28 votes

Where can obsidian be found?

The processes forming obsidian are not well understood because an active obsidian-forming eruption has never been recorded by humans. However, we can make many inferences from the composition of the ...
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  • 2,563
20 votes
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Why do gold deposits form only in certain areas of the earth?

I'll take the form of the question given by another person here and attempt to provide a different answer. So what you are asking is: "How did gold become so concentrated in certain parts of the ...
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  • 22.1k
19 votes
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How do geologists find ore deposits?

That is the multimillion dollar question! "looking for surface formations" is indeed one way, and it was the main method of exploration in the past. This does not necessarily mean that you directly ...
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  • 22.1k
18 votes
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Where can obsidian be found?

Obsidian is formed when a rhyolitic (or felsic) lava flows cool rapidly. This must mean that it's mostly available on the surface (and I think if you go near volcanos you can find pieces of Obsidian ...
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  • 6,356
13 votes

How do geologists find ore deposits?

In addition to the above, what happens is that people look for commonalities between known deposits. So, for example, if you have a lot of gold veins in one area, and lots in another area, then you ...
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  • 441
12 votes
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What determines a mineral's hardness?

The hardness of minerals is diagnostic because the hardness is determined by the strength of bonds and the structure of the mineral lattice. Hardness is basically the stress required to create and ...
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  • 3,780
12 votes

Where can obsidian be found?

I have seen it on the surface in some the lava fields of Iceland. This is consistent with @Neo answer. Obsidian is not that often present, but if present, there is usually plenty around. It occurs in ...
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  • 516
11 votes

How do geologists find ore deposits?

I've been looking for these things over a few decades and along with other geologists doing this we have seen how we find things change somewhat. Many methods used a century ago are still in use, but ...
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  • 425
10 votes
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How and where do double-terminated crystals form?

Double-terminated crystals can from by crystallizing from a melt. The crystallization nucleus has to float freely in the magma chamber. As long as no other crystals obstruct the growth the crystal ...
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  • 3,876
9 votes
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Is it possible for a natural fluorescent mineral to glow from the sun's rays?

The lifetime of a fluorescence excited state is on the nanosecond to microsecond time scale. So once the excitation source light is removed, the emission of light will stop within microseconds. ...
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  • 5,892
9 votes

What is the geologic origin of Cryolite, and could there be more?

Ivigtut Cryolite deposit, Ivittuut (Ivigtut), Arsuk Fjord, Sermersooq, Greenland deposit is the first and largest occurrence of Cryolite but it is not the only location to report Cryolite. Some of ...
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  • 5,861
9 votes

Are there any areas on Earth with purplish-colored soil/sand/rock/land?

Soil color is highly dependant on the oxides and other minerals in the composition. Purplish tones appear to be possible by inclusion of manganese oxide compounds. There are locations in China that ...
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9 votes
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What are the cubic formations found inside Larimar gemstones?

Calcite and hematite may not be the the answer to my question Hematite is not the answer, but calcite is. The inclusions are not cubic, they are rhombohedral. This is precisely how calcite looks ...
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8 votes
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Why is streak different from color?

It mostly has to do with the fact many minerals are partially translucent. Trace impurities or even crystal structure can dominate the color of a translucent material but when ground in to a fine ...
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  • 6,465
7 votes
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How do hematite and magnetite form?

In nature, iron can be either metallic ($\ce{Fe^0}$), ferrous ($\ce{Fe^2+}$) or ferric ($\ce{Fe^3+}$). In hematite, all of the iron is ferric: $\ce{Fe^3+_2O3}$. In magnetite, it is a combination of ...
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  • 22.1k
7 votes

Is it possible for a natural fluorescent mineral to glow from the sun's rays?

I am not a minerals expert, and can't claim expertise on these particular materials. However, from a general physics / materials point of view I'm pretty sure the answer will be that, There is more ...
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6 votes

Why do gold deposits form only in certain areas of the earth?

Gold has primary origin in hydrothermal veins and contact metamorphic deposits and pegmatites. Also occurs in placer deposits of secondary origin. It is more easily found in veins that is related ...
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  • 173
6 votes

Plotting a mineral stability diagram

Remember that you are concerned about stability fields. The lines on your stability diagram are the places where two minerals are in equilibrium. One one side one mineral will be more stable, on the ...
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  • 4,309
6 votes

Muscovite with pleochroic halos?

It's biotite. Not muscovite. That's why you have the pleochroic halos in it. I don't understand why you are saying that the interference colours are like muscovite. First of all, the lack of cleavage ...
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6 votes
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Melting point of minerals

Start with my answer to this very highly related question here: https://earthscience.stackexchange.com/a/2742/725 The melting point of minerals in isolation, or a pure substance is higher than ...
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6 votes

How are minerals classified?

Minerals are defined by chemical composition and crystallography. Dana classification scheme or new Dana classification scheme divides known mineral species in eight broad groups based on primary ...
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  • 5,861
6 votes

How is it possible for rocks to be trapped within another type of rock?

There are three major types of geological formations that contain or are composed of various rocks: breccia conglomerate xenoliths Breccia The word breccia comes from the Italian word for rubble. ...
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  • 20.3k
6 votes

How can the 'crystal cleavage' of apatite have a four-digit number?

This is not a four- digit number but four separare numbers called Bravais-Miller indices. Bravais-Miller indices descrive the orientation of a crystal plane relative to the symmetry axes of a crystal,...
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  • 2,363
5 votes

What kind of minerals have no cleavage plane?

I'm pretty sure a more rigorous answer deserves to come along, but I can give a simple overview of some of the important factors. Cleavage planes have to do with bond strength and bond geometry. If ...
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  • 541
5 votes

What determines a mineral's hardness?

There are some subtleties that I'd like to add, in addition to Mark's answer. When talking about the hardness of a mineral, the nature of the chemical bonds in the crystal structure (e.g. covalent vs ...
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  • 22.1k
5 votes
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How can I normalize this chemical analysis to a mineral with 3 cations?

The first step I did was divide each wt% by their molecular weights to get a molecular proportion: Actually that's not what you did. What you did is how much moles of each oxide you have assuming 100 ...
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5 votes
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Deadly minerals

I assume by "mineral excavation" you mean mineral collecting. I've collected minerals for years. In general the biggest danger is naivete not the minerals themselves. You need proper safety ...
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