Minerals & Igneous Rocks

What is the hardness scale?

1 (talc) → 10 (diamond). Glass/steel ≈ 5.5–6. Minerals harder than 6 scratch glass; softer do not.

What is the hardness of talc?

1

what is hardness of a diamond?

10

What is the hardness of glass/steel?

5.5-6

How do igneous rocks form?

when magma cools

Intrusive

slow cooling, big crystals

Extrusive

fast cooling, small or no crystals

Which rocks are intrusive?

granite, diorite, gabbro

Which rocks are extrusive?

rhyolite, andesite, basalt, obsidian, pumice

Q: What are porphyritic rocks?

Mixed crystal sizes. Started cooling slowly (intrusive, big crystals), then erupted and cooled fast (extrusive, small crystals).

Felsic rocks?

High silica, low iron/mg, light color —> granite (intrusive), rhyolite (extrusive), pumice (extrusive), obsidian (extrusive, glassy/dark)

Q: Mafic rocks?

Low silica, high iron/Mg, dark color → Gabbro (intrusive), Basalt (extrusive).

Intermediate rocks

Medium silica, medium iron/Mg, medium color → Diorite (intrusive), Andesite (extrusive).

Obsidian vs Pumice silica content?

Both high silica (felsic). Obsidian = dark, glassy; Pumice = light, frothy.

How to quickly identify light, medium, dark rocks?

Light = felsic, high silica
Medium = intermediate, gray mix
Dark = mafic, high iron/Mg

What is assimilation?

Magma melts surrounding rock → adds elements → changes composition.

What is fractional crystallization?

Early minerals (dark) form & sink → leftover magma = more silica-rich.

What is partial melting?

Only part of rock melts → silica-rich minerals melt first → magma enriched in silica.

How do minerals form?

1) Cooling magma, 2) Precipitation from water, 3) From shells (recrystallized), 4) Heat & pressure (metamorphism).

What are the 5 requirements for a substance to be a mineral?

Naturally occurring, inorganic, crystal structure, chemical composition, characteristic physical properties