Compositions of fluid inclusions containing halite and sylvite
daughter minerals can define a linear trend of decreasing salinity
when plotted in the NaCl-KCl-H2O ternary system. This is
termed the Halite Trend, and is attributed to halite precipitation
from the fluid before trapping. Salt crystals observed in
vein quartz crystals from mineralized zones in the Capitan Mountains
NM, USA using an electron microprobe are evidence of halite precipitation
causing the halite trend. The crystals (observed on both
polished and broken sample surfaces) are often euhedral and enclosed
in quartz with little suggestion of associated fluid cavities.
Based on the textural evidence, these salt crystals are interpreted
as solid inclusions that were trapped during the growth of quartz.
Given halite saturated fluids, trapping of heterogeneous (liquid
+ halite) fluid inclusions is possible, leading to the potential
to overestimate the fluid inclusion salinities.