5:15 PM - 6:30 PM
[SMP26-P07] Amesite and Al-serpentine accompanied with the serpentinite in Yakiyama, Iizuka, Fukuoka
Keywords:serpentine, amesite, rodingite, chlorite, Al-serpentine
1.Introduction
Serpentine has an ideal chemical composition of Mg3Si2O5(OH)4, and generally contains small amounts of Fe and Al. Amesite (Mg, Al)3(Si, Al)2O5(OH)4 is one of the mineral that has a solid solution relationship with serpentine, and the intermediate composition is called Al-serpentine. Al-serpentine (7 Å) is polymorphic with clinochlore (14 Å) and is considered to be a low-temperature phase (metastable phase), which is mineralogically important. However, the occurrence of Al-serpentine and amesite is relatively rare, and detailed mineralogical descriptions are scarce. In this study, we found amesite and Al-serpentine in rodingites associated with serpentine bodies in this area, and investigated their occurrence and mineralogical properties.
2. samples and experiments
We observed rodingites in serpentinites and examined their constituent minerals in and around Yakiyama, Iizuka City. The samples were observed by naked eye and Stereoscopic microscopes followed by X-ray diffraction analysis (Rigaku Ultima IV, Rigaku RINT RAPID II), polarized light microscopy and scanning electron microscopy (SEM-EDS: JEOL JSM-7001F). TEM (JEOL JEM-3200FSK, JEM-ARM200CF) at the Research Center for Ultramicroscopic Analysis, Kyushu University was also used to observe the microstructure of thin film and powder samples.
3. results
Eleven rodingite outcrops were identified by field observations. Rodingites are composed of Ca- and Al-rich minerals such as epidote-clinozoisite, calcite, grossular, clinochlore, thomsonite, prehnite, and titanite. Amesite was found in three rodingite outcrops (samples Rod3, 7 and 9). Amesite and Al-serpentine are found in two types: veinlets in the rodingites and scattered with calcite and thomsonite around the veins.
Sample Rod3 contains white opaque Al-serpentine with associated light brown transparent amesite. The XRD analysis of both samples is consistent with the pattern of lizardite-6T1. The white opaque part had very small crystals (about 50-300 nm) and was a mixture of plate-like, hollow cylindrical with plate-like crystals inside, fibrous, and irregularly shaped. The chemical composition of each crystalline particle was three phases: amesite, lizardite, and an excess of Si or Al than amesite-lizardite. Microstructural observation showed that these were mixed together. The hollow cylindrical part shows polygonal columnar lattice fringes by TEM observation, but the shape of the sectors is irregular and the lattice fringes at the sector boundaries cannot be confirmed, which is different from polygonal serpentine reported so far. In addition, the hollow cylindrical part has an Al cation number of about 1.3 (O=7), suggesting that it may be a mineral with a kaolin-like structure in which the size of the tetrahedral and octahedral sheets is reversed from that of ordinary serpentine, and the combination of the two sheets is reversed. Inside the hollow cylindrical section, there is a plate-like section bisecting the circle and parallel to the cylinder. The plate-part have an excess of Si or Al, and TEM observation shows a mixture of 7 Å and 14 Å layers.
The light brown transparent part is coarse-grained (200 µm), and at the boundary with the white part, there is a 1 µm filamentous crystal extending toward the white part. The composition of the crystals is non-uniform even within a single crystal, and EDS mapping revealed two types of crystals: one with a mosaic-like non-uniform composition and the other with a clear separation of Al concentration. The former was euhedral with a clear cleavage, while the latter was semi-euhedral, and the filamentous crystals described above were derived from this crystal. The composition of each is generally consistent with a solid solution of lizardite-amecite, but the Al-rich part tends to be Al or Si-rich, and may contain OH-rich material due to the low compound total.
Serpentine has an ideal chemical composition of Mg3Si2O5(OH)4, and generally contains small amounts of Fe and Al. Amesite (Mg, Al)3(Si, Al)2O5(OH)4 is one of the mineral that has a solid solution relationship with serpentine, and the intermediate composition is called Al-serpentine. Al-serpentine (7 Å) is polymorphic with clinochlore (14 Å) and is considered to be a low-temperature phase (metastable phase), which is mineralogically important. However, the occurrence of Al-serpentine and amesite is relatively rare, and detailed mineralogical descriptions are scarce. In this study, we found amesite and Al-serpentine in rodingites associated with serpentine bodies in this area, and investigated their occurrence and mineralogical properties.
2. samples and experiments
We observed rodingites in serpentinites and examined their constituent minerals in and around Yakiyama, Iizuka City. The samples were observed by naked eye and Stereoscopic microscopes followed by X-ray diffraction analysis (Rigaku Ultima IV, Rigaku RINT RAPID II), polarized light microscopy and scanning electron microscopy (SEM-EDS: JEOL JSM-7001F). TEM (JEOL JEM-3200FSK, JEM-ARM200CF) at the Research Center for Ultramicroscopic Analysis, Kyushu University was also used to observe the microstructure of thin film and powder samples.
3. results
Eleven rodingite outcrops were identified by field observations. Rodingites are composed of Ca- and Al-rich minerals such as epidote-clinozoisite, calcite, grossular, clinochlore, thomsonite, prehnite, and titanite. Amesite was found in three rodingite outcrops (samples Rod3, 7 and 9). Amesite and Al-serpentine are found in two types: veinlets in the rodingites and scattered with calcite and thomsonite around the veins.
Sample Rod3 contains white opaque Al-serpentine with associated light brown transparent amesite. The XRD analysis of both samples is consistent with the pattern of lizardite-6T1. The white opaque part had very small crystals (about 50-300 nm) and was a mixture of plate-like, hollow cylindrical with plate-like crystals inside, fibrous, and irregularly shaped. The chemical composition of each crystalline particle was three phases: amesite, lizardite, and an excess of Si or Al than amesite-lizardite. Microstructural observation showed that these were mixed together. The hollow cylindrical part shows polygonal columnar lattice fringes by TEM observation, but the shape of the sectors is irregular and the lattice fringes at the sector boundaries cannot be confirmed, which is different from polygonal serpentine reported so far. In addition, the hollow cylindrical part has an Al cation number of about 1.3 (O=7), suggesting that it may be a mineral with a kaolin-like structure in which the size of the tetrahedral and octahedral sheets is reversed from that of ordinary serpentine, and the combination of the two sheets is reversed. Inside the hollow cylindrical section, there is a plate-like section bisecting the circle and parallel to the cylinder. The plate-part have an excess of Si or Al, and TEM observation shows a mixture of 7 Å and 14 Å layers.
The light brown transparent part is coarse-grained (200 µm), and at the boundary with the white part, there is a 1 µm filamentous crystal extending toward the white part. The composition of the crystals is non-uniform even within a single crystal, and EDS mapping revealed two types of crystals: one with a mosaic-like non-uniform composition and the other with a clear separation of Al concentration. The former was euhedral with a clear cleavage, while the latter was semi-euhedral, and the filamentous crystals described above were derived from this crystal. The composition of each is generally consistent with a solid solution of lizardite-amecite, but the Al-rich part tends to be Al or Si-rich, and may contain OH-rich material due to the low compound total.