By Peter Rosewarne

Sifting through my collection in order to catalogue and assess specimens more fully has got me thinking more about some of their mineralogical and crystallographic properties. Why are some examples of the same mineral one colour and others another? Why are some stubby and others prismatic? What crystal system do they each belong to? What is ilvaite or axinite or vivianite?

I’m fascinated by the interesting habits that some minerals exhibit which in many cases don’t seem to visually bear much resemblance to their crystal class, hence the title of this note. Some photographs of specimens are included below to illustrate the point, using pyromorphite, wulfenite, tourmaline, fluorite, vanadinite and fluorapatite as examples.

Pyromorphite. This lead chlorophosphate crystallises in the hexagonal system and normally forms barrel-shaped aggregates of yellow, green and sometimes orange (arsenian and botryoidal) crystals. Sometimes the crystals take on a hoppered habit where successive crystals grow outward with a stepped interior. Halite and bismuth commonly also crystallise in this habit. The best pyromorphite specimens came from the so-called Jersey Vein in the Bunker Hill Mine in Idaho, USA. A guy called Bob Hopper (who else?) took over the mine in 1980 and proceeded to find a treasure-trove of unprecedented pyromorphite specimens in 1981 that took the mineral specimen market by storm. This was then followed in the early 1990s by even better discoveries. I recall a very good specimen on a dealer’s page in The Mineralogical Record having a sale figure of $225 000 some years ago. Many of the specimens show a hoppered crystal growth as illustrated by the single green crystal in Figure 1. This is apparently caused by faster crystal growth at the edges due to stronger electrical attraction there.


Figure 1

Wulfenite. This lead molybdate crystallises in the tetragonal system. However, most of the iconic wulfenite specimens from Mexico show flattened, tabular habits which always remind me of Liquorice Allsorts. Some also show banding or layering of alternating orange or butterscotch and dark brown looking like a mini-sandwich. Figure 2a shows a crystal intergrowth from the type locality in Slovenia, while Figure 2b shows a typical Mexican Los Lamentos example with the ‘sandwich’ effect.

Figure 2a

Figure 2b

Uvite. This Ca/Mg tourmaline crystallises in the trigonal system but instead of being elongated along the c-axis and heavily striated along their length like most tourmalines, crystals are typically flattened and dominated by the trigonal pyramid terminations. A classic site for uvite is Pomba Pit, Bahia in Brazil, which produces very attractive aggregates of dark green and brown crystals associated with white to clear magnesite. An example from Bahia is shown in Figure 3, which could be mistaken for garnet at first glance? Those faces that look like octahedra are in fact part of the dominant trigonal pyramid terminations with only very short prismatic faces.

Figure 3

Fluorite. Crystallises in the cubic system and is commonly found as large and attractive interlocking and twinned cubes and octahedra. However, at the Nasrik Quarry at Poona, India, it can occur as spheroids on quartz in Deccan basalts. The example shown in Figure 4 illustrates this unusual habit.


Figure 4

Vanadinite. This lead chlorovanadinite crystallises in the hexagonal system and usually occurs as flattened to tabular hexagonal crystal aggregates. The example shown in Figure 5, from Milbladen in Morocco, shows an aggregate of multiple flattened hexagons that have morphed into what looks like a single unusual crystal shape.


Figure 5

Fluorapatite. Crystallises in the hexagonal system and usually occurs as elongated prismatic crystals, embedded in matrix, e.g. calcite. The example in Figure 6 shows dark green flattened tabular crystals sitting proud of the albite matrix, from the Sapo Mine, Brazil. The Panasqueira Mine in Portugal produces crystals of a similar habit but usually blue or green in colour, more translucent and associated with arsenopyrite, fluorite and muscovite.

 Figure 6

Photos and specimens – PR