UNUSUAL CRYSTAL HABITS

March 25, 2019

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

 

FACETIP – QUARTZ

February 21, 2019

By Duncan Miller

(This is a follow-up to a previous article on faceting quartz, to be found with other faceting articles on the club’s website http://ctminsoc.org.za/articles/category/Faceting.) 

Every faceter knows quartz, those great big glassy-looking chunks that seem to cry out to be turned into doorknobs. Or pretty, golden ‘citrine’ that can cut brilliant yellow stones. Or glowing, dark purple amethyst with seductive blue flashes, dreamy rose quartz, or rutilated quartz with geom...


Continue reading...
 

SOME MUSINGS ON SELLING A MINERAL COLLECTION FROM SOUTH AFRICA

February 21, 2019

By Peter Rosewarne

   

Having built-up a mineral collection the question arises, at some stage, as to what to do with it looking to the future. Options include do nothing (and continue to get enjoyment out of looking at and handling the specimens) and let someone else worry about it when you’re gone (i.e. throw it away), give it away, donate it to an institution (probably unwise in SA or anywhere probably), or sell it. This article looks at some aspects of the pricing and selling process ba...


Continue reading...
 

FACETIP – TOPAZ

January 19, 2019

By Duncan Miller

Topaz is a rather under-rated gemstone. This perhaps it due to the fact that pure, colourless topaz is relatively plentiful. Much of it is irradiated and then heat-treated to produce various intensities of bright blue. Natural blue topaz tends to be much paler, although dark blue stones do occur naturally. These are rare and hence more valuable. Natural topaz occurs in a wide variety of colours, including light green, yellow, orange and pink. The famous orangey-pink topaz fr...


Continue reading...
 

Two beached whales were spotted at Yzerfontein this past month, the first on 5th November and the second a few days later

November 23, 2018




And last week this fish appeared at Milnerton Lagoon


Grey chalcedony and aragonite fish


This is the same fish as above, but much prettier “when still alive”, and seen under short wave UV light.


...
Continue reading...
 

"There’s treasure – I just have to find it"

November 23, 2018

Talking about treasure hunting, let me tell you my story … It’s a tale of two parts.

I’ve always been the poster child for the story told by Victor Borge:  “if there’s manure, there must be a pony.”  It’s in my DNA.

Truth be told, my first real life encounter with this approach was doomed. 

To understand it better, you’d have to know that back then Dinner, Bed and Breakfast at a swanky hotel cost R40-00 and a full seafood buffet at the same hotel with all you could eat cos...


Continue reading...
 

FaceTips for December

November 23, 2018
by Duncan Miller

This month I will show you how to scale a GemCad diagram to a different L/W ratio. This is very easy if the diagram is a fully meetpoint diagram, without a preform. You note the initial L/W ratio from the Print Preview and then click on Scale in the Edit menu. Here you check the X box because you want to change the proportions in the X direction, then enter the appropriate numbers to divide by the initial L/W ratio and to multiply by the one you want, and press OK. The next me...


Continue reading...
 

FaceTips for November

November 4, 2018

By Duncan Miller

I started faceting in pre-GemCad days and found cutting ovals very laborious. I would cut the girdles by eye, using various oval templates, and placed the brilliant-style facets by eye too. Producing matching pairs was very trying. The advent of meetpoint faceting and GemCad overcame all these difficulties. Now there are lots of designs for ovals that are meetpoint, requiring no preform, with the girdle outline evolving out of the cutting sequence. You can access some of the...


Continue reading...
 

SEPTEMBER VISIT FROM CONSTANTIA WALDORF SCHOOL

October 25, 2018

The Waldorf School asked us if they could visit the club again this year, and Claire Vaskys organised the day for them. Thank you very much Claire.

Also a big thank you to Rinda who had kept all the little offcuts of stones, and dopped them in preparation for the children to grind and polish, and who managed the workshop while they were busy between machines. 

Thank you to Marsiglio who brought his tools, raw and finished materials, and allowed the kids to take his rock pick and smash it in...


Continue reading...
 

Geological Tour of the Rosh Pinah area

September 25, 2018
Since we have never fully explored the southern parts of Namibia before, we decided to head up to Namibia a few days before the planned start of the FOSAGAMS Namibia 2018 tour to explore the area. Heidi Naudé from the Pretoria Club put us in touch with Gisela Hinder who owns the Rosh Pinah Geo Center and after some discussion with Gisela on our interests, we pre-booked a guided geology tour with her. Our first night in Namibia was spent camping along the Orange River, and a casual walk acros...
Continue reading...
 

Make a free website with Yola