This view from the Mars Hand Lens Imager (MAHLI) on the arm of NASA's Curiosity Mars rover shows texture within a light-toned vein at a site called "Garden City" on lower Mount Sharp.
The area shown is roughly 0.9 inch (2.2 centimeters) wide. It was taken during the 946th Martian day, or sol, of Curiosity's work on Mars (April 4, 2015).
Differences in textures of light-toned veins in the Garden City complex of crisscrossing mineral veins are clues that these veins may result from distinct fluid events. This vein's texture shows indications of crystal growth, suggesting that crystallization may have exerted a force for opening the fracture filled by the vein. Different examples are at PIA19926 and PIA19927.
Mineral veins often form where fluids move through fractured rocks, depositing minerals in the fractures and affecting chemistry of the surrounding rock. At Garden City, the veins have been more resistant to erosion than the surrounding host rock. The fluid movement through fractures at Garden City occurred later than wet environmental conditions in which the host rock formed, before it hardened and cracked.
Malin Space Science Systems, San Diego, built and operates MAHLI. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Science Laboratory Project for the NASA Science Mission Directorate, Washington. JPL designed and built the project's Curiosity rover.
More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl/.