A rock consists of one or more minerals that may be held together by a cementitious binder. Or a rock may be a continuous mass of interlocking crystalline domains.
Igneous and metamorphic rocks are comprised of crystalline phases compacted into an inhomogeneous mass. Amorphous phases may be found as well. Sedimentary rocks are often made of distinct mineral grains or pebbles held into position by cementitious matrix. There is a great deal of variety to be found.
The point is that rocks may have quite complex compositions. If the goal is to use rock for construction, then the composition may not be that important as long as some minimum structural attribute exists.
But if the goal is to extract specific components from a rock, then the details of composition become very important. Rock may be made of simple inorganic compounds. Good examples would be calcium carbonate, sodium chloride, or calcium fluoride. These substances are often found in crystalline form where the crystal consists of cations and anions which are free to solvate in the right solvent system and dissolve. These kind of minerals may be very weak structurally and subject to easy fracture. The geological fate of such minerals is often aqueous transport and deposition to some location where a new mineral may precipitate from component ions in solution.
Some rocks may have appreciable fractions of monomers like silicate and aluminate. Monomeric components are able to form polymer networks which have a large effect on many properties of the mineral. Glass and quartz are silicate network polymers that form rigid matrices. Silicate has 4 attachment points in a tetrahedral array that can form a variety of linkages. These matrices have properties like elevated melting point and rigidity that add or detract from the value of a given material.
Quartz is a pure SiO2 network whereas soda glass contains network terminating additives that alter the connectivity and lower the glass transition temperature and melting point of the material. The additives lend workability to the glass. Chain and network termination no doubt has a major influence on the physical properties of rock.
Most metals are found in nature as an ionic compound in various oxidation states and charge balanced by simple anions like oxide, sulfide, or a halide. Metal cations may also be associated with complex, polyatomic anions like sulfate, molybdate, tungstate, silicate, aluminate, and a few other oxidized species. A few of these polyatomic anions, especially silicate, are held together with substantially covalent bonds. So their network polymer compositions may be very high melting and difficult to mill.
Extraction of desired metals from a rock will follow a path depending on the the type of mineral present. Rocks made of an ionic compound and not subject to network connectivity maybe susceptible to chemical attack and dissolution. Treatment with strong acids or various fluxing agents may cause the digestion of a rock under less than drastic conditions. Such rocks maybe susceptible to weathering as well.
Rocks with substantial polysilicate or polyaluminate compositions are rather more difficult to digest. For the same reason glass resists most chemical attack, so too do silicate and aluminate minerals. But substances that attack glass and alumina may also be useful in digesting rocks high in silicate and aluminate. In particular, hydrogen fluoride stands out. This acid is well known to attack glass by breaking the Si-O bond and making an Si-F bond due to silicons affinity for fluorine. Digestion of silicate minerals with HF or ammonium bifluoride (NH4FHF) has been known for a long time. The use of disulfur dichloride (S2Cl2) has been reported as well.
Silicates and aluminates are also susceptible to attack by hydroxide or carbonate. This is often taken advantage of in the lab through the use of a muffle furnace and crucible. Digestion of a rock sample is affected at high temperature and the resulting digested material is then treated in a manner as to allow the separation of the metal as, for instance a hydroxide or carbonate that can then be ignited in the muffle furnace. This time a purified metal oxide is formed and weighed to give a yield or wt %. Metal oxides can usually be dissolved in aqueous acid and subjected to a variety of tests thereafter.