近日，北京大学鲁安怀教授团队与我校生环国重陈中强教授团队方谦研究员等合作在地学重要期刊American Mineralogist上发表了我国不同气候区发育的矿物膜中黏土矿物形成与转化机制论文（Formation and transformation of clay minerals in Mars-analog rock varnish；https://pubs.geoscienceworld.org/msa/ammin/article/doi/10.2138/am-2024-9585/651334/Formation-and-transformation-of-clay-minerals-in）。论文通过对大量地表矿物膜原位微区观测和对分选的矿物膜粉末样品详细分析（部分图件见图1-3），重点对矿物膜中伊利石和绿泥石形成条件进行了探讨，首次发现伊利石向绿泥石发生转化作用，这种转化关系在地表环境中鲜有报道，提出太阳光辐射作用促进了伊利石向绿泥石发生转化作用的新认识。
“矿物膜”这一概念由北京大学鲁安怀等于2019年在PNAS论文中最早提出，同时报道了地球上广泛分布的矿物膜具有将太阳能转化为化学能的作用，发现了“石头上”进行光合作用的潜在机制。岩石漆是矿物膜中最常见的一类，最早由德国著名地理学家和博物学家亚历山大·冯·洪堡先生于19世纪描述并报道。在随后近两百年里有大量研究深入探讨了岩石漆中锰元素的富集机制。然而针对其中最主要的组分—黏土矿物，仅有少量学者进行过研究，包括加州理工学院George Rossman教授和中国科学院广州地球化学研究所陶奇研究员等，这使得我们对其中黏土矿物所扮演的角色知之甚少。
目前，人类尚未获得火星返回样品，对火星环境演变与潜在生命研究的重要手段之一是研究地球上出现的、与火星上观察到类似的——火星类比物（Mars analog）。地球上各个气候带和火星上岩石表面均广泛分布纳米-毫米级厚的矿物膜，这种矿物膜也越来越多地被视为研究火星环境的良好类比物。原因主要包括：（1）矿物膜在类火星的干旱地区更加常见，发育程度更好；（2）矿物膜在火星上以多种形式广泛出现，其中常见富锰矿物和富黏土矿物的类型；（3）矿物膜中层状黏土矿物可为潜在微生物提供庇护场所，也是生命起源/出现的潜在位置。
图1 矿物膜FIB切片中提取的伊利石形成作用及其向绿泥石转化的信息
图2 不同气候带黏土矿物结晶度与相关化学指数对比
图3 不同厚度伊利石的化学组成分析与特定元素比值对比
受American Mineralogist期刊编辑部邀请，矿物膜研究领域著名专家、美国亚利桑那州立大学Ronald Dorn教授为本文撰写了Highlights and Breakthroughs（亮点与突破）专栏评述，题为“Revisiting the importance of clay minerals in rock varnish”。值得一提的是，2023年和2024年该刊每年仅发表一篇Highlights and Breakthroughs评述。评述对论文正面评价道：“方谦和合作者对岩石漆中这一重要组分的深入研究并提出大量新的见解是过去48年来首次”。指出深入开展地表矿物膜中黏土矿物学特征研究对探索火星环境与潜在生命具有重要借鉴意义，并强调“通过方谦等人对陆地岩石漆中黏土矿物重要意义的再度强调，我希望火星岩石漆研究学者重新关注火星岩石漆中的这一最重要的组分。”同时，他还对未来有关矿物膜的研究提出建议：（1）聚焦极端干冷的地区（如南极和拉达克地区—克什米尔东南），这些地区是炎热沙漠很好的类比物；（2）探索黏土矿物如何随着岩石裂缝到暴露岩石漆位置的改变而改变；(3) 更深入研究与岩石漆中黏土矿物空间关系密切相关的蛋白石。
评述论文
Revisiting the importance of clay minerals in rock varnish
Ronald I. Dorn
Rock varnish is a natural coating on rock surfaces that contains three main ingredients: manganese oxyhydroxides; iron oxyhydroxides; and the largest component — clay minerals. In the nearly half-century since George Rossman and his Ph.D. student Russ Potter discovered the importance of clay minerals in the formation of varnish (Potter and Rossman, 1977; Potter, 1979), only a few (e,g., Chaddha et al., 2021) have conducted research on varnish clay minerals. In the latest issue of American Mineralogist, Qian Fang and colleagues are the first in 48 years to provide substantive new insights into this dominant component of rock varnish.
In “Formation and transformation of clay minerals in Mars-analog rock varnish” Fang et al. (2025) use X-ray diffraction, HAADF-STEM, TEM examination of FIB milled lamella and clay-fraction powders, as well as visible- and near-infrared spectroscopy to analyze clay minerals in rock varnish. Unlike most in varnish research, Fang et al. did not restrict themselves to hot dry deserts, but selected varnishes from different climatic regions in China, discovering that illite and chlorite dominate clays in deserts, whereas kaolinite increases in wetter settings. And unlike many others who limit sampling to exposed surfaces, Fang et al even analyzed varnish that migrated downward into factures within the underlying weathering rind. However, Fang et al. emphasize that their SEM and TEM observations reveal that rock varnish does not originate from the underlying rocks, but is externally applied.
Digging deeper into the texture of clays using high resolution TEM, Fang et al. (2025) discovered that illite thickness influences not only nanoscale chemical variability, but also that much of the interaction with Fe- and Mn-oxyhydroxides occurs with thin, likely authigenetic illite. This explains the “feathering” of authigenetic illite associated with insertion of nanoscale Mn-Fe granules into illite’s outer edges next to microbial casts (Dorn, 2024). Fang et al. also note the importance of thin (<7 nm) illite particles in creating the lamellate texture of nanoscale rock varnish.
Prior to his passing (Mahaney et al., 2018), Dave Krinsley and I were working on a Dual Beam FIB/TEM and HAADF-SEM paper on varnish clays. I am very happy that we were “scooped” by Fang et al. (2025), because I can now report that we replicated all of their findings for our North American samples. The only difference was that we found substantial amounts of mixed-layered illite-smectite, like Potter and Rossman (1977); what remains unexplored and could easily explain this difference are regional variations in desert dust mineralogy (e.g., Gonçalves Ageitos et al., 2023).
Terrestrial rock varnish (including desert varnish) research has had an (almost) obsessive focus on manganese: how it is enhanced (Dorn, 2024); how it informs on Quaternary climatic change and dating surfaces (Liu and Broecker, 2013); its complex mineralogy (McKeown and Post, 2001); and more. A similar focus on manganese dominates varnish research related to Mars, rarely considering its other major components: iron and clays. In contrast, Fang et al. (2025) take on both constituents. For example, Fang et al. point out that observed transformations from chlorite within illite could explain some of the enrichment of iron in varnish.
By Fang et al. reemphasizing the importance of clay minerals in terrestrial varnish, I hope that Martian varnish researchers refocus attention on this largest component of rock varnish. In addition, Fang et al. draw additional attention to transformations of illite to chlorite that occurs within the center of illite particles instead of along edges, as well as through development of interstratified structures between illite and chlorite. The paucity of water on Mars highlights the role of Mg2+ in retaining interlayer water, and hence would be compatible with the illite to chlorite transformation on Mars.
I have three wishes as it relates to clay minerals for scholars like Fang and colleagues who are thinking about rock varnish on Mars. First, please focus on as particularly cold and dry terrestrial sites such as Antarctica (Dorn, 2024) and the Ladakh (Chaddha et al., 2023, 2024) that are far superior terrestrial analogs to warm deserts.
My anecdotal examination of terrestrial varnishes and Martian rover imagery reveals that — what I think looks very much like — rock varnish frequently starts within rock joints, later exposed by spalling of the overlying rock material (Figure 1). Thus, my second wish is for scholars like Fang et al. (2025) to explore how clay minerals might change during this transition. For example, Fang et al. speculate that rock varnish exposure to solar radiation for very long time periods on Mars could promote illite-chlorite transformations in Martian varnish; thus, does the transition from rock joint to exposed varnish on Earth similarly promote this clay transformation?
Figure 1. On the left, Curiosity’s Mars Hand Lens Imager (MAHLI) image on SOL 4025 (June 4, 2024). The white arrow identifies a rock fissure, coated by what looks similar in appearance to terrestrial rock varnish, where the overlying rock material will eventually spall off. From https://mars.nasa.gov/raw_images/1358365/?site=msl .
On the right, rock varnish formed within a rock joint at Devil’s Kitchen, Colorado, with some of the varnish exposed by spalling of the overlying rock. Photo by Ron Dorns
Fang et al. (2025) found nanosized opaline silica in association with varnish clay minerals, and this silica glaze is common terrestrial rock coating unto itself (Langworthy et al., 2010). My observation of Martian rover imagery is that case hardening seems correlated with the presence of what looks like Martian rock varnish. Since terrestrial case hardening is often associated with silica glaze (Dorn et al., 2017), my third wish is a more in depth exploration of pockets of opaline silica surrounded by or perhaps intercalated with rock varnish clays.
In the end, I return to my gratitude to Fang and colleagues for finally, almost a half-century after Potter and Rossman (1977), making a substantial contribution of new insight into the largest component of rock varnish: clay minerals.
References Cited
参考
文献：
Chaddha, A.S., Sharma, A., Singh, N.K., Shamsad, A. and Banerjee, M., 2024. Biotic-abiotic mingle in rock varnish formation: A new perspective. Chemical Geology, 648, p.121961. https://doi.org/10.1016/j.chemgeo.2024.121961 .
Chaddha, A.S., Sharma, A. and Singh, N.K., 2021. Clay minerals identification in rock varnish by XRD: A one-step reduction approach. MethodsX, 8, p.101511. https://doi.org/101510.101016/j.mex.102021.101511.
Chaddha, A.S., Sharma, A., Singh, N.K., Patel, D.K. and Satyanarayana, G.N.V., 2023. Rock Varnish: Nature’s Shield. ACS Earth and Space Chemistry, 7(8), pp.1516-1527.
Fang, Q., Li, Y., Ding, H., Yang, L., Hong, H., Chen, Z-Q., Deng, A., Geng, Q. and Lu, A., 2025. Formation and transformation of clay minerals in Mars-analog rock varnish. American Mineralogist, this issue.
Gonçalves Ageitos, M., Obiso, V., Miller, R.L., Jorba, O., Klose, M., Dawson, M., Balkanski, Y., Perlwitz, J., Basart, S., Di Tomaso, E. and Escribano, J., 2023. Modeling dust mineralogical composition: sensitivity to soil mineralogy atlases and their expected climate impacts. Atmospheric Chemistry and Physics, 23(15), pp.8623-8657.
Dorn, R.I., Mahaney, W.C. and Krinsley, D.H., 2017. Case hardening: turning weathering rinds into protective shells. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 13(3), pp.165-169.
Dorn, R.I., 2024. Rock varnish revisited. Progress in Physical Geography: Earth and Environment, 48(3), pp.389-419.
Langworthy, K.A., Krinsley, D.H. and Dorn, R.I., 2010. High resolution transmission electron microscopy evaluation of silica glaze reveals new textures. Earth Surface Processes and Landforms, 35(13), pp.1615-1620.
Liu, T. and Broecker, W.S., 2013. Millennial-scale varnish microlamination dating of late Pleistocene geomorphic features in the drylands of western USA. Geomorphology, 187, pp.38-60.
Mahaney, B., Langworthy, K., Fischer, R. and Ron, D., 2018. In memory of Professor David Krinsley, University of Oregon. Studia Quaternaria. 35 (1): p. 83. DOI: 10.2478/squa-2018-0005
McKeown, D.A. and Post, J.E., 2001. Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy. American Mineralogist, 86(5-6), pp.701-713.
Potter RM, 1979. The tetravalent manganese oxides: clarification of their structural variations and relationships and characterization of their occurrence in the terrestrial weathering environment as desert varnish and other manganese oxides. Ph.D. Dissertation, California Institute of Technology, Pasadena, 245 pp.
Potter, R.M. and Rossman, G.R., 1977. Desert varnish: the importance of clay minerals. Science, 196(4297), pp.1446-1448.
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