I’m very tired. Please help me understand this.
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The two most common minerals on earth are quartz and feldspar. Granite is a mix of those two with a few other things added, and many other rock types are the result of those mineral being mixed and re-combined with heat and pressure.
Feldspar is actually a family of minerals, but what you get if you heat it to melting is basically porcelain, rather than glass.
When weather breaks rock down over the course of many years, it separates the quartz and feldspar, because of the difference in particle size and weight. Sand is mostly quartz, and clay is mostly feldspar. We make lots of things by heating clay, just like we heat glass, but a mix of the two isn’t really good.
Glass is only about 70% silica, the rest is lime and a mineral called soda ash, which is somewhat similar to fireplace ash. Pure quartz glass is used for some purposes, but it is difficult to work with and requires much higher temperatures to make.
“Big rocks” like iron? We melt “Big rocks” into iron, not glass.
Like melting different sized ice cubes- different sizes melt at different rates, and smaller pieces melt faster, because 100rams of tiny ice cubes has a lot more surface area than a solid cube of 100g of ice. (Which is why ice spheres are popular in whiskey drinking-they have the least surface area and melt the slowest).
A regular kiln for making ceramics (sand-sized pieces of rocks+minerals, that are melted together to fuse them) top out around 2400f/1300c. As many have pointed out, quartz mineral is almost all silica- which is the primary mineral for glass. Quartz melts at about about 3,000f/1670c. And we probably have industrial kilns that can do that temp.
So- like melting a big piece of ice- it would talk more energy for more time to melt a big piece of quartz into liquid than it would to throw that quartz into a grinder, to make a powder with LOTS of surface area, and turn that into the same liquid to pour.
Well, strictly speaking, you CAN turn a rock into glass, but the rock is made of many different minerals which each have their own particularly melting/freezing/crystallization temps that makes the solution (which is what a magma is, a solution in a SiO2-dominant liquid). Using quartz gives a fairly pure solution (SiO2 liquid) which tends to not have the many crystal formation and inhomogeneity problems that happen from very impure liquids. Also, quartz is transparent to visible light so useful as glass.
It is like trying to freeze salty water compared to pure water. The secondary materials cause lots of problems for making whatever the person wants. A little sodium borate is nice as an additive, say, or a bit of lead, which can help lower melting temps and make the glass easier to work with (pure silica glass is a real bear to work with, very viscous and has to be really hot because quartz/SiO2 has a very high melting temperature).
In effect, all magma, lava, is melted rock, and if you look around at what happens in nature, you will find that most of that melted stuff does no make a nice and useful glass. All those other elements muck up the works, so to speak.
In general you can.
It just doesn’t make very nice or useful glass as most big rocks are made of the wrong stuff.
And the big rocks made of the right stuff to give us good glass are harder to melt and deal with due to size. So it’s easier to use the tiny rocks of the good stuff in the right type of sand than to try using bigger rocks.
This is mainly because sand is _not_ just a bunch of tiny rocks. Your initial premise is flawed. There’s tiny rocks in sand, but that’s only a small portion of what sand is.
You could, it’s just not efficient to do so… think of how quickly a handful of snow would melt indoors vs. a solid block of ice that fits in your hand. Same idea for turning sand to glass vs. larger rocks.
Can you flash melt a whole outcrop of rock and then quench it very quickly? Yes? Then you’ve got glass.
Practically, that is very hard to do due to the insane amount of energy required to melt a whole cliff of rock or whatever, then when you cool it it’s going to cool quickly on the outside and slowly on the inside, so that it crystallises beyond a few inches or so of the outer layer which would turn to glass.
The outer glass margin would not be a clear glass due to all the impurities in rock, it would be a dark colour (like obsidian). We often see this in rock which has been intruded by magma and then the magne body has just sat there cooling off until it crystallises. The magma body becomes igneous rock (Google ‘pluton’ to see what I mean) and the margins of both the igneous intrusion and the surrounding rock which was intruded into are often known as ‘chilled margins’ and are an inch or so of glassy material because it cooled off too quick to crystallise into a bunch of minerals.
You may also be interested in [fulgurites](https://en.wikipedia.org/wiki/Fulgurite) which are — poetically speaking — fossilised lightning strikes, or more literally they are the grains of quarts and silica rich minerals on a sandy beach that got struck by lightning, melted and fused together, then quickly cooled in a tube like structure which is made of glass. Its natural, so it’s somewhat messy and ugly compared to glassware or glass ornaments (which have been carefully crafted), but it’s definitely glass in the sense that it is an amorphous solid.
sand is usually all quartz, rocks contain other stuff that hasn’t been withered away yet. you just want the quartz. but even if you had a pure quartzite rock, you’d rather take sand because just like how a big block of ice melts faster if you smash it apart, melting big rocks is a bother.
Most rocks are a mixture of aluminum and silicon oxides. Silicon oxide melts easily (which is why it makes good glass) but also breaks easily. Aluminum oxide is stronger but is more difficult to work with.
When rocks break down from wind and water, the soft silicon rich pieces get broken more easily into smaller pieces. The smaller lighter pieces get carried by wind and rivers while the bigger and heavier aluminum oxide rich pieces stay. The sand that is good for making glass is the sand that has been carried a long way from the mountain, and has a lot more silicon oxide grains.
So, back to your question. Before rocks can make good glass they need to be ground up and the hard/heavy parts taken out. Nature does this with wind and water.
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