These minerals usually make up just a small percentage of the grains in a sandstone. Common accessory minerals include micas (muscovite and biotite), olivine, pyroxene, and corundum. Many of these accessory grains are more dense than the silicates that make up the bulk of the rock. These heavy minerals are commonly resistant to weathering and can be used as an indicator of sandstone maturity through the ZTR index. Common heavy minerals include zircon, tourmaline, rutile (hence ZTR), garnet, magnetite, or other dense, resistant minerals derived from the source rock. Matrix Matrix is very fine material, which is present within interstitial pore space between the framework grains. The nature of the matrix within the interstitial pore space results in a twofold classification: Arenites are texturally clean sandstones that are free of or have very little matrix. Wackes are texturally dirty sandstones that have a significant amount of matrix. Cement Cement is what binds the siliciclastic framework grains together. Cement is a secondary mineral that forms after deposition and during burial of the sandstone. These cementing materials may be either silicate minerals or non-silicate minerals, such as calcite. Silica cement can consist of either quartz or opal minerals. Quartz is the most common silicate mineral that acts as cement. In sandstone where there is silica cement present, the quartz grains are attached to cement, which creates a rim around the quartz grain called overgrowth. The overgrowth retains the same crystallographic continuity of quartz framework grain that is being cemented. Opal cement is found in sandstones that are rich in volcanogenic materials, and very rarely is in other sandstones. Calcite cement is the most common carbonate cement. Calcite cement is an assortment of smaller calcite crystals. The cement adheres to the framework grains, cementing the framework grains together. Other minerals that act as cements include: hematite, limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite minerals. Sandstone that becomes depleted of its cement binder through weathering gradually becomes friable and unstable. This process can be somewhat reversed by the application of tetraethyl orthosilicate (Si(OC2H5)4) which will deposit amorphous silicon dioxide between the sand grains. The reaction is as follows. Si(OC2H5)4 (l) + 2 H2O (l) → SiO2 (s) + 4 C2H5OH (g) Pore space Pore space includes the open spaces within a rock or a soil. The pore space in a rock has a direct relationship to the porosity and permeability of the