The textures of rocks tell the geologist a tremendous amount about how the rock formed. But, sometimes textures can be misleading. The image on the left shows an outcrop of “basaltic” volcanic rock studied by PSI Research Scientist Joseph Michalski located at Swansea, Ariz. – an old mining town just east of Parker that operated in the early 20th century. But, upon closer review, these rocks, which texturally look like the volcanic rock called basalt, are composed of totally different minerals. The center image shows that volcanic rock up close. On the right, you see a mineral map of the rock at microscopic resolution. In the micro-infrared image on the right, carbonate minerals appear as blue-purple, potassium feldspar appears as green, and clay minerals appear as red brown. This is a very special rock because none of those minerals is actually found in basalt! That’s right, this rock has been 100 percent replaced by alteration minerals! How did that happen? The rock that was originally basalt was soaked in hot brine approximately 17 million years ago, while the southwestern part of the continent was extending. That briny fluid completely altered this rock. PSI scientists are studying altered rocks like these to better understand the nature of alteration minerals seen on Mars with spacecraft.
