Abstract The alteration of olivine (forsterite) phenocrysts in three soil profiles that were developed on similar pyroclastic rocks from Trindade Island, at different altitudes and degree of development, were investigated in this study. Optical microscopy, Electron Probe Microanalysis (EPMA), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Differential Thermal and Thermogravimetric Analysis (DTA-TGA) were used to determine the mineralogical, micromorphological and geochemical transformations resulting from the alteration process. Micromorphological analyses showed fractured phenocrysts of colorless olivines with high relief at plane polarized light. These crystals are euhedral and irregularly shaped, and distributed in a dark reddish brown matrix constituted by pyroxenes, magnetites, ilmenites, kaolinite, hematite and anatase. At crossed polarized light, the olivine phenocrysts display a rim of reddish brown material in the fractures and in the borders that extinguishes parallel to their extinction direction. This reddish brown material appears to invade the crystal progressively until a complete replacement towards the top of the soil profile of higher altitude and degree of development. The olivine was identified as forsterite type and the reddish brown material has a chemical composition of Si, Fe, Mg, Al, Ca, Na, K, Mn and Ti, with predominant and variable contents of Fe, Si and Mg. The chemical composition and optical features are consistent with that found in iddingsites which may form through the incipient alteration of olivines due to the oxidation of ferrous iron to ferric iron, and with the coordination of magnesium with hydroxyl groups. The microscopic features of olivine did not allow us to identify any previous alteration by either deuteric or hydrothermal processes. However, the advance of the alteration towards the soil profile, leaving only relicts of olivine crystals or reaching their total transformation in the upper horizons, shows that weathering is the main process of iddingsite formation. This finding is corroborated by the presence of greater alteration of olivine crystals in the topmost soil at the highest altitude on the same lithology.

中文翻译：

---

南大西洋特林达德岛火山岩中橄榄石的变化

摘要 本研究调查了在特林达德岛类似火山碎屑岩上发育的三种土壤剖面中橄榄石（镁橄榄石）斑晶的变化，在不同的高度和发育程度。使用光学显微镜、电子探针微量分析 (EPMA)、扫描电子显微镜 (SEM)、X 射线衍射 (XRD) 和差热和热重分析 (DTA-TGA) 来确定由蚀变引起的矿物学、微形态学和地球化学转变过程。微形态分析显示无色橄榄石的破碎斑晶在平面偏振光下具有高浮雕。这些晶体呈自形且形状不规则，分布在由辉石、磁铁矿、钛铁矿、高岭石、赤铁矿和锐钛矿构成的深红棕色基质中。在交叉偏振光下，橄榄石斑晶在裂缝和边界处显示出一圈红棕色物质，在平行于它们的消光方向熄灭。这种红棕色物质似乎逐渐侵入晶体，直到完全取代较高海拔和发育程度的土壤剖面顶部。橄榄石被鉴定为镁橄榄石类型，红棕色物质的化学成分为 Si、Fe、Mg、Al、Ca、Na、K、Mn 和 Ti，主要和可变的 Fe、Si 和 Mg 含量。化学成分和光学特征与发现的iddingsites一致，iddingsites可能是由于亚铁氧化成三价铁，以及镁与羟基的配位而形成的橄榄石的初期变化。橄榄石的微观特征不允许我们通过氘代或热液过程识别任何先前的改变。然而，随着土壤剖面的变化，仅留下橄榄石晶体的残余物或在上层达到其完全转变，表明风化是iddingsite形成的主要过程。这一发现得到了在同一岩性上最高海拔的最顶层土壤中橄榄石晶体发生更大变化的证实。表明风化作用是iddingsite形成的主要过程。这一发现得到了在同一岩性上最高海拔的最顶层土壤中橄榄石晶体发生更大变化的证实。表明风化作用是iddingsite形成的主要过程。这一发现得到了在同一岩性上最高海拔的最顶层土壤中橄榄石晶体发生更大变化的证实。