Botryoidal Minerals: A Colourful Trip Around the Mineral Kingdom
by
Peter Rosewarne
Introduction
My previous MinChat article on fluorite described a colourful trip around the world. In this article we take a colourful trip around the mineral kingdom, using minerals with a botryoidal habit as the guide. The idea came from the supplement to The Mineral Record of January-February 2020 on Mineral Collectors in Arizona, with the focus of one of the collectors being on botryoidal mineral specimens. The term botryoidal is derived from the Greek word botryios or ‘grape-like’. The terms reniform, globular and mammillary are also sometimes used but for the purposes of this article we’ll stick with botryoidal. This habit in minerals is formed by parallel growth of narrow crystals growing outwards from a central nucleus.
For this article, we are going to be looking at smithsonite, pyromorphite, mimetite, prehnite, orpiment, planchéite, hematite and mottramite, a varied batch of carbonates, molybdates, arsenates, phosphates, oxides, sulfides and silicates (Rosey’s Crystals specimens and photographs except * which are reproduced with permission of The Mineral Gallery and Auction). Simple chemical formulae are given but where these are more complex, a short, written description is given.
Botryoidal Minerals
We’ll start with smithsonite (ZnCO3) as this is the archetypal botryoidal mineral and possibly the mineral with the most varied colours in botryoidal form, e.g. silver, yellow, pink, orange, brown, lavender, green, blue and various hues thereof, and rarely, red. It’s certainly one of my favourite minerals and it was fun trying to collect as many different colours as I could some years ago, and the striking yellow specimens coming out of China fairly recently persuaded me to come out of mineral specimen-buying retirement and buy a couple of those specimens. Colours are due to trace elements, and minerals in some cases, e.g. greenockite, such as cadmium (yellow/orange), cobalt (pink) and copper (greens and blues). Pure smithsonite is white or colourless.
Some of the best botryoidal smithsonites come from Mexico,
principally El Refugio Mine in Choix District, Chihuahua and the San Antonio
Mine, Santa Eulalia, Sinaloa. Wonderful specimens of pink, green, yellow and
orange came from Choix, the pink being the most sought after among collectors
but my pink ones are long sold so we’ll have to do with lavender (Figure 1), green
(Figure 2*), brownish-yellow (Figure
3) and ‘silver’ (Figure 4).
Figure 5 shows the satiny lustre that is associated with the best
botryoidal smithsonites; if only this one were pink or blue…
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
The robin’s-egg blue
smithsonites from the Kelly Mine, Magdalena District of New Mexico are among
the most sought after among mineral collectors. A modest example with calcite
crystals is shown in Figure 6 although the best ones have more
pronounced botryoids and a satiny lustre like that shown in Figure 7
(with permission of the Mineralogical Record). I’ve broken with my usual
approach here by including someone else’s specimen and photograph to show a
world-class example. These smithsonites are regarded as classics with
five-figure (US$) price-tags.
Figure 6
Figure 7
One
of my favourites, among many, is the deep apple-green
botryoidal smithsonite from the 79 Mine, Gila County, Arizona, of which an
example is shown in Figure 8*. The combination of the silky lustre and
colour make this smithsonite irresistible, to my eye at least.
Figure 8
Relatively recent finds in China include yellow botryoidal smithsonite from Weinan, Yunnan Province, an
example of which is shown in Figure 9*. These first surfaced in numbers
in the West at the Denver Show of 2016 but apparently come from a hoard much
older than that.
Figure 9
The
Masua Mine in Sardinia, Italy, is famous for its yellow smithsonite (drool over the cover of Smithsonite - Think
Zinc!) with a modest example shown in Figure 10.
Figure 10
Closer
to home light blue botryoidal
smithsonite was found at the Skorpion Mine, Namibia (as shown in Figure 11) and green, yellow and pink from Tsumeb,
although the latter mine is more famous for its crystalline smithsonite, among
the “best-of-species” in the world, if not the best, rather than the botryoidal
habit. However, the latter habit was quite common apparently, mainly the
green-tinted variety. I had a modest green example originally purchased from
Johann de Jongh at Open Day many years ago but someone else owns it now (the
owner of RC116 can claim their mystery prize from Jo!).
Figure 11
Figure 12
Moving
away from smithsonite, and in no particular order, pyromorphite (lead chloro-phosphate) was found in attractive brown to orange botryoidal masses at the Bunker Hill Mine in Idaho, USA. The
orange colouring is due to the presence of arsenic. The best orange examples
seem to glow like red-hot metal. An example is shown in Figure 13*.
Figure 13
Mimetite (lead chloro-arsenate), so named
because it imitates pyromorphite, with which it forms a solid solution, is
largely known from prismatic crystals from Tsumeb but also occurs in attractive
yellow and orange botryoidal form. The San Pedro Corralitos Mine in Chihuahua,
Mexico produces beautiful yellow
examples with the botryoids having an almost ‘glowing’ lustre, as shown in Figure
14. Less well known are orange
examples from the Chah Milleh Mine, Anarak District of Iran, as shown in Figure
15.
Figure 14
Figure 15
Prehnite (calcium/aluminium silicate) is mostly known to local collectors
from the light green botryoidal whorls and masses that occur in cavities in
basalts in the Brandberg and Goboboseb areas of Namibia. It is also well known
from the Prospect Park Quarry, New Jersey, USA and a light green example from
that site is shown in Figure 16. This one has an attractive silky lustre
similar to the smithsonites mentioned previously.
Figure 16
Figure 17
Planchéite is a hydrated copper silicate
mineral and the sky-blue example
shown in Figure 18 comes from the Tantara Mine, Katanga, Zaire. Locally,
it is more commonly associated (non-botryoidal) with dioptase from the Kunene
Region of Namibia.
Figure 18
Hematite (Fe2O3) is
another archetypal botryoidal mineral, at least when it occurs as ‘kidney ore’.
The example in Figure 19 is a typical reddish-brown one from the famous Egremont site in Cumbria,
England.
Figure 19
Finally, we have the more drab-coloured mottramite as our last mineral. The first example
is dark green with
lighter contrasts and comes from the prolific Ojuela Mine, Durango, Mexico, as
shown in Figure 20. And the final one for this article is from Tsumeb,
which had to be featured here somewhere, and is shown in Figure 21. This
one is dark grey-green and not very colourful but the mine has a very
colourful history, so a reasonably appropriate place to finish this article.
Figure 20
Figure 21
We’ve
covered just about all the colours in the visible spectrum, excluding red and
substituting lavender for indigo/violet. But what about malachite I hear you
say? Well the only botryoidal specimen I have of that is polished and surely everyone
knows what botryoidal malachite looks like anyway?
References
Hughes, T. Liebetrau, S. and Staebler, G. Eds. (2010). No. 13: Smithsonite – Think Zinc! Lithographie, LLC. Denver.
Von Bezing, L. Rainer, B. and Steffen, Jahn. (2014). Namibia I. Minerals and Localities. Bode, Germany.
The Mineralogical Record (2020). January-February 2020, Vol 51 No. 1. Supplement: Mineral Collections in Arizona-II. Tucson.
The Mineralogical Record (2020). 50 Years of What’s New in Minerals: Vol II, 2004-2019, p1623. Tucson.
In : Mineralogy
