Blackhead Quarry

Blackhead Quarry –Zeolites (Thanks to Heather Wilson)

Blackhead is located ~5 km southwest of the city centre of Dunedin, South Island, New Zealand (latitude 45º55.8’S, longitude 170º26’E). The Blackhead body is part of the Dunedin Volcanic Group, which is centered upon the Miocene Dunedin shield volcano.Blackhead Quarry is located within a peripheral volcanic vent and is hence included with the Dunedin Volcanic Group. Blackhead Quarry exposes massive basanite as well as other subordinate rock types that host many of the zeolite associations. The Quarry has changed from this description as it has been well worked down many layers.

Highly vesicular (50 –75 vol.% vesicles) altered basanite is restricted to the top of the exposed sequence. A pale grey basaltic lithic tuff is restricted to the northern side of the quarry and a highly vesicular(50 vol.% vesicles) pale grey crystal tuff is exposed elsewhere in the quarry. The Blackhead volcanic centre erupted through Upper Cretaceous to Neogene marine sediments. Spectacular sub-vertical columnar jointing over 30 m high is evident throughout the quarry, though the basalt is generally massive and devoid of vesicles. The lowest exposed basalts are overlain by highly vesicular spatter cone material in which abundant zeolite-rich vesicles occur. Natrolite, phillipsite and calcite are found at the top of the quarry, within either vesicular basanite or highly vesicular altered basanite.Chabazite and thomsonite are restricted to the tuffaceous volcanics from the northeastern part of the quarry. Rare gismondine is only found in one small area of the quarry within massive basanite, below the natrolite-bearing zone.

Here are a few of the more common minerals that can be found in the basalt.

Natrolite

Natrolite is the main secondary mineral within the Blackhead Quarry. It occurs as hemispherical aggregates of glassy, colourless to white, well terminated striated (parallel to the c axis) thin prismatic crystals, commonly radiating from a central region of gonnardite as epitaxial overgrowths. There is a sharp contact between the two phases. Many prism faces have a very thin coating of montmorillonite as botryoidal aggregates In early quarrying operations, natrolite was found in large vugs up to 30 cm in diameter, from the upper part of the quarry.As time went on ( as club members will verify) the natrolite has been harder and harder to come by.

Thomsonite

Thomsonite typically occurs in small vesicles within pale-grey basaltic lithic crystal tuff as interpenetrating radial aggregates of thin, glassy, colourless to dull-silky-white, well terminated bladed crystals up to 1 mm in length. It is commonly associated with calcite, montmorillonite and phillipsite. In places, the crystals are coated with a thin layer of montmorillonite. There was Thomsonite found on this trip but once again smaller areas of it inside the vugs.

Gonnardite

Is very common, usually as pearly white centres (up to 10 mm in diameter) within hemispherical natrolite aggregates in massive basanite. It occurs on either glassy, colourless, equant fourling-twins of phillipsite or grained montmorillonite. It commonly has an epitaxial overgrowth of colourless, glassy, prismatic crystals of natrolite. Gonnardite also occurs within highly vesicular, pale-grey, altered basanite as silky white hemispherical aggregates and radiating sprays of thin, elongate, prismatic crystals. These aggregates are commonly hollow and have epitaxial overgrowths of colourless, glassy, natrolite.

Phillipsite

Phillipsite is very common and occurs in the following forms:

(1) as lustrous, glassy, short prismatic, moderately striated fourling twins, generally forming dense interpenetrating crystal aggregates lining vugs in fresh basanite In some specimens, the phillipsite is associated with siderite and clay alteration of the basanite. (2) It occurs as linings of densely matted tiny crystals (with occasional larger crystal aggregates of elongate prismatic fourling twins) completely lining vugs in altered basanite; (3) as botryoidal radial aggregates (up to 2 mm) of densely matted tiny, glassy, prismatic phillipsite crystals completely coating vesicle walls in basanite; (4) as avesicle-lining of crystalline glassy equant crystals within massive basanite

Phillipsite

Calcite

Calcite is abundant in a wide variety of habits in both massive basanite and pale grey basaltic crystal tuff. It occurs with most of the zeolite species or as the only phase within the vesicles and vugs, usually in crystals averaging 0.1 –2 mm. An unusual aspect is that each vesicle or vug contains calcite crystals of the same habit, with no other habit present. Calcite ranges from completely transparent and colourless, to opaque, white crystals. The different habits noted from this locality are:

(1) most commonly as glassy colourless and massive crystalline

(2) commonly as partially etched, glassy, colourless, nail-head crystals on a thin siderite lining within massive basanite.

(3) as colourless, glassy, complexly twinned acute rhombohedral crystals, often with etched and corroded faces. The main elongate faces are also highly striated, with the striations parallel to the long axis of the face. Interpenetrating crystals and inclusions of vermicular pale emerald-green chlorite are common. This form occurs either on a lining of siderite on massive basanite or, within the pale grey basaltic lithic crystal tuff;

(4) as glassy, colourless, highly modifed scalenohedral crystals on a thin crust of tiny phillipsite crystals overlying fine-grained siderite in massive basanite

(5) as colourless, glassy, zoned (consisting of alternating colourless and cloudy layers along the rhombohedral faces), interpenetrating simple rhombohedral crystals (up to 2 mm) with pronounced indented etching along the crystal edges

Olivine.

Olivine can be found in the basalt and is usually named for its typically olive-green color (thought to be a result of traces of nickel), It has a conchoidal fracture and is rather brittle. The hardness of olivine is 6.5–7, its relative density is 3.27–3.37, and it has a vitreous luster. It is transparent to translucent.

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