By Theresa Williams
Slackjaw Sally here to clear up some confusion about rocks and minerals. I come from the mesopelagic ocean layer and didn’t know that rocks and seafloor even existed before I was reeled in for this expedition. Boy, did I have a lot to learn!
You can think of a rock as a granola bar with each ingredient a mineral. Just like granola bars can be made of any combination of ingredients in any size or proportion, rocks can be made of any combination of minerals in any size and proportion. Okay, so rocks are made of minerals, but what exactly is a mineral?
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered atomic arrangement. That sounds deep, so let’s break it down. Minerals are natural and they were never alive. Minerals are solid. A specific mineral is always made with the same ingredients in the same proportions. Each mineral has a specific crystal structure determined by the way its atoms connect to each other. Think about it. Ice is a mineral, but liquid water is not! Why?
There are about 4,000 different minerals in the world, made from various combinations of atoms. Some minerals are made of only one type of atom like copper, but most minerals are made from combinations of two or more atoms. For example, halite (table salt) is a mineral made of sodium and chlorine atoms. Each sodium atom connects to six chlorine atoms and vice versa in a repeat cubic arrangement forming the mineral halite. When you look at table salt under a microscope, you see a pile of tiny cubes. You know it’s a mineral because it occurs naturally, it’s inorganic (doesn’t come from living things), it’s solid, there is exactly one chlorine atom for every sodium atom, and it always forms crystals.
When humans describe minerals, they include many physical properties like color, hardness, crystal structure, density, and luster. When humans describe rocks, they include the types and proportions of minerals in the rock as well as the mineral crystal sizes. In the gabbro samples, they’re mostly looking at the minerals plagioclase, pyroxene and olivine. The proportions of those minerals in the rock tell us about how hot the magma was that the rock came from.
There’s a lot of rock describing going on around here!
P.S. When water is liquid it doesn’t have a crystal structure, but it always freezes into a solid with the same specific crystal structure.