炭酸塩岩中の自生石英
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概要
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Acid insoluble residues from sedimentary carbonate rocks contain usually quartz, feldspars, apatite, pyrite, clay minerals and organic matters. Most of quartz grains, the most abundant mineral in the acid insoluble materials, are of detrital, and are dominantly fine-sand to silt size angular shape. However, the existence of well-formed quartz crystals of sedimentary origin was found in the acetic acid insoluble residues of some limestone sand dolomites. In this kind of quartz crystals of microscopic size and larger, crystalline outlines with straight edges and sharp corners with many fine calcite inclusions are excellent indication of growth in place. A randomly oriented quartz sample has the two strongest X-ray diffraction peaks from the 101 (d=3.34A) and 100 (d=4.26A) crystallographic planes. Although the ratio of the intensities of these peaks I (100)/I(101) is given as 0.30-0.35 from the X-ray powder data, the samples prepared from acetic acid insoluble residues containing authigenic quartz crystals give considerablly greater ratios of intensities of the two diffraction peaks. The occurrences of the authigenic quartz crystals in the thin sections of sedimentary carbonate rocks suggests the facts as follows: 1. Authigenic quartz occurs typically in the fine carbonate grains such as micrite, oolitic layer composed of very fine carbonate grains, and so on. This kind of quartz crystal is characterized by many fine calcite inclusions in it. Some of limestones and dolomites studied that contained authigenic quartz were partially or completely recrystallized. 2. The crystallization of authigenic quartz was diagenetically carried out after deposition of micritic calcareous ooze and oolite under the rather quiet condition. 3. In the case of the authigenic quartz is found in dismicritic rocks, the quartz crystals were formed after the formation of sparitic calcite, and the crystallization of the authigenic quartz was interrupted with pre-existing sparitic calcite grains. Authigenic quartz crystals are ubiquitous in sedimentary carbonate rocks. In the past their abundance has gone unnoticed because of the small dimensions of most of them, and the fact that has been broken by use of strong acid such as hydrochloric acid.
- 日本地質学会の論文
- 1978-11-30
著者
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