Basic Geological Concepts
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Basic concepts of geology

What is geology?
The etymology gives us a first approximation: ge:"earth" and -logia:"study of". The study of the earth. As often with science, the starting point of the study of the solid Earth is done through observations and experiments. In geology, there are essentially two scales of observation:
  • Large scale observation, where the study of the Earth is done in situ. This usually involves going to very beautiful places (like here, and here, and here for example), and making broad observations on the shape and characteristics of the Earth's crust at that point. In scientific parlance, this type of observation forms the empirical component of lithology.
  • Microscopic scale observation, where certain rocks (small, manageable fragments of the Earth's crust) are recovered, brought to a laboratory, and studied under the microscope. In scientific parlance, this type of observation forms the empirical component of petrology.
Now, you might look at this distinction and think :

"Lithology is the study of the Earth's observable crust, so lithology being a part of geology makes sense. On the other hand, petrology is the study of rocks; why is that a part of geology, and not chemistry or physics for example?"

The best answer is probably that the observations in themselves are not a part of any field of study. It's the type of conclusions one draws from these observations which determines the field of study. In the case of petrology, geologists are interested in the creation, history and evolution of the Earth by looking at microscopic traces left within the rocks observed. This heavily influences which minerals are deemed interesting to observe (typically, those stably present on Earth or, more recently, on other celestial bodies), and which properties are deemed important within the context of geology. The transmitted light and reflected light datatables on this website list a collection of properties for each mineral that a geologist would find important - these might not be so relevant to a chemist for example.

What is a mineral?
Heuristically, minerals are the "fundamental building blocks" which compose any and all rocks on Earth. Unlike chemistry where the "fundamental building blocks" - called atoms - can be neatly arranged into a periodic table small enough to be comprehensible to the human mind, there are far too many minerals, and there is far too much diversity in the properties of each mineral for geologists to build a "geological periodic table" which could be taught in high school. With this analogy in mind, here is an actual definition of a mineral:

Solid compound which can occur naturally in pure form.

What does that actually mean? Is for example frozen water a mineral? Let's break it down:
  1. Compounds are collections of molecules (themselves collections of atoms) gathered together.
  2. We say that a compound is solid if its spacial structure bears strong resistance to exterior forces.
  3. A priori, a solid compound can be made from chemically different molecules: a compound is of pure form if all the molecules within it are chemically identical. When a generic compound becomes solid, we call the process solidifaction. When this compound is in fact of pure form, we call that process crystallisation.
  4. The final term in the defintion - "naturally" - is somewhat more outdated. For a long time, the main struggle for geology and geologists has been the difficulty of reproducing the formation of minerals, or in other words, to reproduce cristallisation of minerals. However, this is slowly changing, and now more and more compounds can be cristallised.
So, is frozen water a mineral? Technically, yes ! But ask any geologist, and they might pause for a bit before giving that answer, because geologists are interested in minerals which reveal something about the creation, history and evolution of the Earth, and as we all know, water becomes liquid far too quickly and often for the study of ice to have any kind of interest in geology, as a mineral.

What are famous examples of minerals, and can I find them in the datatables?
Here are a list of popular gemstones:
Can a mineral be suitably observed both in transmitted and in reflected light? Coming soon..