Deformed displacive halite crystals: Diagenetic or tectonic origin?

Abstract

ABSTRACT Displacive halite crystals in fine-grained sediments are a common feature of evaporitic sequences worldwide. The crystal habit of halite precipitated from pure aqueous solution under laboratory conditions is a cube: equal side lengths (a = b = c) and rectangular angles (ab = bc = ac = 90°). Crystals with deformed-looking shape appear in mudstone of Permian evaporites of the Eastern Alps. They expose platy, rod-like, parallelepiped-like or rhomb-like shapes. The aim of the study was to explain the origin of these shapes. Samples were taken in the Altaussee, Berchtesgaden, and Hallstatt mines. Representative mudstone samples were scanned by high-resolution X-ray CT, and halite shapes were restored by 3D software. Additionally, 327 euhedral halite crystals were dissected from mudstone and subjected to individual shape analysis. Their sizes range from 3 to 30 mm in thirteen specimens. Crystals are compact, but a slightly platy preference prevails in all samples. Approximately 80% of all objects expose angles > 1° deviating from the right angle, but only ∼ 15% contain angles > 20°. The maximum deviation from the right angle was ca. 33°. Most objects combine a platy shape (a ≈ b ≠ c) with only one angle deviating from the right angle (2× 90°). The acute angle is preferably oriented parallel to the layering. We interpret a primary rectangular crystal growth to platy shapes in the mudstone (reduced fluid circulation). The displacive halite crystals subsequently deformed during compaction of the mudstone. Halite in deformed shapes was replaced by datable polyhalite ca. 15 My after deposition, which rules out deformation during Alpine tectonics.