Final answer:
Radiolucent stones are those that allow the passage of X-rays and include uric acid and cystine stones. Urinary stones like struvite and carbonate apatite form in urine with an increased pH. Stoneworts, a type of algae, can also precipitate calcium carbonate and raise pH.
Step-by-step explanation:
In the context of medical imaging, a radiolucent stone refers to a stone that does not block the passage of X-rays or any similar radiation and, thus, appears darker on a radiograph. Urinary stones such as those formed from uric acid or cystine are typically radiolucent. In contrast, calcium-containing stones, like calcium oxalate or calcium phosphate stones, are generally more radio-opaque.
Pertaining to stones with an increased pH, we can refer to certain biological stones such as those formed in the urinary system. Struvite stones, which are composed of magnesium ammonium phosphate, often form in a urine with an elevated pH, usually above 7.0. This alkaline pH can be a result of a urinary tract infection caused by urea-splitting bacteria. Additionally, carbonate apatite stones can also form under similar alkaline conditions.
From an environmental point of view, some species of charophyte algae like Chara, known as stoneworts, can precipitate calcium carbonate within their cell walls in alkaline environments with a pH of 7 or greater. This process essentially creates stony deposits that can increase the local pH as well.