Take Home Midterm GSC310
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.
3) Phyllosilicates are a prime example to show the relationship between silicate
structure and cleavage. Phyllosilicates such as Biotite and Muscovite have a silicate
structure that forms in sheets. This structure forms in one plane and it is very weak
because the tetrahedral atomic structure does not attach well in a vertical direction, it
attaches well horizontally. Meaning the layers do not form well to one another, they
simply form well as a layer. Basically, any Phyllosilicate peels off in sheets when it is
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.
"University of Minnesota's Mineral Pages: Potassium Feldspar." University of Minnesota's
Mineral Pages: Potassium Feldspar. University of Minnesota, n.d. Web. 14 Oct. 2014.
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.
3) Phyllosilicates are a prime example to show the relationship between silicate
structure and cleavage. Phyllosilicates such as Biotite and Muscovite have a silicate
structure that forms in sheets. This structure forms in one plane and it is very weak
because the tetrahedral atomic structure does not attach well in a vertical direction, it
attaches well horizontally. Meaning the layers do not form well to one another, they
simply form well as a layer. Basically, any Phyllosilicate peels off in sheets when it is
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.
"University of Minnesota's Mineral Pages: Potassium Feldspar." University of Minnesota's
Mineral Pages: Potassium Feldspar. University of Minnesota, n.d. Web. 14 Oct. 2014.