Cannot be mistaken for : other
Mouse-ears because this is the only one which grows in Keen of Hamar (your Author thinks...?)
Not to be semantically confused with :
Alpine Mouse-ear (Cerastium alpinum).
Cerastium nigrescens hybridizes with :
Common Mouse-ear (Cerastium fontanum) to produce Cerastium × richarsdonii which is mostly sterile and has been found in Snowdonia and the Scottish Highlands (no mention of Keen of Hamar, probably because this is not a hybrid of Cerastium nigrescens var. nigrescens)
Serpentinic rock is not one mineral; there are several different types of serpentine. All originally from the normally deeply buried mantle. Only very rarely the ultramafic rock magma from the mantle reaches the earth surface, as it has done in the Gower peninsula in Wales and on some Shetland islands. On its way up from the deep this magma becomes altered by hydration processes and metamorphism to become a type of rare rock called serpentine when it reaches the earths surface. The exact minerals present in this serpentinic rock depend upon the starting composition of the magma and the transformation processes it undergoes as it slowly reaches the surface, which includes hydration and oxidation.
Serpentine is formed from the mineral olivine. Basically
serpentine is a hydrated magnesium silicate, Mg3Si2O5(OH)4. But on its way it may also have
Quartz SiO2 and especially much more water incorporated into it, as well as other metals. The minerals Antigorite, Lizardite, and Chrysotile (which can contain asbestos type minerals) are different minerals coming under the 'serpentine' moniker. Your Author does not yet know exactly which particular serpentinic rock that from Unst belongs, but suspects it is Chrysotile because of its golden-yellow colouration which some forms of 'asbestos' have (there are 3 differing 'asbestos' minerals too; with Blue asbestos being the most dangerously carcinogenic).
The serpentine rocks at Keen of Hamar are ultrabasic and called dunite serpentine. The soil in amongst the fractured rocks is mildly acidic with moderately high magnesium to calcium ratios. There are elevated levels of trace heavy metals such as Nickel, Chromium and Cobalt present, the first two in potentially toxic concentrations. But low concentrations of some essential macronutrients such as Nitrogen, Phosphorus and Potassium. Iron is one of the most usual metals in serpentine soils and has been linked with Phosphorus deficiency. The hill of ultrabasic rocks upon which these plants grow reaches a height of 89m.
Included in the list of plants to be found growing here, apart from the one on this page, are
Arctic Sandwort (Arenaria norvegica ssp. norvegica) [RRR],
Stone Bramble (Rubus saxatilis), and others which are not so rare such as Kidney Vetch (Anthyllis vulneraria), Early-Purple Orchid (Orchis mascula), Frog Orchid (Coeloglossum viride),
Hoary Whitlow-grass (Draba incana), Sea Plantain (Plantago maritima), Glaucous Sedge (Carex flacca) and Thrift (Armeria maritima) amongst others. These plants are specially adapted to the poor soil quality and chemical constituents (or lack of them in some cases).
Many of the plants growing on the Keen of Hamar on Unst differ in form to their counterparts which grow elsewhere on mainland UK. This could be due both to the rather extreme arctic climate way up north and to the available soils which are deficient in certain minerals and over-endowed with other more toxic heavy elements on the serpentinic rock upon which many species to be found on Keen of Hamar grow. The fact that Thrift (a reknown metallophyte which is able to sequester many differing heavy metals and stash them out of harms way) also grows on the Keen of Hamer is surely testament to the protection afforded by heavy metal concentrations if the plant has some mechanism for dealing with the heavy elements.
The erosion of the soil does not seem to be an issue on the Keen of Hamar: it is slowly eroded, but perhaps the dying plants at end of season slowly replenish that which is eroded.