Mineral Cements in Sandstones

Introduction

Mineral cements are the materials that bind framework grains together and reduce porosity in sandstones. They are entirely secondary - that is, none of them existed at the time of deposition. They precipitate from pore fluids during burial, filling the interstitial space between framework grains. The type of cement that forms depends on the chemistry of the pore fluid, the burial temperature, and the mineralogy of the surrounding grains. Cementation is one of the two main processes of diagenesis in sandstones, alongside compaction; it is the primary control on whether a sandstone becomes a useful petroleum reservoir or a tight, impermeable rock.

Silicate Cements

The framework grains in most siliciclastic sedimentary rocks are bound together by some type of mineral cement. These cementing materials may be silicate minerals such as quartz, opal, and clay minerals, or nonsilicate minerals such as calcite and dolomite. ^[1]

Quartz is the most common silicate cement. In most sandstones, the quartz cement is chemically attached to the crystal lattice of existing quartz grains, forming rims of cement called overgrowths. Overgrowths that retain crystallographic continuity with the host grain are said to be syntaxial. Because syntaxial overgrowths are optically continuous with the original grain, they extinguish in the same position as the host grain when the stage is rotated on a polarising microscope. Overgrowths can be recognised by a line of impurities or bubbles that marks the surface of the original grain. Quartz overgrowths are particularly common in quartz-rich sandstones. ^[1]

Less commonly, quartz cement occurs as microcrystalline quartz - a fine-grained crystalline silica with a texture similar to chert. When deposited as microcrystalline quartz, silica cement forms a mosaic of very tiny quartz crystals filling interstitial spaces among framework grains. The crystals next to framework grains are commonly small, slightly elongated, and oriented normal to the grain surfaces. ^[1]

More rarely, opal occurs as a cement, particularly in sandstones rich in volcanogenic materials. Like quartz and microcrystalline quartz, opal is also composed of SiO₂, but unlike those minerals, opal contains some water and lacks a definite crystal structure - it is therefore amorphous. Opal is metastable and crystallises over time to microcrystalline quartz. ^[1]

Carbonate Cements

Carbonate minerals are the most abundant nonsilicate cements in siliciclastic sedimentary rocks. Calcite is a particularly common carbonate cement, precipitated in pore spaces among framework grains and typically forming a mosaic of smaller crystals that adhere to the larger framework grains and bind them together. ^[1]

Less common carbonate cements include dolomite and siderite (iron carbonate). ^[1]

Other Cements

Other minerals that act as cements in sandstones include the iron oxide minerals hematite and limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite minerals. Zeolites are hydrous aluminosilicate minerals that occur as cements primarily in volcaniclastic sedimentary rocks. ^[1]

All cements are secondary minerals that form in sandstones after deposition and during burial. ^[1]

The variety of possible cements reflects the diverse chemistry of basinal pore fluids at different burial depths and temperatures. Silica cements are most common in deeply buried, quartz-rich sandstones where pressure solution at grain contacts releases dissolved silica into the pore fluid. Carbonate cements are most common in shallower or more open-fluid systems where calcium-bicarbonate waters percolate through the rock. Iron oxide cements are typical of oxidising, shallow-burial environments - the red colouring of many continental red-bed sandstones is iron oxide cement. Clay mineral cements - particularly chlorite and illite - are important diagenetically because they line grain surfaces and may either preserve or destroy porosity depending on their morphology and distribution.

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