Loughlinite

loughlinite

shortite

northupite

searlesite

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Formula: Na₂Mg₃Si₆O₁₆.8H₂O
Phyllosilicate (sheet silicate), sepiolite group
Crystal System: Orthorhombic
Specific gravity: 2.165 measured, 2.22 calculated
Hardness: 1
Streak: White
Colour: White
Luminescence: Not fluorescent under UV
Solubility: Decomposed by hydrochloric acid
Common impurities: Ti,Al,Fe
Environments

Sedimentary environments

Localities

At Killik, Mihaliççik District, Eskişehir Province, Turkey, the volcano-sedimentary units consist mainly of altered tuff and claystone, with thin layers of dolostone and silica nodules and horizons. Sepiolite and loughlinite are dominant in the claystone of the study area, generally associated with opal-CT, analcime, feldspar, dolomite and calcite.
Loughlinite and sepiolite occur as fibres 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, in arid and semi-arid climatic conditions. Under these conditions, volcanic glass as well as dolomite release silicon Si and magnesium Mg, which are used for the formation of sepiolite. Increasing evaporation causes the dominance of magnesium with sodium and potassium in the depression zone, 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 environments, rather than being the product of a transformation of one to the other (CM 40.1091-1102).

At the type locality, the Westvaco mine, Green River Basin, Sweetwater County, Wyoming, USA, loughlinite is found in medium- to low- grade oil shale, formed after compaction of the sediments had taken place. Loughlinite is found in veins, usually less than 3 cm long, that vary in thickness up to 1 cm, which is the maximum length of the fibres. In thin section, it is readily seen that loughlinite replaces the dolomitic shale, isolated small islands of the shale being frequently observed within the area occupied by loughlinite. Shortite, northupite and searlesite are likewise seen being replaced by loughlinite. In no instance has loughlinite been observed in the process of replacement by another mineral (AM 45.270-281).

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