原文網址：https://phys.org/news/2024-09-electricity-generated-earthquakes-secret-giant.html
金塊經常發現成團堆積於石英脈當中，科學家長久以來都對其形成過程深感興趣。由蒙納許大學的科學家主持的新研究，認為過程可能比我們之前所想的還要「刺激」。
金塊因為產量稀有及美麗的色澤而價格不斐，數個世紀以來都是驅動淘金熱的核心動力。
此研究即將發表於《自然—地球科學》(Nature
Geoscience)，主持人為蒙納許大學地球、大氣與環境學院的Chris
Voisey博士。
「高溫且富含水分的液體流過地殼的裂隙時會把金沉澱出來，這是標準的解釋，」Voisey博士表示。
「當這些液體冷卻或是經歷化學變化，金便會分離出來並且包裹在石英脈裡面，」他說。
「雖然大多數人接受這道理論，但它無法完全解釋大型金塊如何形成，尤其是考量到金的濃度在這些液體當中其實非常的低。」
研究團隊測試了一道新的想法，其跟壓電性有關。這些堆積下來的金塊一般位於石英這種礦物當中，它有一種特殊的性質稱為壓電性，意味著石英受到壓力的時候會產生電荷。其實此現象對我們來說並不陌生，在某些日常用品，像是石英錶和烤肉用的點火槍當中，只要施加一點機械力就能產生強大的電壓。如果地震產生的壓力也能在地球內部達成類似的功用呢？
為了測試這道假說，研究人員設計了一項實驗來複製石英在地震時可能經歷的情況。他們首先把石英晶體浸泡在富含金的流體當中，然後用馬達模擬地震造成的晃動來施加壓力。實驗結束後，他們把石英晶體放到顯微鏡下，檢查是否有金子堆積下來。
「結果讓我們很驚訝，」共同作者，蒙納許大學地球、大氣與環境學院的教授Andy
Tomkins表示。
「受壓的石英不只會因為電化學反應而有金堆積在表面，而且還有聚集成團的奈米金粒。」
「值得注意的是，金傾向堆積在已經出現的金粒表面，而不是形成新的顆粒。」
原因在於雖然石英是電的絕緣體，但金卻是導體。
一但有些金堆積下來，就會變成之後的成長中心，實際來說就是把這些金的顆粒鍍上更多金子。
「我們的發現提供了一個可行的說法，來解釋為什麼石英脈裡面會形成大型金塊，」Voisey博士表示。
隨著石英因為地震而反覆受壓，由壓電性產生的電壓便能還原周遭液體中的溶解金，使其堆積下來。
此作用經年累月下來可以聚集大量的金，最後產生讓寶藏獵人以及地質學家都著迷不已的巨大金塊。
「基本上，石英的作用就像是自然的電池，金則是電極，隨著每一次的地震事件就能累積越來越多金子，」Voisey博士表示。
這道過程可以解釋為什麼大型金塊如此常見於和地震有關的沉積物當中的石英脈。
研究人員對於含有金塊的地層得出的新理解，不只闡明了這道歷時已久的地質難題，也顯現了地球的物理作用與化學作用之間有著相當密切的關係。
Electricity
generated by earthquakes might be the secret behind giant gold nuggets
Scientists have long been fascinated by
the formation of gold nuggets, often found nestled within quartz veins. New
research led by Monash University geologists suggests that the process might be
even more electrifying than we previously thought—literally.
Gold nuggets, prized for their rarity and beauty,
have been at the heart of gold rushes for centuries.
The study is led by Dr. Chris Voisey from the Monash
University School of Earth Atmosphere and Environment and will be published in Nature Geoscience.
"The standard explanation is that gold
precipitates from hot, water-rich fluids as they flow through cracks in the
Earth's crust," said Dr. Voisey.
"As these fluids cool or undergo chemical changes,
gold separates out and becomes trapped in quartz veins," he said.
"While this theory is widely accepted, it
doesn't fully explain the formation of large gold nuggets, especially
considering that the concentration of gold in these fluids is extremely
low."
The research team tested a new concept,
piezoelectricity. Quartz, the mineral that typically hosts these gold deposits,
has a unique property called piezoelectricity—it generates an electric charge
when subjected to stress. This phenomenon is already familiar to us in everyday
items like quartz watches and BBQ lighters, where a small mechanical force
creates a significant voltage. What if the stress from earthquakes could do
something similar within the Earth?
To test this hypothesis, researchers conducted an
experiment designed to replicate the conditions quartz might experience during
an earthquake. They submerged quartz crystals in a gold-rich fluid and applied
stress using a motor to simulate the shaking of an earthquake. After the
experiment, the quartz samples were examined under a microscope to see if any
gold had been deposited.
"The results were stunning," said study
co-author Professor Andy Tomkins, from the Monash University School of Earth,
Atmosphere and Environment.
"The stressed quartz not only electrochemically
deposited gold onto its surface, but it also formed and accumulated gold
nanoparticles," he said.
"Remarkably, the gold had a tendency to deposit
on existing gold grains rather than forming new ones."
This is because, while quartz is an electrical
insulator, gold is a conductor.
Once some gold is deposited, it becomes a focal point
for further growth, effectively "plating" the gold grains with more
gold.
"Our discovery provides a plausible explanation
for the formation of large gold nuggets in quartz veins," said Dr. Voisey.
As the quartz is repeatedly stressed by earthquakes,
it generates piezoelectric voltages that can reduce dissolved gold from the
surrounding fluid, causing it to deposit.
Over time, this process could lead to the formation
of significant gold accumulations, ultimately producing the massive nuggets
that have captivated treasure hunters and geologists alike.
"In essence, the quartz acts like a natural
battery, with gold as the electrode, slowly accumulating more gold with each
seismic event," said Dr. Voisey.
This process could explain why large gold nuggets are
so often associated with quartz veins formed in earthquake-related deposits.
This new understanding of gold nugget formation not
only sheds light on a longstanding geological mystery but also highlights the
interrelationship between Earth's physical and chemical processes.
原始論文：Christopher
R. Voisey, Nicholas J. R. Hunter, Andrew G. Tomkins, Joël Brugger, Weihua Liu,
Yang Liu, Vladimir Luzin. Gold nugget
formation from earthquake-induced piezoelectricity in quartz. Nature Geoscience, 2024 DOI:
10.1038/s41561-024-01514-1
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