Keywords Concretion, Dakota Formation, Navajo Sandstone, Nebraska, In concretions or diagenetic beds, the carbonates characteristically are pore-filling cements the decreasing concentration of which from centre (up to 85 or 90% by volume) to rim (as little as 25%) reflect the decreasing porosity at the time of precipitation (Hesse et al., 2004).For porosities estimated in this way, the term “minus-cement porosity” (Taylor, 1950) is commonly used. Concretions are a mysterious geologic phenomenon that happens in a variety of sedimentary rock such as s hale, sandstone, siltstone, and limestone. Concretions (conjunctival lithiasis) The CMGs are guidelines on the diagnosis and management of a range of common and rare, but important, eye conditions that present with varying frequency in primary and first contact care. It is estimated that one pearl is found for every 10,000 conchs collected. They do not have the pearly lustre of true pearls and are often called calcareous concretions. In some cases, the center or nucleus of the concretion is still present within it and it may even contain an intact fossil. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Hematite and magnetite are two common iron-oxide minerals. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. Geodes, Nodules, Vugs, Concretions and Thundereggs. Concretions are often orange and brown The cement preserves everything and freezes the rock and its contents in time. Geodes, nodules, vugs, concretions and thundereggs are all sites in the earth where substances dissolved in subsurface waters precipitate to form crystals or rounded objects. See more ideas about rocks and minerals, minerals, gems and minerals. Iron-oxide nodules or concretions are the most common kind of meteorwrong sent to us. These objects share many common features and form by similar processes. A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Not only are concretions the most amazing shapes, they contain valuable information about the time they formed. Apr 5, 2014 - Explore spring aling's board "Concretions", followed by 111 people on Pinterest. Most iron ore deposits consist mainly of hematite, magnetite, or both. Conjunctival concretions (Conjunctival lithiasis) are small vascular, granular, yellowish-white deposits being produced due to conjunctival degeneration and are found in the sub-epithelium of palpebral conjunctiva and fornix (junction between palpebral and bulbar conjunctiva) in the elderly group or in patients with chronic inflammatory conditions. complete oxidation upon exposure, so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate. In a particular region of upstate New York, concretions have stunning shapes. concretions typically includes other miner-als like calcite, dolomite, silica and pyrite. Indigenous to the Caribbean Sea and a small area of the Atlantic Ocean, natural conch pearls are extremely valuable. Otherwise, the nucleus has dissolved over time leaving a void in the center of the concretion. 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