A geologist finds a well-sorted sandstone composed of over 95% quartz grains with almost no feldspar, pyroxene, or other minerals. What does this mineral composition tell her about the sediment's history?
AThe original rock was granite, which is naturally quartz-rich and would produce quartz-dominated sediment
BThe sediment underwent intense and prolonged weathering and transport, destroying all less-resistant minerals until only chemically durable quartz survived
CThe deposit formed quickly from a nearby source, as rapid burial preserves original mineralogy
DThe sediment came from an oceanic crust source, which is naturally quartz-rich
Mineralogical maturity — dominance by quartz — is a record of weathering history, not simply original source composition. Even granite (rich in both quartz and feldspar) would initially produce sediment containing both minerals. Prolonged weathering destroys feldspars, pyroxenes, and amphiboles (which decompose to clay minerals), while quartz — fully bonded in a three-dimensional SiO₂ framework with no weak cleavage planes — survives nearly indefinitely. A quartz-dominated sandstone is thus a 'mature' sediment reworked extensively over time. Option C is backwards: rapid burial actually preserves less-stable minerals (immature sediment).
Question 2 Multiple Choice
Mica flakes into perfect thin sheets along a single plane, while quartz fractures irregularly in all directions. Both are silicates — what best explains this difference?
AQuartz is harder than mica (higher Mohs hardness), making it resistant to splitting
BMica's sheet silicate structure shares three of four oxygens in flat layers, creating planes of weak interlayer bonding; quartz shares all four oxygens three-dimensionally, leaving no preferential planes
CMica contains water molecules trapped between layers that act as a lubricant for splitting
DMica is an isolated silicate where tetrahedra do not connect, making it easy to split
This is directly determined by silicate framework structure. In sheet silicates (micas), each tetrahedron shares three of its four oxygens with neighbors in the same flat layer. The layers bond to each other only through weaker forces — these interlayer bonds define the perfect single-plane cleavage. In quartz, all four oxygens are shared three-dimensionally with equal strength in every direction, leaving no weak planes — it fractures conchoidally like glass. Option D is wrong: isolated silicates (like olivine) also lack cleavage for a different reason. The real contrast is the two-dimensional sheet structure versus the full three-dimensional network.
Question 3 True / False
Quartz and feldspar are both framework silicates that share most four oxygens in a three-dimensional network, so neither mineral exhibits cleavage.
TTrue
FFalse
Answer: False
Quartz (pure SiO₂) has no cleavage because every oxygen is shared equally in a uniform three-dimensional network, leaving no preferential weak planes. Feldspars, however, have two distinct cleavage planes at approximately 90° — one of their key diagnostic properties in the field. The cleavage in feldspars arises because aluminum substitutes for some silicon in the framework (Al³⁺ for Si⁴⁺), and the resulting structural distortions create planes of slightly weaker bonding. Both are framework silicates, but the aluminum substitution breaks the symmetry that would otherwise eliminate cleavage.
Question 4 True / False
Mafic minerals like olivine and pyroxene are denser and darker than felsic minerals like quartz because they contain a higher proportion of silicon.
TTrue
FFalse
Answer: False
Mafic minerals are denser and darker because they are rich in iron (Fe) and magnesium (Mg) — the word 'mafic' derives from Magnesium and Ferric (iron). Iron and magnesium atoms are heavier than the aluminum, potassium, and sodium found in felsic minerals, giving mafic minerals densities of ~3.0–3.5 g/cm³ versus ~2.6–2.7 g/cm³ for felsic minerals. The dark color also comes from iron. Felsic minerals actually have more silicon per formula unit — this is the opposite of the misconception.
Question 5 Short Answer
Why does the way silicon-oxygen tetrahedra connect — the silicate framework type — control physical properties like cleavage, density, and weathering resistance?
Think about your answer, then reveal below.
Model answer: The silicate framework type determines how many oxygen atoms each tetrahedron shares with neighbors and how bond strengths are distributed through the crystal. In isolated silicates (olivine), unshared oxygens bond to metal cations in all directions — no planes of weakness exist, producing dense, cleavage-free minerals. In sheet silicates (micas), three-of-four oxygens link tetrahedra within a flat layer, while the fourth oxygens connect layers through weaker bonds — this creates a preferential cleavage plane. Framework silicates (quartz) with all oxygens shared have no weak planes, hence no cleavage and high chemical stability. Greater oxygen sharing also produces lower density because the open three-dimensional network is less compact than densely-packed isolated units.
This structural logic is the key to reading rock history: the silicate structure of a mineral determines how long it survives weathering, how rocks break down, and what minerals appear in sediments far from their source.