Why do some weathered rocks have rust streaks?
Rust streaks arise when rocks containing iron are weathered. In particular, rocks containing reduced ferrous iron will often produce rust streaks. Fe2+ reacts with oxygen and water in the environment, producing Fe3+ ions. Rust is a mixture of Fe3+ containing oxides and hydroxides, so it forms as a result of this oxidation. Pyrite grains are one common progenitor of rust streaks because they are easily oxidized to ferric sulfate, which is soluble in water. However, this salt hydrolyzes easily in water (especially slightly basic water) to form a precipitate of rust. This leads into your next question. What remains after weathering depends on the solubility of the weathering products. Limestone and marble are made of calcium carbonate, which is slightly soluble in water and soluble in dilute acids*. These rocks can just dissolve away (and may be reprecipitated elsewhere, as in stalactites and other cave features) upon weathering. Other rocks contain highly insoluble elements, things like
