MALTESE MINERALS

INTRODUCTION

Due to the sedimentary origin of Maltese Islands rocks, there are very few minerals worth of mention. The bulk of the Islands is made up of various forms of limestone. These have been used extensively in building and lime making. The Greensand strata contains grains of glauconite, felspar, quartz, augite, zircon, hornblend, magnetite and tourmaline. These are however relatively unimportant since the diameter of these minerals barely exceeds 0.2 mm. Minerals worthy of note are those derived from calcium: calcite forms, gypsum and anhydrite.

CALCIUM MINERALS

CALCITE is formed by the redeposition of carbonate after this is dissolved by rain-water. It may occur in numerous variety of forms including stalactites, stalagmites or incrustations in caves; as crystal aggregations in fault crevices or the Blue Clay strata. It is one of the most common rhombohedral carbonates of calcium, the crystals appearing either as a minute indistinguishable mass; a needle-pointed mass termed NAIL-HEADED SPAR; or as a large crystal mass. It may be colourless, white, or more commonly tinged with yellow, red or brown because of iron and other impurities. It may have a vitreous to earthly lustre. Its hardness lies in the 3.5-4.0 range.

GYPSUM or SELENITE, usually found in the Blue Clay strata, are colourless crystals with a pearly or silky lustre. The crystals are lozenge-shaped often twinned to give an arrow-head shape. Cleavage is perfect in one direction giving flexible unelastic flakes. Its hardness and specif gravity are 2.0 and 2.3 respectively. It is soluble in dilute acid. Associated with gypsum is ANHYDRITE, distinguished from it by its greater hardness (3.0-3.5) and more massive character. Its appearance is prismatic and tabular. Cleavage gives rectangular fragments. Its specific gravity is 2.89-2.98. It is only with difficulty soluble in dilute acid.

IRONSTONES

Iron oxide is a common constituent of Maltese rocks, particularly in the Blue Clay strata. It is however very finely mixed and forms no definite aggregate, except in mineralized fossils and occasional deposits of very marked red Globigerina Limestone. A number of IRON NODULES (IRONSTONES) are found in the Blue Clay strata, these being composed of a peroxide of iron. Such mineralization is very commonly associated with organic remains - in fact most fossils obtainable from the Blue Clay strata are markedly mineralized. Ironstone may also be met occupying fissures and irregular cavities. LIMONITE is also associated with iron rich areas of Globigerina Limestone. When pure, Limonite is a compact fibrous or statactitic aggregate, in which form it usually occurs in viens and cavities. When appearing as a bedded rock it is usually earthy and porous, and full of impurities.

PHOSPHATE MINERALS

A number of substrata of Globigerina Limestone contain PHOSPHATIC NODULES. These nodules vary from pea-size to about 7 inches diameter. They consist of an agglomeration of rounded or oval pebble-like concretions with a chocolate -brown colour and generally a smooth glazed surface. The origin of these nodules is not clear, but the "Challenger Expedition" dreged many similar nodules from depths of 150-1900 fathoms off the Cape of Gode Hope. They are formed at the bottom of the sea along the continental slopes and are derived from the bodies of marine animals.

SILICATES

In the Globigerina Limestone strata, there may be sometimes found quantities of siliceous concretions which are irregular-shaped masses of 2 feet or more in length and 6-12inches thick, or spheroidal-shaped nodules with symetrical outlines. The larger masses consist of pure flint known as SILEX and the smaller nodules of chert - PHTANITE. The latter is an impure calcareous variety of flint. The colour is grey, fawn, blue or black, and under the magnifying glass sections of foraminifera, small molluscs and other organisms may be seen. Flint forms break with a marked conchoidal fracture, and may be translucent along the sharp cutting edges. The precise mode of formation is not clear, but it would appear to be partly of organic, partly of chemical origin. Sponges and other organisms secret soluble silica from seawater, and when they die additional silica is deposited upon and within their skeleton and exuviae. Calcareous shells, and even chalk itself in which these are embedded, have often been partially or whooly replaced by silica.

ORGANIC-DEPOSITS

Associated with the Globigerina Scutella Transition bed in a number of localities in Malta and Gozo, quantities of rounded boulders and angular fragments of BLACK LIMESTONE have been found. These vary in size from a walnut to a medium-sized pumpkin. They occur in the gorges beds, in the field soils and in all those localities where the Globigerina Limestone has been eroded away. Evidence of Black Limestone in-situ was found by J.H. Cooke opposite Il-Mara (Malta) - between Benghisa and Wied il-Mixta. The thickness did not appear to exceed one foot and there was a gradual transition of the black limestone into the characteristic yellow variety. Cooke also found a patch or view with a diameter of 4 feet at a site 200 feet to the west of Ghar Hassan Cave (Malta); and irregular patches near the Madonna della Qala Church (Gozo). A chemical examination of the rock by Dr. J.A. Smythe gave traces of manganese whilst the black matter was found to be of a bituminous nature. A thin section showed the limestone to be unaltered but smudged by an impregnation of the carbonaceous matter, which was finely divided carbon rather than organic matter or hydrocarbons. Emmons test for petroleum-bearing rocks carried out on a specimen of Black Limestone collected from Il-Mara proved negative. The origins of the rock remains speculative.

The prospects of petroleum exploration in the Maltese arcipelago area have long been explored both on the Islands and in off-shore areas, but with no economic success to date.