Earth Materials Lecture 6: Felsic Minerals Flashcards
Define Silicate minerals
Minerals that contain [SiO4]−4 as the fundamental unit of their chemistry and structure.
Define Felsic minerals
Felsic minerals are silicate minerals, magma, and rocks which consist of lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.
What is the chemical composition of Quartz?
SiO²
Describe Quartz
Quartz is a silicate mineral (SiO²) made up of pure silica.
Several polymorphs - three main ones
Describe Micro and Cryptocrystalline quartz
Opal
SiO2 .nH2O
Various types of Opal:
− Opal-C, well ordered
− Opal-CT, disordered
− Opal-A, ~amorphous
Chalcedony;
chert, flint, agate
(Chalcedony are micro quartz, meaning < 20 μm quartz crystals with sub-microscopic pores)
Describe the Paragenesis of Quartz
-Mostly in acid igneous rocks – granites, dacites, rhyolites etc.
-Hydrothermal veins; hot springs.
-Various metamorphic and sedimentary rocks
Describe the Paragenesis of the Polymorphs of Quartz
Tridymite and cristobalite:
− acid volcanic rocks: obsidian, rhyolite and related ash deposits,
pitchstone, dacite. Can crystallize much later.
− Unknown why they exist beyond their stability fields.
Coesite and stishovite:
− high pressure so meteorite impact sites, high pressure and ultra high pressure metamorphic rocks.
What is the feldspar group
The Feldspars is a group of rock-forming aluminium tectosilicate minerals.
Also containing other cations such as sodium, calcium, potassium, or barium.
Commonest minerals in
Earth’s crust.
Define a tectosilicate
Chemical compound with a three-dimensional structure of silicate tetrahedrons that are linked together by sharing oxygen atoms
Central silicon atom surrounded by four oxygen atoms at the corners.
Each oxygen atom is shared with another tetrahedron.
And each tetrahedron is connected to four others.
Describe the solid solution series of Feldspars and how it works
Solid solution series of Feldspars is a ternary system (meaning a system of three parts).
The three components are K (Potassium), Na (Sodium) and Ca (Calcium).
When the solid solution is between K and Na, the term is Alkali feldspars.
When the solid solution between Na and Ca, the term is Plagiocase feldspars.
When it is purely K, the mineral is Orthoclase (K,Al,Si³O⁸)
When it is purely Na, the mineral is Albite (Na,Al,Si³O⁸)
When it is purely Ca, the mineral is Anorthite (Ca,Al²,Si²,O⁸)
Describe the Paragenesis of Feldspars
Basic igneous rocks:
− Ca-rich plagioclase, usually labradorite.
Intermediate igneous rocks:
− Andesine/labradorite; +/- anorthoclase, sanidine /microcline/orthoclase
Acid igneous rocks:
− Albite/oligoclase; anorthoclase, sanidine /microcline
/orthoclase
