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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 rock and settings in which it is found. Obsidian is more than 70% weight percent SiO2 (i.e. rhyolitic), but has less than 0.5 weight percent H2O, and almost 0% ...


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) ...


19

Sound-like waves are routinely used to image the subsurface, but mostly well below the ultra-sound band. Several methods involve sound-like vibrations: Reflection seismic — the most important subsurface remote sensing tool we have Refraction seismic — important, but niche, and has a lot in common with reflection Acoustic logging — uses ultrasound, which ...


18

I'd like to add to Brian's answer, and also point out some inaccuracies. First of all, it is not true that felsic minerals have lower melting temperatures than mafic minerals. Here are some melting temperatures of common minerals, sorted from high to low: Forsterite (mafic): 1890 °C Quartz (felsic): 1713 °C Anorthite (felsic): 1553 °C Diopside (mafic): 1391 ...


18

Good question! As you know, Bowen's reaction series describes the order of crystallization of silicate minerals in a cooling magma. The complex anion of silicates is a tetrahedron of four oxygen atoms surrounding one silicon atom, connected with strong covalent bonds. Each tetrahedron may be isolated from one another or they may be bonded together ...


18

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 on the ground) because molten rock cools much faster above ground than it does below, allowing the melt to cool with small crystals (as opposed to intrusive ...


18

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 ...


16

I am going to assume that you are referring to recent new stories with titles like "Rare Diamond Confirms That Earth's Mantle Holds an Ocean's Worth of Water" (Scientific America). These articles are referring to research published in Nature: Pearson, D. G. et al. (2014). The researchers found an inclusion of ringwoodite inside a diamond. Here is a quote ...


15

I think Azzie's answer describes the situations where gold is formed and found as Lode deposits. But native gold in lode can then be subject to weathering when it is exposed, which releases the gold into the environment as flakes, grains and nuggets. This gold is then subject to erosive and sedimentary processes. As gold is denser than most minerals it can ...


14

why would India have so much thorium on it's beaches...? Because monazite. Monazite is a rare earth element phosphate, with the formula CePO4 (where Ce stands not only for cerium, but all of the are earth elements and yttrium as well). It is one of the main ore minerals for the rare earths. It's a very common mineral in granites and similar rocks, and it's ...


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

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 rather large pieces. This photo is from Landmannalaugar.


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 ...


12

It is bond strength, not hardness, that determines how easily oxygen can attack and burn a material, allowing me to burn a diamond in a pool of liquid oxygen resting in a block of graphite. Diamond is hard because its bonds form an inflexible, three-dimensional lattice. However, the strength of these bonds themselves is not even as strong as graphite, ...


11

Why you should not do it The QAPF and related diagrams are intended for classification of rocks in the field, or preliminary classification with modal proportions as seen in the optical microscope. They are not designed with the chemical composition of the rocks in the mind. Furthermore, these diagrams are merely descriptive and not genetic. They do not ...


11

First of all, Earth is not a random assortment of atoms. While it probably formed along like that in the beginning, somewhere along the way it differentiated into distinct core, mantle and crust, which are not chemically the same. That's a story for another question though, and there is an excellent answer to it on this site. Back to your question, Why are ...


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

I've heard that gold ore is often associated with quartz veins. What geological processes enable gold particles to naturally cluster together like this? First of all, gold is mostly found in quartz veins in and around igneous intrusions. It happens with a very hot hydro-thermal fluid of silica, oxygen and concentrated gold that is created during ...


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

First, a short introduction to incompatible elements The Earth's mantle is mostly composed of the minerals olivine, pyroxene, anorthite, spinel and garnet. These minerals are made from the elements Si, Al, Fe, Mg and Ca. In the figure below I've put them in the MRFE field (Mantle Rock Forming Elements). The trace elements, the elements that occur in very ...


9

The most common explanation for microbial activity in serpentinites is the exploitation of abiogentic hydrogen and methane formed during the serpentinisation process. Quoting the abstract of a recent authoritative review: The process of serpentinization creates strongly reducing conditions and produces fluids that are highly enriched in molecular ...


9

To answer the first part, there are many minerals on earth which do not involve silicate bonded structures. For instance, non-silicates (minerals of carbonates, sulfides, sulfates, phosphates, and oxides are common non-silicate varieties). For a detailed list of mineral species, you should check out the Dana Classification here: Minerals Arranged by the New ...


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

Are there probably decent oil fields located in the middle of the ocean? tl;dr: no. Hydrocarbon deposits form by thermal maturation (i.e. slow and mild heating) of buried organic matter (i.e. dead things) in sedimentary rocks. This is exactly why you find hydrocarbons... very near the country's shore, on the continental shelf ...because that's exactly ...


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 ...


8

In iron oxides, titanium commonly substitutes for iron in the crystal structure. Two solid solution series are important: the ulvöspinel-magnetite and ilmenite-hematite series. The composition of a solution changes continuously, without any change in crystal habit. Your reference indicates its the size of crystals than matter for being efficient nucleation ...


8

The phosphorous sinks are the ocean and ocean sediments. The ocean holds $3 \times 10^{15}$ moles of phosphorous. The annual amount of phosphorous input into the ocean, as well as the amount of burial of phosphorous as sediment is on the order of $10^{10}$ moles. The sediment is of three categories: Phosphorous associated with calcium carbonate ...


8

According to Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon, the Xe is trapped in crystalline cage-like structures formed of the aluminum phosphate that are similar to zeolites. The reference also explains that the aluminum phosphate structures grow quickly under hydrothermal conditions of 270-300 degrees C.


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