coral clasts cemented by multiple concentric layers of finegrained collophane. Note that the fibrous aragonite which forms the trabeculae of the coral show some loss of definition. Plane polarised light; x320. 52. structure. The extent to which this primary texture has been lost varies both between and within individual clasts (Fig. 2). Some cavities appeared to contain small quantities of phosphatic sediment, poreward of the cement encrustation, which seemed similar in some respects to that described by Braithwaite (1968) from comparable phosphatic carbonates of the Indian Ocean. In the present sections it was not clear if this material was part of the powdery cement surface on a more weathered portion of the rock, or other than a natural deposit, being a function of the manner in which the section was prepared.

The handspecimen fluoresces a pale lemon yellow in longwave U.V., the colour seeming to intensify under shortwave. It is the clasts rather than the cement which obviously fluoresce but in thin section the clasts show little excitation under the microscope while details of the cement structure are enhanced.

Mineralogy

The x-ray powder di~fraction pattern of the cement shows the signature of apatite underwritten by that of high magnesium calcite and/or aragonite. The last two minerals compose the bulk of the clasts. A pure separate of the cement could not be obtained.

Most of the refelctions in the apatite pattern show a broad line width which, when combined with the patterns of either two carbonates, makes it difficult to identify the signatures of some other phosphate minerals which have been recorded from comparable insular settings (Hutchinson, 1950; McConnell, 1950). For example, while brushite and monetite appear to be absent, minor amounts of whitlockite may be present. The three strongest reflections of this mineral lie in a position occupied by the edge of prominent broad apatite reflections in the present pattern.

Comparison of the apatite reflections with ASTM standard patterns, the data of Lehr et ~. (1967) and that of McConnell (1973), suggest that 53. the phosphate is dahllite, carbonate h ydroxyapatite. However, it is not possible to make an unambiquous interpretation of the collophane pattern as being one of several apatite species due both to the width of the reflections and their maximum intensities not being clearly defined. Braithwaite (1968) regards this apparent blurring of similar patterns as resulting from the "low grain size and poor crystallinity" of collophane. However, the effect may well reflect a range of compositions amongst the cement crystallites. The closest signature match for the mid-point of the various reflections is with t he dahllite of McConnell (1960, 1973). Semiquantitative energy dispersive analysis using a scanning electron microscope confirmed that fluorine was below 1% in both of two small fragments examined and hence the cement is not fluorapatite as implied by Warin (1968 p.129) and, in this connection, McConnell's (1957) stricture against using the term "fluorapatite" in a casual way should be noted.

DISCUSSION

It would be unwarranted to attempt to draw any conclusions as to the origin of the Amatuku phosphate based on this study of a single sample divorced from its outcrop. White and Warin (1964), and Warin (1968), followed earlier workers such as Hutchinson (1950) in regarding all such insular phosphates of the Pacific as being derived from avian guano decomposing in situ under warm arid conditions. Slightly acid solutions leached from the guano are regarded as reacting directly with the underlying carbonate rocks with rainwater playing an active role in the metasomatism. No evidence for or against this notion was found in the present study.

The fact that the phosphate is dahllite, rather than the less soluble fluorapatite or francolite, offers some hope that the deposit's nutrient value may be able to be tapped for agriculture. However, it should be noted that in Tuvalu little intensive agricultural production occUrS and the requirements for fertilizers are small. Phosphorus has not been shown to be limiting except in situations where small scale intensively utilized composted systems have been established. In these 54. latter situations fertilizers consisting of blends of sulphur and the crushed phosphate rock may prove very effective, if the major nutrient limitations (potassium and iron) are overcome.

The problem of increasing the availability of phosphorus to plants resolves itself essentially around reducing the pH of the carbonate groundwaters from their present value near 8.4 (Krauskopf, 1967) which inhibits breakdown and solution of the phosphate. The sulphur may have the effect of dropping the pH sufficiently in the vicinity of plant roots to promote more efficient dissolution of phosphate (Dr John Rogers, pers. comm., July 1986).

ACKNOWLEDGEMENTS

Thanks are due to Sam Rawlins for supplying the sample, and the New Zealand Ministry of Foreign Affairs' diplomatic bag which helped to deliver it to the Department of Geology. Sue Courtney, Nan Howett and Dave Stringer provided technical expertise.

REFERENCES

Braithwaite, C.J.R. 1968. Diagenesis of phosphatic carbonate rocks on

Remire, Amirantes, Indian Ocean. ~ Sed. Petrol., 38, 1194-1212.

Cooksey, T. 1896. Rock specimens from Funafuti. 73-78.

Cullis, C.G. 1904. Mineralogical changes observed in cores of Funafuti borings. In: The atoll of Funafuti. Borings into a coral reef and the results. Report, Coral Reef Committee, the Royal Society, Harrison & Sons, London. 392-420.

David, T.W.E. and Sweet, G. 1904. The geology of Funafuti. In: The atoll of Funafuti. Borings into a coral reef and the results. Report, Coral Reef Committee, the Royal Society. Harrison & Sons, London. 61-124. 55.

Hutchinson, G.E . 1950. The biogeochemistry of vertebrate excretion. Bull. Am. Mus. Nat. Hist. 96, 554pp.

Judd, J.W. 1904. The chemi cal examination of materials from Funafuti . In The a toll of Funafuti. Borings into a coral reef and the

results. g~~ Coral Reef Committee, the Royal Society. Harrison & Sons, London. 362-389.

Krauskopf, LB. 1967. Introduction to geochemistry. McGraw-Hill , New York.

Lehr, J.R., Brown, E . H., Frazier, A.W . , Smith, J.P. and Thrasher, R.D. 1967. Crystallographic properties of fertilizer compounds.

Tennessee Valley Authority, Chem. ~ Bull., £, 166pp.

McConnell , D. 1950. The petrography of rock phosphates. Jour. Geol., 58, 16-23.

McConnell, D. 1958. The apatitelike minerals of sediments. Geol., 53, 110-111.

McConnell, D. 1960 . The crystal chemistry of dahllite. Mineral., 45, 209-216.

McConnell, D. 1973. Apatite: its crystal chemistry, mineralogy, utilization and geologie and biologic occurrences. Springer- Verlag, Wien. 111pp .

Sollas, W. J . 1904. Narrative of the expedition of 1896. l!l : The atoll of Funafuti. Boring into a coral reef and the results. Report, Coral Reef Committee, Royal Society. Harrison & Sons, London. 1-28.

Warin, O.N. 1968. Deposits of phosphate rocks in Oceania. United

Nations Min. Res . Dev. Ser. , ~, 124-132. 56.

White, w.e. and Warin, O.N. 1964. A survey of phosphate deposits in the south-west Pacific and Australian waters. Bur. Min. Res. Geol. Geophys. Bull., 69, 172pp.

Accepted 15 September, 1986.