FORMATION OF PARTING IN QUARTZ

This paper presents hydrothermal quartz displaying macroscopic planar partings shown as large, mm-spaced continuous planar penetrative faces with pearly luster. The studied quartz comes from hydrothermal quartz veins from the Mesoproterozoic Modum complex in Southern Norway containing albite, clinochlore, hornblende and hydroxyl apatite intersecting well foliated and banded amphibolite-mica gneiss at high angle. The quartz is anhedral, inequigranular and has undulose extinction and sub-grain rotation microstructure. Thin sections oriented perpendicular to the most pronounced planar parting show lamellas that extinguishes at approximately 2 degrees to each other.

EBSD mapping has been conducted on sections perpendicular to and parallel to the parting. Sections parallel to the parting display two orientations {0-111} (r-face) and {1-101} (z-face) separated by irregular boundaries. The misorientation between these two crystallographic orientations on the parting is a 60 degree rotation on [001], in accordance with the Dauphiné twin law. In sections normal to the parting the Dauphiné twins are cut by the parting, but display irregular boundaries in the quartz between the partings.

SEM-CL imaging documents three generations of quartz and two planar structures. The most prominent of the three appears to cut across the recrystallized quartz causing an apparent displacement. Some correlation between the recrystallized quartz and the Dauphiné twins are seen, but appear rather inconsistent.

Etching has been conducted in attempt to reveal polysynthetic Brazil twins. This, however, yielded no evidence of such twinning, but confirmed the irregular Dauphinè twins.

The parting is penetrative and cutting through all preexisting features, such as fluid inclusion trails. We propose, in our working theory, that a sufficiently high differential stress must have been produced to be able to break the strong Si-O bonds in optimally oriented crystals. A rather quick deformation mechanism is suspected. No pre-deformation weakness zones, such as polysynthetic Brazil twins, have been revealed to ease the formation of a parting plane in either direction.