從條紋狀閃玉－龍紋玉探討臺灣閃玉的新成因

在花東地區有普通閃玉、貓眼閃玉以及蠟光閃玉互呈條紋狀分布的閃玉集合體，當地俗稱「龍紋玉」：普通閃玉中可以發現具有蠟狀光澤的岩脈，而兩者之間還可發現垂直於岩脈邊界的纖維狀結晶。過去研究中僅張郇生於2006年報導了龍紋玉的特殊產狀並對其進行了岩相觀察，其研究中僅說明了岩脈的閃玉及圍岩閃玉結晶長軸具有明顯的方向差異，並無更加深入的分析及探討。本研究針對龍紋玉進行放大觀察、岩相觀察、拉曼光譜檢測、X光粉末繞射分析、掃描式電子顯微鏡觀察、雷射剝蝕－電感耦合等離子體質譜分析。研究結果顯示龍紋玉的可能形成過程為：（1）蛇紋岩經換質作用後形成透鏡狀的普通閃玉，（2）因應力作用，普通閃玉產生裂隙，（3）熱液充填於裂隙之中，環境溫度下降後沉澱產生閃玉結晶，（4）裂隙中充填的熱液異質成核，由裂隙壁向中心垂直生長，且同時受到剪力作用，因此形成具有明顯方向性的彎曲貓眼閃玉結晶，（5）貓眼閃玉結晶生長一段時間後，溫度下降，結晶缺乏溫度梯度難以快速生長，待熱液降溫至結晶溫度以下形成過冷現象，均質成核形成隨機取向的蠟光閃玉。

關鍵字

條紋狀閃玉；龍紋玉；臺灣閃玉新成因；斑馬構造

並列摘要

The ＂Dragon-like Nephrite,＂ commonly known in the Hualien area, has three types of Taiwan Nephrite existing in the same area: the zebra-like veins can be found among the lens-like Common Nephrite. The color of veins is lighter than that of the Common Nephrite of country rocks, and the gloss of veins is different from that of country rocks. It has the characteristics of Cat's-eye Nephrite that the fibrous crystal is vertical on both sides of the veins and the characteristics of Waxy Nephrite that the vein center has waxy luster. In previous research, only Zhang (2006) reported on the special occurrence of the Dragon-like Nephrite and has done the optical mineralogy. The study only shows that the c-axis direction of Nephrite of the vein and the Nephrite of country rock are obviously different. There is no more in-depth analysis and discussion. In this research, the analyses of Dragon-like Nephrite were performed at macroscopic observation, optical mineralogy observation, Raman spectroscopy, X-ray powder diffraction analysis, scanning electron microscope observation, laser ablation-inductively coupled plasma mass spectrometry and electron backscatter diffraction analysis. The results show that the formation process of the Dragon-like Nephrite is as follows: (1) Serpentite was formed into Common Nephrite as lens after the metamorphism. (2) Due to the stress, the Common Nephrite produced cracks. (3) The hydrothermal filled the crack, and the precipitation produced a Nephrite by crystallization. (4) During the crystal growth process in the crack, it was subjected to shearing force, thus forming a Cat's eye Nephrite crystal with obvious directionality. (5) After a period of Cat's-eye Nephrite crystal growth, the temperature dropped, and the crystal lacked the temperature gradient, which was difficult to grow rapidly, and the hydrothermal was cooled down to the crystallization temperature. Supercooling happened, which made homogeneous nucleation to form a randomly oriented waxy nephrite.