ORIGIN OF SEPIOLITE AND LOUGHLINITE IN A NEOGENE VOLCANO-SEDIMENTARY LACUSTRINE ENVIRONMENT, MIHALlCClK ESKISEHIR, TURKEY

Abstract

The Middle to Upper Miocene volcano-sedimentary units in the Mihalıççık-Eskişehir area of Turkey consist mainly of altered tuff and claystone, with thin layers of dolomite and silica nodules and horizons. Ripple marks and desiccation cracks are common in the siliceous tuff and dolomite units. Green to yellowish green claystone exhibits either hard layered or friable features. Sepiolite and loughlinite are dominant in the claystone of the study area. These minerals are generally associated with opal-CT, analcime, feldspar, dolomite and calcite. At Mihalıççık, loughlinite and sepiolite are identified by d(110) reflections at 12.9 and 12.3 Å, respectively. Loughlinite and sepiolite occur as fibers in dissolution voids and along the sides of dissolved volcanic glass particles and casts of silicic nodules. The area is represented by a lacustrine environment of variable depth and salinity, controlled by synsedimentary step-faulting in arid and semi-arid climatic conditions. Under these conditions, volcanic glass as well as dolomite release Si and Mg, which are used for the formation of sepiolite. Increasing evaporation causes the dominance of Mg with Na and K in the depression zone in the Killik area, favoring the formation of loughlinite under alkaline conditions. Field observations and mineralogical determinations indicate that sepiolite and loughlinite are both formed authigenically and independently in different physicochemical environments rather than being the product of a transformation of one to the other.

ON THE OCCURRENCE OF LOUGHLINITE (Na-SEPIOLITE) IN PANS OF THE NORTHERN CAPE PROVINCE, SOUTH AFRICA

“…As mentioned earlier, loughlinite has only been described from three localities world-wide, all of which are playa-lake settings (Eugster and Hardie, 1975;Kadir et al, 2002;Buch and Rose, 1995) similar to that described here. Interestingly, the mineral analcime has been described from all of these localities as well (Echle, 1967;Buch and Rose, 1995;Iijima and Hay, 1968), which would suggest similar physico-chemical requirements for the formation of both loughlinite and analcime.…”

“…The mineral has subsequently only been described from two additional localities, viz. i) Killik (Turkey), where it occurs within a Middle to Upper Miocene volcanosedimentary lacustrine basin (Echle, 1967;Kadir et al, 2002), and ii) from the Etosha Pan region in Northern Namibia (Buch and Rose, 1996). The limited distribution of the mineral is confirmed by Anthony et al (1995), who make mention of only the occurrences in Wyoming and Turkey.…”

“…In the Green River occurrence, loughlinite was found in medium-to low-grade oil shale of the Wilkins Peak member of the Green River formation, where it occurs as veins replacing dolomite, shortite, northupite and searlesite (Fahey et al, 1960). In Killik, Turkey, loughlinite and sepiolite is found in claystone where according to Kadir et al (2002) it is deemed to have formed authigenically and independently under different physicochemical conditions. Echle (1978), however, prefers a model in which sepiolite may have been loughlinitized and loughlinite sepiolitized in the deposits of Killik, based on his experimental work and field observations from the area.…”

“…The presence of numerous, idiomorphic calcite crystals occurring in association with loughlinite within the sediments of Koi Pan argues against a similar process (cf. Kadir et al, 2002). Furthermore, Velde (1985) suggests that sepiolite is rapidly destroyed during weathering, making it an unlikely component of the pan environment if it was introduced as a result of weathering of the Kalahari calcrete.…”

On the Chemical Composition of Sepiolite and Palygorskite

“…Loughlinite is the Na-sepiolite, in which Mg is partially replaced by Na and also contains Na in the channels (Fahey and Axerod, 1948). Kadir et al (2002) Suarez and GarciaRomero (2006) reported that isomorphic substitutions in octahedral palygorskite sheets occur only in M2 posi tions. Fe may occupy M2 positions whereas Mg can occupy all possible sites: M1, M2, and M3.…”