Orbicular Granite vs. Rapakivis – Amazing Geology EarthCache

  球状花岗岩与环斑花岗岩 – 上海的惊人地质
 

概述：
 

在现场，您会发现由火成岩制成的抛光石凳，以及一种以球形且清晰可辨的结构为特征的迷人地质现象。在这个地球宝藏中，我想向您展示 Rapavikis（卵形）花岗岩和 Orbilur 花岗岩之间的区别。在这里您可以近距离观察这块特殊的岩石并了解更多有关其形成的信息。
 

要记录这个地球宝藏，请转到给定的坐标并仔细查看下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. 描述圆形结构的外观（例如大小、颜色、与周围岩石的对比）。

2. 它是环斑花岗岩还是圆状花岗岩？您怎样证明您的决定是正确的？

3. 你发现的最大圆形结构的直径是多少？这说明了什么关于形成和冷却的时间？

4. 必须：在给出的坐标处拍摄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。
 

介绍
 

火成岩是由熔岩（岩浆）冷却结晶而成，根据凝固位置可分为深成岩（深层岩石）和火山岩（火成岩）。它们的化学成分，特别是二氧化硅含量决定了它们的矿物组成，并进一步将其分为酸性岩、中性岩、基性岩和超基性岩。冷却速度显著影响微观结构特征，缓慢冷却导致粗晶粒结构，快速冷却导致细晶粒或玻璃状结构
拉帕基维花岗岩和球状花岗岩都是具有典型圆形结构的火成岩，但其成因不同：

1 岩石类型及成分
 

拉帕基维花岗岩

• 始终为花岗岩（碱性长石 + 斜长石 + ≥20% 石英）
• 圆形卵形体由碱性长石（正长石/微斜长石）构成，通常带有绿色斜长石（例如，奥长石）边缘。基质含有石英、黑云母和角闪石，通常伴有图形共生体。
   

球状花岗岩

• 可以有不同的成分（例如闪长岩、花岗闪长岩、正长岩）
• 方晶石显示出深色矿物（黑云母、角闪石、辉石）和斜长石的同心壳，围绕着长石和石英的核心。
   

2.结构与尺寸

拉帕基维花岗岩（黑白照片）	Orbicular Granite (black and white photo)

3. 起源
 

拉帕基维花岗岩

• 由岩浆混合形成：较热的玄武岩浆渗透到花岗岩浆中，导致压力下降和长石溶解/重结晶
• 卵形体在岩浆早期形成并继续生长。在岩浆缓慢冷却的过程中，大型长石晶体生长
• 这些卵形体通常不像球状花岗岩的球状体那样界限分明，而是与周围的岩石基质更加平滑地融合。
   

球状花岗岩
 

• 通过围绕外来核（例如岩石碎片）结晶形成。这发生在岩浆房中的特殊条件下，例如温度波动或岩浆中的化学变化。
• 同心壳是由残余熔体中的矿物质有节奏地沉淀形成的
• 这些同心层形成了引人注目的图案，并且通常与周围的岩石有明显的界限。

资料来源：
维基百科
结晶的
地质公园-hblo

 Orbicular Granite vs. Rapakivis – Amazing Geology in Shanghai
 

Overview
 

Here you'll find polished stone benches made of igneous rock, a fascinating geological phenomenon characterized by spherical and clearly recognizable structures. In this EarthCache, I want to show you the difference between rapavikis (ovoid) and orbicular granite. Here you can see this special rock up close and learn more about its formation.
 

Logging Conditions:

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

1. Describe the appearance of the round structures (e.g., size, color, contrast with the surrounding rock).

2. Is it rapakivis or orbicular granite? How would you justify your decision?

3. What is the diameter of the largest round structure you found? What does this tell you about its formation time and cooling?

4. Mandatory: Take a photo of yourself or a personal item at the given coordinates, without spoiling any information about the questions asked.
 

Introduction
 

Igneous rocks form through the cooling and crystallization of molten rocks (magma) and are divided into plutonites (plutonic rocks) and volcanics (igneous rocks) based on their location of solidification. Their chemical composition, particularly the silica content, determines their mineralogical composition and leads to a further classification into acidic, intermediate, basic, and ultrabasic rocks. The cooling rate significantly influences the microstructure, with slow cooling resulting in coarse-grained structures and rapid cooling resulting in fine-grained or glassy structures.
Rapakivi granites and orbicular granites are both igneous rocks with characteristic rounded structures, but differ in their formation:
 

1. Rock type and composition
 

Rapakivi granite

• Always a granite (alkali feldspar + plagioclase + ≥20% quartz)
• The round ovoids consist of alkali feldspar (orthoclase/microcline), often with a rim of green plagioclase (e.g., oligoclase). The groundmass contains quartz, biotite, and hornblende, often with graphic intergrowths.

Orbicular granite

• Can have various compositions (e.g., diorite, granodiorite, syenite)
• The orbicules show concentric shells of dark minerals (biotite, amphibole, pyroxene) and plagioclase around a core of feldspar and quartz.

2. Structure and size

Rapakivi Granite (black and white photo)	Orbicular Granite (black and white photo)

3. Formation
 

Rapakivi Granite

• Forms through magma mixing: Hotter basalt magma penetrates granitic magma, leading to a drop in pressure and the dissolution/recrystallization of feldspar.
• Ovoids form early in the magma and continue to grow in a rounded shape. During the slow cooling of the magma, large feldspar crystals grow.
• These ovoids are generally not as sharply defined as the orbiculars of orbicular granite and blend more smoothly into the surrounding rock matrix.

Orbicular Granite

• Forms through crystallization around foreign nuclei (e.g., rock fragments). This occurs in magma chambers under special conditions, such as temperature fluctuations or chemical changes in the magma.
• Concentric shells form through the rhythmic precipitation of minerals from the residual melt.
• These concentric layers create a striking pattern and are usually well demarcated from the surrounding rock.

Sources
Wikipedia
kristallin
geopark-hblo