This species was discovered in 1971 by Alfred Lau, in a location south of Tocopilla, Chile. It took some time before it was officially described because it shares many characteristics with both Copiapoa and Eriosyce s.l. (s.l. meaning “sensu lato”, as in the expanded view of that genus), while at the same time possessing many particular characteristics other members of those genera do not. In the nineties it was included in the genus Eriosyce s.l., although some felt it belonged in Copiapoa, while others felt the genus Islaya (included in the sensu lato version of Eriosyce) was the best fit. Roy Mottram felt the species was too distinct to belong in Eriosyce, and erected the genus Rimacactus for it in 2001. Later phylogenetic molecular studies have shown this to be the most correct option, as the species is not closely related to Eriosyce. The same studies show that it is most closely related to Yavia (another monotypic genus) and Neowerdermannia (a genus with two species).
It is an attractive species with white needle like spines on a greenish-brown body, and with copious amounts of wool. There is, however, a very great difference between plants in habitat and grafted plants in cultivation. The plant is often grafted because it is fairly difficult on its own roots, being particularly vulnerable to overwatering. The upside of grafting is that it flowers easily and willingly, while also developing a lot of attractive wool. On the other hand it does not particularly resemble habitat plants when grafted.
I sowed some 10 seeds 18 months ago and two germinated. They grew very slowly and as seeds of this species are usually quite rare I decided to graft the two seedlings in order to try and produce seeds of my own in the future. Only one of the grafts were successful however, but in exchange it has grown very well and has now flowered just a little over a year after being grafted. The stock plant is Pereskiopsis diguetii which is an excellent stock plant for seedlings and will greatly accelerate growth.
The petals are a nice shade of yellow, while the sepals contrast well with an attractive brownish-red colour. The flowers last a good deal longer than in most cacti. The flowers also contrast nicely with all the white wool.
I am going to sow more seeds of this species soon and try to grow them on their own roots. The species will probably do best in a completely inorganic mixture with somewhat infrequent watering.