19

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 see the ore deposit in front of your eyes. Many ore-forming systems are accompanied by different kinds of alteration (for example potassic or argillic alteration) ...


17

Your rocks are marcasite nodules. These nodules weather out of the chalk beds along the coast. Here is a reference to UK chalk beds. If you are interested, I would like to trade for one. I'll trade you a nice pyrite nodule I found here in northern Ohio near Lake Erie.


14

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 world?" So yes, gold is all around but the concentration is too low to make extraction of it worthwhile. You need some process to take small amounts of gold from a ...


13

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 look at what is needed for these veins to form. Once you have a reasonable idea of possible ways to form your ore, then you have something that you can look for. ...


12

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 grow extended lattice defects such as micro-fractures, stress twins, and dislocations. Diamond, quartz, and framework silicates, such as feldspar, are hard ...


11

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 with the addition of refinements. So looking at how large and small deposits were found will change over time. (Simple example of that would be that Romans and ...


10

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 will grow in its own characteristic form (euhedral). This happens for example with feldspar crystals. (Example: http://www.erdwissen.ch/wp-content/uploads/2010/12/...


9

Based on the brassy color, it's metallic luster, and it's resistance to breakage, I would say it's pyrite. Try to scratch glass with it (be careful not to break the glass). If it scratches, it is likely pyrite. If a nail can't scratch it, it is likely pyrite. You could test it's streak by rubbing it on a piece of hard unglazed ceramic. If it leaves a ...


9

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. Note that fluorescence is distinct from phosphoresence, and phosphorescence can involve longer lifetime excited states. Spectroscopic Characterization of the ...


9

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 these other locations listed below have produced collectible specimens but cryolite does not occur in large enough quantities to be mined. Other important ...


9

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 have markedly violet soils. If you broaden your search to sands as well as soils, there are plenty of examples of garnet-based sands that can appear markedly ...


9

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 like. but why they stand out from the rest of the stone is beyond me. Because calcite is a translucent mineral, and pectolite is not. You can drop some ...


7

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 than enough UV in sunlight to make the minerals fluoresce, but... The amount of visible light from the fluorescence is low enough to be undetectable by eye on ...


7

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 powder (streak) they can no longe do so and you can see the true color. In other minerals surface oxidation can mask the true color.


7

Your mineral specimen is most likely goethite after pyrite, a common pseudomorph found in many different geological environments. A quick google search revealed pyrite and goethite can be found in Normandy. Mindat - Goethite (map before from Mindat) Soumont-Saint-Quentin mine, Calvados, Normandy, France


6

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 other side the other one will be. Let's talk about $\ce{Ca}$ first. The reactions all have the same $\ce{Ca}$ so it's activity isn't a factor in their relative ...


6

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 suggests you are looking down (or close to) the x axis, so this means the interference colours you see are going to be very low. Let's say 1st order white or ...


6

It is a quartz crystal artificially coated in thin layer of metal called aura quartz or rainbow quartz. They come in various colours and often can be found in new age shops. More information on the topic can be found on wiki: https://en.wikipedia.org/wiki/Metal-coated_crystal


6

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 mixtures of minerals. For example - a (well-mixed) mixture of quartz and pyroxene will melt at a lower temperature than pure quartz or pure pyroxene. The exact ...


6

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 chemical properties and crystal form. Organic vs inorganic Silicates vs non-silicates Subdivided silicates into smaller groups by crystalline structures ...


5

Given your description of the base rock and the age of the mine I would suggest that what you are seeing is what in a cave we would refer to as flow stone (A deposit of Calcium Carbonate in one or more of many crystal structures). Simplified Process: Ground water often has varying degrees of dissolved carbon dioxide in it. This carbon dioxide within the ...


5

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 equipment, proper tools, and proper planning. If you're breaking rocking with a hammer, then the hammer should be a soft steel. Hardened steel can splinter. Since ...


5

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 grams of mineral. But that's less important now. You are mostly correct in your steps. Eventually you have 0.972 Fe, 0.375 Ti, and 0.972 + 2×0.375 = 1.722 O. ...


5

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 to igneous rocks rich in silica. The main sources of gold are in hydrothermal quartz veins with pyrite and other sulfides. Gold is mechanically mixed with ...


5

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 there isn't a plane of bonds that can be cut through, then you won't get cleavage. When a mineral is fractured, the fracture "wants" to take the path where the ...


5

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 ionic) are not the only important thing. Crystal morphology is also important. For example, Si-O-Si and Al-O-Al bonds usually cause minerals to be hard, such ...


5

I'd like to add to Gordon's answer. A phase change in this context does not only refer to the change in the state of matter (e.g. liquid to solid) but a change in different solid states as well. Olivine, $\ce{(Mg,Fe)2SiO4}$, is the stable magnesian silicate in pressures down to those prevailing at a depth of ~410 km below the surface of the Earth. As the ...


5

You have a specimen of gypsum, CaSO4 · 2H2O. Reference


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