Amanda Shoemake GSC 310
Take Home Exam
1) Potassium feldspars are a group of polymorphs, polymorphs being minerals that have the same chemical composition but slightly different crystal structures. (University of Minnesota) Similar to the chemical variations in the phase diagram for Olivine, potassium feldspar is formed with substitutable elements. These are calcium, potassium, and sodium. As a polymorph, these elements are able to substitute themselves within the chemical structure, and create different compositions. During formation in magma, minerals focus on one element at a time. For example, potassium feldspar is a feldspar variation with mainly potassium. Once all the potassium in an area of crystalline formation is gone, another element is transitioned into focus such as sodium. If two feldspars form simultaneously exsolution lamellae occurs when the magma cools. Perthite, potassium feldspar polymorphs, and the continuous plagioclase feldspar series are interrelated because Perthite is formed by alternating bands or layers of potassium feldspar and plagioclase feldspar. This gives Perthite it’s striped pattern. (University of Minnesota)
2) In reference to the phase diagram for Olivine, Olivine has a chemical composition that fits between a “pure” forsterite (Fo) and “pure” fayalite (Fa). This happens because in the series, Iron and Magnesium have the ability to replace each other. They are about the same size and can substitute within the atomic structure, therefore what determines the chemical composition of an Olivine sample is the concentration of Iron or
Magnesium in the magma from which it forms.
Amanda Shoemake GSC 310
Take Home Exam
tested for cleavage. Phyllosilicates have cleavage in one plane because the atoms form together in sheets.
4) Crystal system refers to the shape in which a mineral forms such as isometric, hexagonal, tetragonal, orthorhombic, etc. Crystal habit is the arrangement of crystals within a particular mineral. When it comes to crystal systems, these can be hard to identify at times because as we know earths crust is ever changing and has various pressures throughout, which does not always make for perfect crystallization areas. When a crystal grows and hits another body this is called an impeded crystal face. These are really common and make finding out crystal system difficult. Luckily, crystal system and habit tie in with one another because when a mineral is given free range to grow it will form in similar systems and habits. A good example of this would be Fluorite. Fluorite forms in a cubic crystalline habit, and when it is unimpeded, has an isometric crystal system. An example of this with a hexagonal crystal system would be Beryl because it has the most common crystal habit of a hexagonal prism as well.
5) “What doesn’t kill you makes you stronger” could not be more true for minerals that have a hardness greater than 8.0. For any substance to be that tough, it had to go through hundreds of thousands of years of heat and pressure throughout its journey in earth’s crust. In order to reach an 8 on the hardness scale the mineral at hand, for example Diamond, has a strong crystal structure that could withstand the intense heat and pressure required to create a mineral with that level of hardness. Minerals that are rated lower on the hardness scale can be scratched by their harder counterparts because the forces holding their crystal structure are weak in comparison. Only a few minerals have a hardness greater than 8.0 because only a few minerals can withstand that kind of pressure and heat.
