蛇纹岩 – 上海的惊人地质

概述

蛇纹岩是一种非凡的岩石，起源于地幔深处，为了解塑造我们星球的动态过程提供了见解。蛇纹岩由超镁铁质岩石的水化作用形成，在板块构造和地壳内水循环中起着至关重要的作用。其特有的绿色色调和光滑、通常呈鳞片状的纹理使其视觉上引人注目。蛇纹岩通常出现在俯冲带或古代海洋地壳残余等地质环境中，但它也可能出现在意想不到的地方——比如上海的抛光外墙，这种独特的岩石被重新用作装饰元素。参观这个地方提供了一个难得的机会，可以近距离观察蛇纹岩，欣赏它从地球深处到城市环境的旅程。你需要一块磁铁。

要记录此 EarthCache，请转到给定的坐标并仔细查看上图中用红色标记的区域。回答以下问题并通过电子邮件或消息中心发送给我：
 

1. 根据列表，蛇纹岩是什么颜色，您能看到什么结构（结晶、纤维、角砾状、风化）。此外，请仔细查看标记的区域 1 和 2。

2. 根据列表，蛇纹岩通常与其他矿物（叶蛇纹石、温石棉和蜥蜴石）有关。考虑到您所在的位置，您可以立即排除哪种矿物？

3. 列表中描述蛇纹岩也可以具有磁性。使用磁铁在现场进行测试并告诉我结果（无反应、弱反应、中等反应或强反应）。根据您在列表中看到的内容，您能证明它可能存在或不存在吗？

4. 必须：在给定的坐标处拍摄您自己或个人物品的照片，但不要泄露有关所提问题的任何信息。
 

关于蛇纹岩

蛇纹岩是一种变质岩，通过蛇纹石化过程形成，在此过程中，来自地球上地幔的超镁铁质岩发生化学转变。这些超镁铁质岩，例如橄榄岩和纯橄榄岩，主要由橄榄石和辉石矿物组成。
超镁铁质岩属于火成岩类别，源自熔岩（岩浆）的冷却和凝固。它们的特点是镁铁质矿物含量高，占其体积的至少 90%。
蛇纹岩通常存在于蛇绿岩复合体中——古代海洋地壳的碎片，通过构造过程被抬升到地球表面。

蛇绿岩这一术语源于古希腊语，由 ὄφις (ophis) = 蛇和 λίθος (lithos) = 石头组成，指的是质地类似蛇皮的绿色岩石。该术语主要描述蛇纹岩，但也包括相关岩石，例如碎石。
 

蛇纹石的形成——蛇纹石化过程
 

蛇纹石化是一种低温变质过程，涉及无水铁镁硅酸盐矿物（如橄榄石和辉石）转化为含水硅酸盐矿物（蛇纹石）以及次生矿物，如水镁石和磁铁矿。

当前体岩石的镁含量较高时（例如纯橄榄岩），就会形成水镁石。

磁铁矿在富含铁的环境中形成（例如辉石岩），使蛇纹石具有其特有的深色。

这种转变主要发生在俯冲带、大洋中脊和其他构造活跃地区。

1. 水合和化学反应

橄榄石是超镁铁质岩中的主要矿物，它与水（H₂O）发生反应，形成蛇纹石矿物，例如：

• 叶蛇纹石——绿松石色
• 温石棉——纤维状白绿色
• 利蛇纹石——苹果绿色

此外，还可以形成水镁石（Mg(OH)₂）等次生矿物。

2. 变质作用

在低温至中温（约 200–500°C）和低压条件下，橄榄石和辉石会变质转化为蛇纹石矿物，主要是叶蛇纹石、温石棉和利蛇纹石。

3. 化学反应：

导致蛇纹石矿物形成的化学反应可简化如下：

2(Mg,Fe)2SiO4+3H2O→(Mg,Fe)3Si2O5(OH)4+(Mg,Fe)(OH)2
橄榄石 (Mg,Fe)2SiO4 转化为蛇纹石 →(Mg,Fe)3Si2O5(OH)4 和水镁石 +(Mg,Fe)(OH)2。

结晶的	叶状的, 纤维状的

抛光, 碎裂	uncut rock

蛇纹岩的特性
 

外观
 

蛇纹岩通常以其斑驳的绿色为特征，这是由于蛇纹石矿物的存在而产生的。这种岩石具有光滑、蜡质的感觉，类似于硬化的蜡烛。其纹理可能因变质的强度和类型而异，从闪亮的结晶细粒到片状或纤维状。

1. 网状纹理：

o 当橄榄石转变为细粒蛇纹石矿物时，会形成这种纹理，从而形成独特的网状图案。
 

2. 片状或纤维状纹理：

o 根据存在的具体蛇纹石矿物，蛇纹岩可能会形成片状（层状）或纤维状（类似石棉）结构。
 

3. 风化和表面特征：

o 在地球表面，由于风化，蛇纹岩会形成特有的绿色和光滑、肥皂般的感觉。
o 如果岩石具有角砾结构，棱角状岩石碎片之间的空间可能会充满方解石、绿泥石、菱镁矿或温石棉等矿物，从而增强视觉对比。
 

(角砾岩是一种由棱角状岩石碎片组成的岩石，这些碎片由细粒基质胶结而成。与包含圆形岩石碎片的砾岩不同，角砾岩由棱角状碎片组成.）
蛇纹岩出现在地球表面经历过重大构造活动和中度至高度变质的地区。由于它起源于地壳的较深处，因此与更广泛的沉积岩层相比，它通常出现在相对较小且局部的区域。它通常与大陆边缘的先前俯冲带、断裂带和褶皱山脉有关。此外，蛇纹岩是海洋地壳的组成部分，尤其是在洋中脊和板块边界沿线。
 

物理特性
 

• 硬度：蛇纹岩相对较软，莫氏硬度为 2.5–4。
• 密度：这种岩石的密度约为 2.5 至 3.3 g/cm⊃3;。
• 用途：蛇纹岩通常用作装饰石或雕塑。它也可以作为镁的提取来源。
• 颜色：虽然蛇纹岩通常与鲜艳的绿色色调有关，但其色谱可能会有很大差异。它也可能呈现出深红色、红棕色、深棕色、黑色、黑绿 色甚至浅绿色。
• 磁性：蛇纹岩通常不具磁性。但是，岩石中的少量磁铁矿有时会产生局部磁性。一个简单的测试是将磁铁靠近岩石 - 如果岩石中含有足够的磁铁矿，磁铁可能会被吸引。

来源
steine-und-minerale.de
mineralienatlas.de
www.geodienst.de
wikipedia
jsjgeology.net
自己的图片

Serpentinite – Amazing Geology of Shanghai
 

Overview
 

Serpentinite is a remarkable rock that originates deep within the Earth's mantle, offering insight into the dynamic processes shaping our planet. Formed through the hydration of ultramafic rocks, it plays a crucial role in plate tectonics and the cycling of water within the Earth's crust. Its characteristic greenish hues and smooth, often scaly texture make it visually striking. While typically found in geological settings such as subduction zones or remnants of ancient oceanic crust, serpentinite can also appear in unexpected places—like the polished facade in Shanghai, where this unique rock has been repurposed as a decorative element. Visiting this site provides a rare opportunity to observe serpentinite up close and appreciate its journey from the depths of the Earth to an urban setting. You will need a magnet.
 

To log this EarthCache, go to the given coordinates and take a close look at the area marked in red in the image above. Answer the following questions and send them to me by email or via the message center:

1. What colors are the serpentenites and what structures (crystalline, fibrous, brecciated, weathered) can you see according to the listing. Additionally have a close look at the marked areas 1 and 2.
    
2. According to the listing, serpentinite is often associated with other minerals ( antigorite, chrysotile and lizardite). Given the location you are in, what mineral can you rule out right off the bat?
    
3. The listing described that serpentenite can also be magnetic. Carry out a test on site with a magnet and let me know the result (no reaction, weak, middle or strong). Based on what you read in the listing, can you justify the possible presence or absence of it?
    
4. Obligatory: Take a photo of yourself or a personal item at the coordinates given, without spoiling any information about the questions asked.
    

About Serpentinite
 

Serpentinite is a metamorphic rock that forms through the process of serpentinization, in which ultramafic rocks from the Earth's upper mantle undergo chemical transformation. These ultramafic rocks, such as peridotite and dunite, are primarily composed of the minerals olivine and pyroxene. Ultramafic rocks belong to the category of igneous rocks, originating from the cooling and solidification of molten rock (magma). They are defined by their high content of mafic minerals, making up at least 90% of their volume.

Serpentinite is commonly found in ophiolitic rock complexes—fragments of ancient oceanic crust that have been uplifted to the Earth's surface through tectonic processes. The term ophiolite is derived from ancient Greek, combining ὄφις (ophis) = snake and λίθος (lithos) = stone, referring to greenish rocks with a texture reminiscent of snake skin. This term primarily describes serpentinites but also includes related rocks such as spilites.

Formation of Serpentinite – The Process of Serpentinization
 

Serpentinization is a low-temperature metamorphic process that involves the transformation of anhydrous ferromagnesian silicate minerals (such as olivine and pyroxene) into hydrous silicate minerals (serpentine) along with secondary minerals like brucite and magnetite.
 

• Brucite forms when the precursor rocks have a high magnesium content (e.g., dunite).
• Magnetite develops in iron-rich environments (e.g., pyroxenite), giving serpentinite its characteristic dark coloration.
• This transformation primarily occurs in subduction zones, mid-ocean ridges, and other tectonically active regions.

1. Hydration and Chemical Reactions
 

Olivine, the dominant mineral in ultramafic rocks, reacts with water (H₂O), leading to the formation of serpentine minerals such as:

• Antigorite – turquoise-colored
• Chrysotile – fibrous whitish-green
• Lizardite – apple-green
• Additionally, secondary minerals such as Brucite (Mg(OH)₂) can develop.

2. Metamorphism
 

Under conditions of low to moderate temperatures (approximately 200–500°C) and low pressure, olivine and pyroxene undergo a metamorphic transformation into serpentine minerals, primarily antigorite, chrysotile, and lizardite.3. Chemical reaction:

The chemical reaction that leads to the formation of serpentine minerals can be simplified as follows:

2(Mg,Fe)2SiO4+3H2O→(Mg,Fe)3Si2O5(OH)4+(Mg,Fe)(OH)2

Olivine (Mg,Fe)2SiO4 is converted into serpentine →(Mg,Fe)3Si2O5(OH)4 and brucite +(Mg,Fe)(OH)2.

crystalline	foliatet, fibrous

polished, brecciated	uncut rock

Properties of Serpentinite
 

Appearance
 

Serpentinite is typically characterized by its mottled greenish color, which results from the presence of serpentine minerals. The rock has a smooth, waxy feel, similar to hardened candle wax. Its texture can vary depending on the intensity and type of metamorphism, ranging from shiny and crystalline-fine-grained to flaky or fibrous.

1. Mesh texture:
   • This texture forms when olivine transforms into fine-grained serpentine minerals, creating a distinctive mesh-like pattern.
      
2. Flaky or fibrous texture:
   • Depending on the specific serpentine mineral present, serpentinite may develop a flaky (lamellar) or fibrous (asbestos-like) structure.
      
3. Weathering and surface features:
   • At the Earth's surface, serpentinite can develop a characteristic greenish color and a smooth, soapy feel due to weathering.
   • If the rock has a brecciated structure, the spaces between angular rock fragments may be filled with minerals such as calcite, chlorite, magnesite, or chrysotile, enhancing the visual contrast.

(A breccia is a rock composed of angular rock fragments cemented by a fine-grained matrix. In contrast to conglomerates, which contain rounded rock fragments, breccias consist of angular fragments.)

Serpentinite occurs at the Earth's surface in regions that have experienced significant tectonic activity and moderate to high degrees of metamorphism. Since it originates from deeper parts of the Earth's crust, it is typically found in relatively small and localized areas compared to more extensive sedimentary rock formations. It is commonly associated with former subduction zones along continental margins, fracture zones, and fold mountain belts. Additionally, serpentinite is an integral part of the oceanic crust, particularly in mid-ocean ridges and along plate boundaries.

Physical Properties
 

• Hardness: Serpentinite is relatively soft, with a hardness of 2.5–4 on the Mohs scale.
• Density: The rock has a density of approximately 2.5 to 3.3 g/cm⊃3;.
• Uses: Serpentinite is often used as a decorative stone or in sculptures. It can also serve as a source of magnesium extraction.
• Colors: While serpentinite is commonly associated with vibrant green hues, its color spectrum can vary significantly. It may also appear in shades of burgundy red, reddish-brown, dark brown, black, black-green, or even light green.
• Magnetism: Serpentinite is generally non-magnetic. However, small amounts of magnetite within the rock can sometimes create localized magnetic properties. A simple test involves holding a magnet close to the rock—if it contains sufficient magnetite, the magnet may be attracted.

Sources
steine-und-minerale.de
mineralienatlas.de
www.geodienst.de
wikipedia
jsjgeology.net

own pictures